AN ABSTRACT OF THE THESIS OF
Eric J. Hanson
in
for the degree of
Horticulture
Doctor of Philosophy
presented on
June 5, 1984
Title: Boron Nutrition and Fruit Set of 'Italian' Prune
Abstract approved:
_
Pal^itk i: Breen
v
Foliar B sprays have increased fruit set in 'Italian' prune
(Prunus domes tica L.) by 2 fold in some trials and had no effect
in others.
This work was conducted to identify reasons for this
inconsistent response.
Since B levels in flowers are associated
with the amount of fruit set,
factors affecting B supply to
flowers were also studied.
Foliar B sprays (500 ppm) were applied to 'Italian' prune
trees in the fall.
Results
from 2 trials suggest
that
the
response to B is influenced by the overall fruit set and B levels
in flowers in a given year.
Boron sprays had no effect on set
during a warm spring when overall fruit set was high (12.2%), but
increased set by 32% during a cool year when the overall set was
low (3.2%).
The B content of flowers forced on excised branches was
similar to that of flowers on intact trees,
indicating that
flowers derive most of their B from reserves in nearby branch
tissues.
Simulated
rain and
relative
humidity
treatments
indicated that B in flowers is not readily leached by rain and
that
transpiration
rate has little effect on B
accumulation
in
flowers.
Boron
weeks
accumulated
(5
Xylem
differentiated in the axes of buds as they began to swell.
Prior
this,
the discontinuous xylem vessel connection may impede
movement into buds.
K,
Ca,
and
S
before bloom,
that
B
elements.
but rapidly from then until
swelling
bloom.
to
before bloom),
slowly in buds prior to bud
in
The observation that the concentrations of P,
in the xylem exudate increased markedly
while B concentrations remained constant,
branches
The
B
is
amount
not as
readily
remobilized
of B supplied to buds via the
5
weeks
suggests
as
xylem
other
was
estimated from the accumulations of B in buds, the B concentration
in
Only
xylem
exudate,
and the calculated transpiration
from
buds.
a fourth of the B entering buds during the 2 months prior to
bloom was supplied by the xylem.
BORON NUTRITION AND FRUIT SET
OF 'ITALIAN' PRUNE
By
Eric J. Hanson
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Doctor of Philosophy
Completed June 5, 1984
Commencement June 1985
APPROVED:
Associate Professor of Horticulture^"in charge of major
^ _| |
"
I a" !-• ^^*^'
—
— »■»■
*
Head of Department oy Horticulture
i "^^ »'•• t
Dean of Graditabe School
01
<J
Date thesis presented
Typed by Eric J. Hanson for
June 5, 1984
Eric J. Hanson
Acknowledgements
I would like to thank Dr.
Michael H.
Chaplin for serving as
my major advisor during the early part of my program.
express
my
deepest
appreciation to Dr.
Patrick
serving
as
advisor during the second half of
I'd like to
J.
my
Breen
for
program.
His
encouragement and sincere concern were invaluable to me, while his
technical
advice
and foresight (and intense
interest
nutrition) directed and strengthened this research.
is
also
extended to my committee members,
Drs.
in
plant
Appreciation
Larry
Boersma,
Norman Goetze, David Loomis and Daryl Richardson.
I'd
like
to
thank Jim Wernz and Fred Dixon
of
the
Plant
Analysis Lab for their advice and prompt analysis of samples,
acknowledge
the financial support provided by the
Department
and
of
Horticulture and the Computer Science Department.
Finally,
of
the
Department of Horticulture for their support and
friendship,
and
extend
Their
my
I'd
warmest
like
to
thanks
thank the graduate students
to
my
parents.
encouragement and unquestioning support cannot be repaid.
constant
Note:
This thesis is presented in two papers written in
in the format of the Journal of the American
Society for Horticultural Science.
TABLE OF CONTENTS
Page
Chapter 1.
Introduction
1
Chapter 2.
Review of Literature
3
Introduction
Proposed Functions
Boron Availability and Absorption
Boron Transport
Xylem Transport
Phloem Transport
Boron and Fruit set
3
4
6
7
9
11
15
Chapter 3.
Effects of Fall Boron Sprays and Environmental
Factors on Fruit Set and Boron Accumulation in
'Italian' Prune Flowers
Abstract
Introduction
Materials and Methods
Results
Discussion
Literature Cited
Chapter 4.
Xylem Differentiation and Boron Accumulation
in 'Italian' Prune Flower Buds
Abstract
Introduction
Materials and Methods
Results
Discussion
Literature Cited
Chapter 5.
General Discussion and Conclusions
19
19
20
21
24
33
38
40
40
41
43
45
52
58
60
Bibliography
65
Appendices
76
LIST OF FIGURES
Figure
Page
Chapter 4
4.1
Radial section of procambial tissue in the axis
of dormant 'Italian' prune flower bud collected
in January.
46
4.2
Radial section of the axis of 'Italian' prune
flower bud collected at green tip stage (March
10.
47
4.3
Boron accumulation in 'Italian' prune flower
buds and concentrations in xylem exudate.
50
4.4
Ca accumulation in 'Italian' prune flower buds
and Ca concentrations in xylem exudate.
51
Appendices
A.l
Accumulation of K, P, and S in 'Italian' prune
flower buds and concentrations in xylem exudate.
76
A.2
Changes in boron concentrations in
prune leaves treated with 500 ppm B
on July 29 or September 19.
'Italian'
solutions
77
A.3
Effect of 500 ppm B sprays applied in autumn,
1981 on fruit set of 'Italian' prune in 1982.
78
LIST OF TABLES
Table
Page
Chapter 3
3.1
Effect of 500 ppm B spray applied in autumn,
1981 on fruit set, yield and fruit B concentration of 'Italian* prune.
25
3.2
Effect of 500 ppm B and 0.5 M Ca sprays applied in autumn, 1982 on bud and flower dry
weights, pistil length and fruit set of 'Italian' prune in 1983.
26
3.3
Effect of 500 ppm B spray applied in autumn,
1982 on pollen tube growth in 'Italian' prune
flowers in 1983.
27
3.4
Effect of 500 ppm B and 0.5 M Ca sprays applied
in autumn, 1982 on B levels in 'Italian' prune
trees in 1983.
29
3.5
Boron levels in excised branches from B treated
(500 ppm B spray in autumn, 1982) and control
'Italian' prune trees forced to bloom in February.
30
3.6
Effect of simulated rain applied during the
prebloom period to excised 'Italian' prune
branches on flower B levels and size.
31
3.7
Effects of low and high relative humidity (RH)
during the prebloom period on transpiration and
flower B levels of excised 'Italian' prune
branches.
32
Chapter 4
4.1
Boron movement through the cut end and distribution in excised twigs of 'Italian' prune
(Prunus domestica L.) and chokecherry (Prunus
virginiana L.).
48
BORON NUTRITION AND FRUIT SET
OF 'ITALIAN' PRUNES
Chapter 1
INTRODUCTION
'Italian'
variety
grown
prune
erratic
annual
domestica
in the Northwest.
suited for drying,
to
(Prunus
million lbs.
is
canning or fresh consumption,
prune
the
principal
It is a high quality
fruit set and periodic crop
Oregon
L.)
variety,
but it is prone
failures.
production has ranged from
Since
a
low
1960,
of
6.8
to a high of 88 million lbs. (89) due largely to the
variability in 'Italian* yields.
Because
boron has improved fruit set in a number of species,
trials
were begun in 1971 to evaluate the potential of
sprays
for
contained
symptoms
increasing
adequate
B
set
by
in
'Italian'
August leaf
of B deficiency.
prune.
analysis
foliar
Trees
and
set 115% and yield 40 to 100% the following year (31).
trials
Set
increased
by B sprays in 4 orchards (44
unaffected in 2 others (29).
set
could not be determined.
to
no
increased
Additional
were conducted over 2 years at 5 sites In western
was
used
showed
Sprays applied in the fall
B
Oregon.
125%)
but
The mechanism by which B influenced
Pollen tube growth was
unaffected
and pollen viability was slightly reduced by B sprays.
These
sprays
but
inconsistent
trials demonstrated the potential benefit from fall
also the variability in response.
response
are
Reasons
for
B
the
unclear, but several factors might be
important.
levels
in
applied
response.
First,
if
the flowers,
and
endogenous
the fruit set response is dependent on
then factors affecting the
B
the
An understanding of how and when B enters flowers
may
would
The response to B may also depend on
the overall set on a given year.
in
flowers
of
influence
help identify these factors.
into
movement
B
Two orchards not responding to B
previous trials had high overall set (29).
The
purpose of this work was to determine why the fruit
set
response to B sprays is variable and to identify factors which may
limit the B supply to flowers.
Chapter 2
REVIEW OF LITERATURE
Introduction
Boron
is essential for the growth of angiosperms
gymnosperms (81).
diatoms
(80),
(130)
and
Other organisms benefiting from B include some
ferns
(22).
and the nitrogen
fixing
bacteria,
Azotobacter (45).
The
amount of B required for optimum growth of higher plants
varies
with
plants
develop deficiency symptoms when leaf B levels drop to
ppm (58),
oats
species.
Rutabaga
(Brassica
napobrassica
Mill.)
20
whereas leaf B levels as low as 6 ppm are adequate
(Avena
sativa
L.)(55).
Adequate B levels in
for
fruit
tree
concentrations inducing toxicity symptoms also
vary
leaves are typically between 15 and 30 ppm (108).
Leaf
widely
B
with
species.
exceeds 20 ppm (55),
no
Growth of oats may be reduced if
leaf
whereas rutabaga leaves with 250 ppm B
adverse effects (58).
Leaf concentrations inducing
B
show
toxicity
symptoms in fruit trees may range from 56 ppm B in plum to 100 ppm
B in cherry (Prunus avium L.)(108).
The
still
specific function or functions of B in higher plants are
not
suggested.
The
literature and current theories of B function in plants have
been
thoroughly
(99).
A
known although many roles have
been
reviewed recently by Dugger (45) and Parr and Loughman
brief overview of the more prominent theories
will
be
given here.
This will be followed by more detailed discussions of
B absorption and transport in plants,
and B involvement in pollen
germination, pollen tube growth, and fruit set.
Proposed Functions
An
death
early symptom of B deficiency is cessation of growth
of shoot meristems and root tips.
This led
to the
and
early
theory that B was involved in carbohydrate transport (53).
Later
work
sugar
indicated
translocation
strength
that
B
deficiency
indirectly
(47,132),
and
probably
by reducing growth
influences
and
altering
this is a commonly accepted
sink
view
today
tips
cease
(45).
The
observation
that
shoot meristems and root
growth and die when the B supply is removed suggests a role for
in cell division and elongation.
studies
of
B
deficient
root
It is not clear from anatomical
tips
whether
cell
elongation is affected first by low B (33,34,77,111).
B
deficiency on nucleic acid metabolism,
content (2,70),
(36,37) and
in
cell
14
B
increased incorporation of
division
or
Effects of
including decreased RNA
32
P into nucleic acids
C-uridine into RNA (32), suggest an involvement of B
division.
Symptoms similar to B deficiency
have
been
induced by applying an inhibitor of uridine synthesis (2), while B
deficiency
symptoms
have been alleviated by applying
uracil
or
orotate, a uracil precursor, to B deficient cotton ovules cultured
in
vitro (17).
It is still not clear whether the effects
of
B
nutrition on nucleic acid metabolism involve a primary function of
B or secondary responses to altered growth and metabolism.
An
based
involvement
been
proposed
largely on the observation that meristerns cease growth when
B is limiting.
of
of B in auxin metabolism has
Although there was an early report that
symptoms
B deficiency could be partly alleviated by IAA applications on
cotton (48),
deficient
suggest
this work could not be repeated (84). Reports that B
plants
that
accumulate
high
auxin
levels
B deficiency may be equivalent to
(35,40,67,102)
auxin
toxicity.
Although the morphological and gross anatomical symptoms of
toxicity are similar to B deficiency (102,105),
auxin
Hirsch and Torrey
(62) argued that distinct ultrastruetural differences exist.
Changes
(3,98,112)
in
cell
wall
thickness
in
B
deficient
plants
have suggested an involvement of B in the synthesis or
deposition of cell wall materials.
cellulose
(92),
lignin
Differences in the content of
(46,92) and phenolic lignin
(41,131) have been reported in B deficient tissue.
precursors
The
evidence
that B functions in the process of lignification has been reviewed
by Lewis (81).
with
Torssell (124) hypothesized that B forms complexes
cell wall components to control the orientation of cellulose
microfibrils and allow cell elongation.
Recent
membrane
reviews
function
have focused attention on B
(99,100).
involvement
Low B supply rapidly
reduces
in
ion
absorption by roots (101,105), possibly by reducing membrane bound
ATPase
activity
membrane
function
(101).
Several
bean
root
phenomena
are strongly influenced by
electrical potential of mung bean
mung
other
tips to
B
related
nutrition:
to
the
hypocotyls (117); attachment of
negatively
charged
glass
(118);
the
6
nyctinastic
closing
of
Albizzia
julibrissin
stomatal
closure and Rb uptake by guard cells
Loughman
(99)
membranes,
pinnules
(106).
hypothesized that B is a functional
possibly
forming
linkages between
(119);
Parr
and
component
of
glycoproteins
or
glycolipids.
The
with
exact function of B is still
insufficient
B
undergo
a
wide
physiological, and biochemical changes.
the
unclear.
Plants
range
of
supplied
anatomical,
Dugger (45) believes that
involvement of B in these various processes is linked to
ability
to
form
complexes
with
a
variety
of
its
poly-hydroxy
compounds.
Boron Availability and Absorption
Most
native
soil
B
is present
as
tourmaline
relatively insoluble boro-silicate minerals (14).
contain 14 to 40 ppm total B,
B
and
other
Soils commonly
but between .1 and .2 ppm available
when estimated by hot water extraction procedures
(14).
Most
crop plants require .1 to .5 ppm hot water extractable B (12)
Boron availability dependents on soil pH and organic
Boron
becomes
fixed and less available in alkaline
Although the mechanism of fixation is unclear,
of
(56).
hydroxy
(57).
and may
with broken Si-0 or Al-0 bonds on the surface of
particles (14).
B
soils
maximum adsorption
B to layer silicate clays occurs at pH 9.0 (109),
associated
matter.
Berger
be
clay
Soil organic matter may be an important source of
and Pratt (13) suggested B forms complexes
groups on organic matter,
necessary for its release.
and microbial activity may
with
be
Because B adsorption by clay minerals
declines with pH, B retained in organic matter may be an important
source in acid soils.
Although
soils
may
early
have
a
studies reported that applications of Ca
negative effect on
(44,86), later work showed that
raising
soil
influence
pH
the
(57).
absorption
by
plants
Ca affects B uptake indirectly by
The
physiological
B
to
amount of Ca in plant
B
requirements.
tissue
Tobacco
may
plants
containing the same B concentrations develop B deficiency symptoms
more readily when the tissue Ca concentrations are high (44).
Ca/B
The
ratio of tissue may describe the B nutrition of plants
more
accurately than B concentration alone (44,104).
Boron
may become deficient in sandy or acid soils which
heavily leached by rain (133).
excessively dry soils.
(12),
limited
matter
and
B
are
Crop deficiencies also develop on
This may be due to B fixation in dry soils
root growth in the surface horizons where
contents are usually highest (12),
or
organic
limited
B
uptake due to reduced transpiration rates (56).
Boron Transport
Boron
(H,B0, )
and
may
be
present in soils and tissues
and undissociated boric acid (H-BO,).
cellular
pH ranges,
undissociated boric
(H,B03 + 2H20 = H.BO," + H-O"*", pK
of
(95)
as
as
acid
solution
absorption
pH
increased
above
The primary form
Oertli
7.0.
soil
predominates
and
Grgurevii
found B absorption by excised barley roots declined
the
ion
At typical
= 9.25)(103).
B absorbed,by plants has been argued.
borate
The
rapidly
decrease
was closely associated with the dissociation of
in
boric
8
acid to borate, suggesting that borate was not as readily absorbed
as boric acid.
(Saccharum
Evidence suggests that B is absorbed by sugar cane
officinarum
L.) leaf tissue primarily as
borate
ion
(19,20), whereas characteristics of B absorption by carrot (Daucus
carota)
root
absorbed
Passive
diffusion of B across
membranes
by Oertli and coworkers (15,94,95).
excised
barley
concentrations
with
acid
may
be
metabolic requirement for B transport across membranes is
clear.
argued
suggest both borate and boric
(135).
The
not
discs
roots
and
increases
external concentrations (95),
been
Boron absorption
linearly
appears to approach a
has
with
solution
diffusion
by
B
equilibrium
whereas varying the
solution
temperature or adding metabolic inhibitors (KCN or 2,4-DNP) had no
detectable effect on B absorption (15).
Bowen and Nissen (23,24) also studied B absorption by excised
barley roots,
but concluded that transport was an active process.
They believe that as much as 90% of the B absorbed by barley roots
was actually in the free space, possibly associated with a film on
the
surface
reversibly
was
of roots,
to cell walls (23).
cells,
and
B
bound
They felt that a 30 minute
rinse
necessary to remove free space B before intercellular B could
be quantified,
rinse
free space between
(15)
diffusion
and suspected that previous work using a 30 second
and a 1 minute rinse (95) may
into
the
free
space
and
have
not
been
uptake
describing
by
Intercellular accumulation of B was found to be sensitive to
low temperature and metabolic inhibitors,
cells.
both
suggesting transport is
an
active process (24).
also
inhibited
Boron uptake by carrot root tissue
by low temperature,
anoxia,
and
the
was
metabolic
inhibitor 2i)4-dinitrophenol(135), and B uptake by sugarcane leaves
is sensitive to low temperatures and metabolic inhibitors (19).
Indirect
been
found
tomato
evidence of metabolic involvement in B
at the whole plant level.
xylem
leaves
in
in the root solutions were varied from 0.5 to
1.0
the
plants have some
regulatory
control
over
A B efficient tomato cultivar transported more B to
and
efficient
concentrations
the
indicating
uptake (66).
has
sap remained unchanged (0.68 to 0.82 ppm) when
concentrations
ppm,
The B
uptake
had
higher xylem sap B concentrations
cultivar
growing in the same nutrient
than
a
solution
less
(28).
Reciprocal grafts demonstrated that this regulation was located in
the roots.
Bowen (21) compared the amount of water transpired and
the
B absorbed by sugarcane seedlings.
air
temperatures
transpiration
37 "C to 80C reduced B
from
rate,
Decreasing the root
while
increasing
uptake
the
transpiration rate much more than B uptake.
more
humidity
and
than
reduced
Their findings show B
uptake is controlled by the plant and not directly proportional to
water use.
It can be concluded that plants regulate B uptake and
transport and this regulation has an energy requirement.
Xylem Transport
Boron
is transported from roots to shoots in the xylem
the transpirational stream.
Oertli (94) has shown that
with
movement
of B to the shoots of barley plants is more strongly influenced by
transpiration
rate
than
is
transport
of
K,
Mg,
Ca
or
Fe.
10
Increasing
temperature
margins.
transpiration
resulted
Modifying
in
rate by altering relative
a
greater accumulation of
environmental
factors
to
B
in
or
leaf
increase
pressure also increases B transport to shoots (94),
are
humidity
root
and if plants
given supraoptimal B supply they may lose large quantities of
B from leaves in guttation drops (93).
Estimates of xylem B concentrations vary widely.
Jones
(28) collected xylem sap flowing from the excised stems
tomato
Later
and
of
plants under root pressure and found sap from one cultivar
contained
B
Brown
1.0 ppm B while another cultivar contained 5.2
ppm
B.
work using the same cultivars but varying the root solution
concentrations showed B concentrations in xylem exudate
from 0.1 to 16 ppm (26).
ranged
Sap collected in a similar manner
from
sunflower plants contained less than 1.0 ppm B when the roots were
exposed
to
0.5
to
1.0
ppm
B
solutions
(66).
Others
have
questioned whether the root pressure extraction technique actually
samples
xylem sap (103).
Boron
the
concentrations
rates of transpiration and B accumulation in
(103),
from
plants.
Raven
using reported values of water transpired per g dry weight
accumulated (1000 ml/g D.W.
B
in the xylem sap can be estimated
for herbaceous C_ plants) and typical
concentrations in the leaves of C_ plants (1 to 65 ug/g
D.W.),
estimated the B concentrations in the transpirations stream should
range from 0.01 to 0.6 ppm.
the
concentrations
pressure
extraction
These concentrations are lower
measured in sap,
technique
and suggest that
may not
accurately
the
sample
than
root
the
11
contents
rates
of
and
xylem elements.
Bowen (21) measured
B accumulation in the shoots of
sugar cane seedlings exposed to a variety of
intensities and relative humidities.
transpiration
hydroponically
grown
temperatures,
light
Boron concentrations in the
xylem sap were calculated to be between 2.2 and 12.6 ppm. The high
calculated
concentrations
in the sap may have resulted from
the
the high B concentrations in the root solutions (2.0 ppm).
Raven
(103) stated that although B transport from the
to the shoots occurs
absorption
fraction
by mass flow in the xylem,
does occur in the roots.
roots
regulation of
B
Evidence that a considerable
of free space B may be bound in cell walls
(23,116,120)
suggests that B-carbohydrate complexes may influence B movement in
the apoplast.
Phloem Transport
The
The
phloem
first
mobility of B is limited under most
symptoms
of B deficiency are usually
conditions.
seen
in
young
tissue of actively growing plant parts indicating B is not readily
remobilized
from older tissue to supply deficient growing
points
(49).
Several
been
explanations for the low phloem mobility of
suggested.
excluded
Epstein
(51)
hypothesized the B
is
B
have
actively
from the phloem in order to prevent the accumulation
callose (a&-l,3-linked glucan) which may plug the pores of
elements.
Injection
Cucurbita
of
sieve
of 0.5 to 10 ppm B solutions into the sieve
elements
of
maxima petioles induced
heavy
plugs
of
callose,
but assimilate movement in the phloem was unaffected
or
12
accelerated
callose
(52).
(125),
Boron
deficient bean leaves also
so the exact relationship between B
accumulate
and
callose
accumulation is unclear.
Oertli and Richardson
limited
movement of B out of leaves based on B
adjacent
enters
high
xylem and phloem tissue.
the
and
diffuses
carried
between
They proposed that B
readily
moves in the phloem towards
decrease
further
from
the
concentrations
petiole.
the
leaf
are
Apoplastic
margin
and
B
out of the sieve elements and into adjacent xylem and is
back to the leaf margins with the
transpiration
stream.
countercurrent exchange system may prevent B movement out of
leaves and redistribution throughout the plant.
hypothesis,
to
explain the
exchange
phloem at the leaf margins where
concentrations
This
(96) developed a model to
the
To support their
they demonstrated that B moves readily from the xylem
adjacent phloem in the stem of cotton plants.
They
also
showed that B moved through a strip of bark and into the stem when
the
end
suggesting
of
the
strip was immersed in a
100
that phloem mobility is possible.
(66)
saw
the
transpiration
ppm
B
solution,
Husa and
Mcllrath
no evidence of B movement out of sunflower leaves
countercurrent
stream
exchange
was
reversed,
suggesting
between the xylem and phloem is not
when
that
the
only factor limiting B movement out of leaves.
Indirect evidence of phloem transport and remobilization of B
has
sand
been seen in several species.
If grape seedlings
culture with sufficient B are placed in minus B
grown
in
conditions,
the B concentrations in the older leaves declines from 77-123
ppm
13
(dry weight) to 36-42 ppm,
old
if
presumably because B is remobilized in
leaves to supply the growth of new leaves (107).
Similarly,
B is removed from the root solution of sand cultured
plants,
old
accumulate
leaves
B (11).
lose
B
while
young
continue
to
Mcllrath (88) demonstrated B movement out
of
mature cotton leaves containing 33 ppm B
leaves
root
and out of mature turnip
containing 50 ppm B when plants were placed in a
media.
No
species studied.
was
leaves
brocolli
observed
remobilization
B
remobilization was observed in
minus
seven
B
other
No evidence of remobilization from cotton leaves
following
from
old
foliar
B
applications
(4),
leaves of tomato was seen
and
no
B
was
when
removed from the root media (28).
Evidence
of
phloem mobility has been obtained by
comparing
the relative rates of nutrient accumulation in fruits.
and
Van Lune (127)
Goor
suggested that most nutrient ions enter young
apple fruit through the xylem,
primarily
Van
but there is a gradual change to a
phloem supply as the fruit develops.
They
used
this
assumption to estimate the phloem mobility of different nutrients.
Boron
and
continued
have
K
were
classified
as
phloem
mobile
because
to accumulate in the fruit even after the fruit
switched
to
a phloem supply.
In contrast to
B,
they
should
Ca
was
classified as phloem immobile because movement into fruit declined
as fruit developed.
The
not
underground fruits of peanut and subterranean clover
transpire
requirements
at common soil moisture levels so
their
do
nutrient
are thought to be absorbed directly by the fruit
or
14
transported to the fruit via the phloem.
Campbell et
al.,
(30)
showed that B absorbed by roots can be transported to fruit, while
the
Ca
requirement
of fruit must be absorbed
directly
by
the
fruit.
Extensive
when
phloem transport of B may occur
in apricot
tissue concentrations approach toxic levels.
(50)
found
that
quantities
of
apricot
B
when
fruit
the
and
roots
were
accumulated
exposed
to
large
high
much of this must have been supplied through the
phloem.
B sprays applied in the fall to prune trees have
B concentrations in dormant buds the
and flowers the following spring (29).
and
B
supply is small,
was
transpiration
al.
Assuming
increased
fruit
Eaton et
concentrations.
Foliar
that
bark
trees
xylem
following
winter
The possibility that the B
absorbed directly by buds and not remobilized from leaves
to
buds was not studied.
Although
phloem
mobility
of B in plants is
normally
very
limited, evidence of phloem transport has been reported in several
species under a variety of experimental conditions.
Environmental
conditions and species characteristics which affect B movement
the
phloem
phloem
Scott
only
are not understood.
mobility
may
An important factor
be the concentration of B
and Schrader (107)
influencing
in
felt that B moved out of
the
tissue.
grape
leaves
if concentrations were above a certain level.
Ohki (4) saw no evidence of phloem mobility in cotton,
previously
suspected
reported
to
remobilize
B
from
leaves
in
Andersen and
a
species
(88),
and
that tissue concentrations in their study may have been
15
too low.
Eaton (49) demonstrated that as the total B content
leaves increases,
sap.
Skok
nondlalyzable
content
zero
as
a greater proportion of B is found in expressed
and
Mcllrath
B fraction,
changes,
of
(110)
found
leaves
contain
which remains constant as the total
and a dialyzable B fraction,
B
which declines to
symptoms.
The
proportion of dialyzable to nondlalyzable B in leaves varies
with
species
the plants begin to develop
a
and
B supply (88).
deficiency
Although the
relationship
between
dialyzable and nondlalyzable B and phloem mobility is unclear,
is
possible
increase
that
the
high
proportion
movement in the phloem.
to
concentrations of
species (116),
in
of dialyzable B) may be
tissue
(which
necessary
for
Evidence that a fraction of B is adsorbed
cell walls (23,120),
with
B
it
and that the adsorption capacity
indicates that B movement in plants
varies
may
be
restricted by factors in the apoplast as well.
Boron and Fruit Set
A
may
severe
affect
metabolism.
fruit
deficiency
fruit
set
by
of
any
essential
reducing
plant
nutrient
vigor
and
element
altering
Plant B levels seem to have a more direct effect
set than other nutrient elements.
on
Boron applications have
increased seed set in white clover (Trifolium repens L.)(90), rape
(Brassica napus L.)(61) and corn (1),
and fruit set in
'Stamen'
apples (25), 'Anjou' pear (9) and 'Italian* prune (29,31).
Boron
effects
on pollen germination and pollen tube
have been well documented.
often deficient in B.
Vasil (129)
growth
suggested that pollen is
This is supported by numerous reports of B
16
enhanced pollen germination and pollen tube growth in vitro. The B
content
of
pollen varies,
and species that normally
have
high
endogenous B levels in pollen (pear) often show a greater response
to supplemental B in vitro than species with low endogenous levels
(pine)(114).
apple,
Boron
peach,
stimulation
has improved in vitro pollen germination
apricot,
of
pollen
'Stanley' plum (121).
cherry,
and
germination
pear (113,121).
in
vitro
tube
B,
small
reported
Pollen tube growth rate in vitro has
enhanced by B in nearly every species tested.
cherry,
was
A
in
in
been
In pear, apple, and
the greatest stimulation of pollen germination and pollen
growth rate in vitro occurred in solutions of 2.5 to 40
with
concentrations
above
160
ppm
resulting
in
ppm
apparent
toxicity (121).
The
B
response
plants
vitro
of
contained
10.8
(1).
plant
may
influence
the
Pollen from B deficient
corn
5.8 to 7.5 ppm B and had higher germination
when B was added,
hybridum
the parent
of pollen to B Jji vitro.
contained
vitro
nutrition
whereas pollen from B sufficient
ppm B and did not respond to
In
vitro
germination of alsike
supplemental
clover
plants
B
in
(Trifolium
L.) pollen is stimulated by supplemental B much more
parent plants were B deficient (90).
in
if
Antles (5) found that pollen
from pear trees did not germinate in sugar solutions alone, but if
B
fertilizers
were applied to the trees for 4
years,
in
vitro
germination in sugar solutions was 100%.
Although
the
pollen tube growth
B
requirement
for
in vitro is apparent,
pollen
germination
the influence of
and
B
on
17
these
processes JLn vivo is less clear.
growth
through
clover
styles was not affected by
pollen or flowers (134).
B
deficient
for
The rate of
pollen
B
tube
levels
Johri and Vasil (72) suggested that even
pollen may derive ample B from the stigma and
normal
germination
applications
have
in
and tube
growth.
No
reports
style
that
increased pollen tube growth ija vivo could
B
be
found.
Critical B concentrations in leaves have been established for
most
crop
optimum
species,
(71,136),
flowers
normally
(71).
prune
suggests
when
a
B
'Bartlett'
pear
disorder,
require
normally.
develop
higher
Alsike
maximum vegetative growth in 0.1 to 0.25
leaf
B
pear
deficiency
pear flowers
than leaves to develop
produced
for
(29),
whereas leaves containing 10 ppm B
This
concentrations
solutions,
'blossom blast',
ppm B,
required
Flowers typically contain higher
cherry (136) and clover (90).
developing
18
concentrations
than leaves in apple (39,136),
sweet
contained
(90).
flower B
fruit set have not.
concentrations
plants
but
B
clover
ppm
concentrations were between 23 and 37
B
ppm
Seed production was greatest in 0.5 to 1.0 ppm B solutions,
when flowers contained 59 to 61 ppm B.
corn
delayed
production
anthesis,
and viability,
Marginal B deficiency
shortened anthers,
and
reduced
in
pollen
but had no effect on vegetative growth
(1).
In prune trees, sufficient B for vegetative growth may not be
adequate for optimum fruit set.
sprays
increased
Chaplin et al., (31) found fall B
fruit set of 'Italian' prune the
next
spring.
18
The
trees
sufficient
standards.
flower
showed
no
symptoms of
B levels (29 ppm),
The
B
deficiency
and
contained
by August mid-shoot leaf
analysis
authors felt that there was a B requirement
development
and fruit set which is not
supplied
for
by
the
general B nutrition of the tree and can't be diagnosed from August
leaf
analysis.
contain
ppm),
Callan
et al.
(29),
showed that prune
flowers
much higher B concentrations (87 ppm) than leaves
(35-38
and that 500 ppm B sprays applied in the fall raise
flower
levels
to
between 165 and 185 ppm and increased fruit
set.
No
effect of fall B sprays could be seen in leaf levels the following
August.
Pollen
tube
affected
by B sprays.
growth
through excised
flowers
require
was
Viability of pollen from B treated
was lower than pollen from control trees.
prune
flowers
more
normally supplied by the tree;
not
trees
This work indicates: 1)
B for optimum
fruit
set
than
is
2) fall B sprays increase reserves
in the tree which then supply B to flowers in the spring;
3)
The
increase in fruit set could not be attributed to B enhanced pollen
viability or tube growth.
19
Chapter 3
EFFECTS OF FALL BORON SPRAYS AND ENVIRONMENTAL FACTORS
ON FRUIT SET AND BORON ACCUMULATION IN 'ITALIAN* PRUNE FLOWERS
Abstract
Foliar
'Italian'
sprays
B
prune
have
studies.
spring
fruit
set
had
a
variable effect on
by
cut
fruit
the
32% during a cool year when set
cool year,
set
in
but
was
B sprays increased flower
pistil length,
B
forced
to
why
previous
a
increased
low
(3.2%).
levels
and
but had no effect on the rate of pollen
Calcium sprays (0.5 M) applied in the
fruit
set,
pollen tube growth or flower Ca or B levels.
branches
fall
investigate
12.2%),
prior to the cool spring did not affect
length,
in
application had no effect on fruit set during
tube growth through styles.
fall
ppm) were applied
when set was high (average,
the
decreased
(500
(Prunus domestica L.) trees to
Boron
warm
During
sprays
pistil
Flowers on
to bloom indoors accumulated as much
B
as
those on intact trees, indicating that flowers are supplied B from
reserves in nearby branches.
readily
branches
Simulated rain showed that B was not
leached from flower buds.
Flowers blooming
on
excised
in high relative humidity (86%) contained 13% less B per
flower than flowers in low humidity (29%). The amount of set and B
concentrations
in
flowers
response to fall applied B.
in a given
year
may
influence
the
20
Introduction
Flowers
typically
vegetative organs
contain higher concentrations of
(7,12,15,24).
B
than
Plants marginally deficient in B
may exhibit blossom wilting and necrosis (2),
poor seed set (15),
or reduced pollen production and viability (1), whereas vegetative
tissue
is unaffected.
Reports of B enhanced pollen
germination
(13,22) and pollen tube growth (8,20) in vitro are numerous.
Chaplin
flowers
may
et
al.
suggested that the B supply
often be inadequate for optimum fruit
consecutive trials,
fruit
(5)
to
set.
prune
In
two
foliar B sprays applied in the fall increased
set (115%) and yield (40 to 100%) of 'Italian* prune
not deficient in B by leaf analysis standards.
trees
Callan et al. (4)
applied similar sprays to 'Italian' trees not deficient in B
and
found fruit set was increased in 4 of 6 trials (44 to 125%) during
2
years.
tube
Boron applications had no effect on the rate of
growth
viability
by
in
excised
40%.
flowers at 10
0
C,
but
reduced
Fall B applications increase B
pollen
pollen
reserves
in
dormant buds and spurs (4) which supply B to developing flowers in
the spring.
The mechanism through which B enhances fruit set
in
prune is not known.
The
the
work.
purpose of this work
was to investigate the reasons for
variable fruit set response to B treatments seen
Since
the
fruit set response may be associated
amount of B in flowers,
in
earlier
with
the
environmental factors which affect floral
B accumulation were studied.
21
Materials and Methods
Field
trials
established
All
were conducted during 2 consecutive
'Italian'
prune trees in a western
years
Oregon
orchard.
trees contained sufficient pretreatment B in midshoot
sampled
in
early autumn (30 to 33
ppm).
In
1981,
on
leaves
treatments
included an unsprayed control and a 500 ppm foliar B spray applied
to
the
point
Solubor
77,
on
of drip on September 16.
applied
as
(78% Na2B8O13.4H20 and 20% Na2B407.5H20) with 300 ppm
X-
a nonionic wetting agent.
single
8
adjacent
was
Treatments were replicated 6 times
year old prune trees
on
(Plrunus persica Batsch.) rootstocks.
an
Boron
'Lovell'
peach
seedling
Treatments were repeated in
group of 10 year old prunes on 'Marianna 2624'
plum
(Prunus cerasifera Ehrh. x P^ munsoniana Wight & Hedr.) rootstocks.
Fruit
set was measured through the season by counting flowers
anthesis
and remaining fruit on 4 branches per tree (500 to
flowers
per tree).
analyses.
Fruit were collected on June 22,
at
1500
1982 for
Trees were mechanically harvested to measure yield.
Because tissue Ca concentrations affect the B requirement
plants (17),
tissue
the
following
control;
Boron
4
treatments
trees on 'Marianna
2)
500 ppm B;
were applied
2624'
3) 0.5 M Ca;
on
rootstocks:
In 1982,
September
1)
wetting agent.
10
to
unsprayed
4) 500 ppm B and 0.5 M Ca.
was applied as Solubor and Ca as Ca(0H)_ with 300 ppm
randomized
of
Ca sprays were added in 1982 to determine if varying
Ca concentrations may affect the response to B.
'Italian'
B
X-77
Treatments were replicated 8 times in a completely
design.
Fruit
set measurements
were
made
on
4
22
branches
growth
per tree (1500 to 3000 flowers per tree).
was
Flowers
measured
in vivo on B
treated
and
Pollen
control
tube
trees.
were tagged at anthesis and 5 pistils were collected from
each tree 4,
7, 10, 15 and 19 days after anthesis.
The length of
the longest pollen tube in each style was measured as described by
Callan
et al.
dormant
(4).
buds
Samples collected for B
and spur tissue (January
28,
analyses
1983),
included
flowers
and
pistils (April 1), and fruit (May 25).
Boron
reserves
in
prune branches were studied in
1983
by
removing a 0.8 m long branch on February 10 from 8 control and 8 B
treated trees.
distilled
Branch ends were recut under water and immersed in
water in plastic containers.
at 20oC.
bloom
flower
buds
Branches were forced
to
Tissue collected for B analysis included dormant
and spurs (February 10),
and flowers and
spurs
at
bloom (February 19).
The
flower
were
possibility
that
heavy prebloom rain
buds was studied in 1984.
from
0.8 m
long,
collected on January 20 from 5 'Italian' prune
trees
which
recut
porcelain
Branches
under
water
crocks.
Tops
were
temperatures.
and
B
Thirty branches,
had received 500 ppm B sprays on September 10,
were
leaches
90
cm
Applications
placed
in
and
of
immersed
in
1982.
distilled
crocks were covered
a greenhouse at
Branch ends
with
17° /20o C
water
plastic.
day/night
Three simulated rain treatements (totaling 0,
water)
were
applied
to
of 0,
4.5,
or 9.0 cm were made at 2 day
until branches bloomed 19 days later.
groups
of
10
in
45,
branches.
intervals
Distilled water was applied
23
as
a
fine
spray
with
a stainless steel
sprayer
delivering the equivalent of 5.0 cm water/hr.
changed
after
10
days to prevent
at
30
psi,
Water in crocks was
excessive
bacterial
growth.
Flowers were collected at anthesis for B analysis.
The effect of flower bud transpiration rate on B accumulation
in
flower buds
and
flowers
was also studied.
Two 0.8 m
branches
were collected from each of 9 'Italian' prune
February
24,
1984.
In the laboratory,
long
trees
branch ends were
on
recut
under
water and immersed in distilled water in small polyethylene
bags.
Bags were supported inside flasks and tied around the base
of
the branch to prevent water from evaporating.
relative
buckets
humidity
sealed
Low
and
(RH) chambers were constructed from
loosely with inverted
polyethylene
high
12
liter
bags.
High
humidity (averaging 86%) was maintained by bubbling compressed air
through
bottom
an
aquariun type air stone in a beaker of water
of the bucket.
without
the
Supplying the same flow of compressed
air
bubbling through water provided a low humidity
(averaging
29%).
state porometer.
high
in
Relative
treatment
humidity was measured with a
A branch from each tree was placed in
steady
separate
and low humidity chambers and kept at 180C until full bloom.
Flowers
were then collected for B analysis and transpiration
branch was determined by measuring the volume of water
All
tissue
analyses
coupled plasma spectrometer.
were
performed
with
an
per
remaining.
inductively
24
Results
Weather was relatively warm and dry during the 3
period
in
week
bloom
C,
total
0.71 cm) and overall set was high (8-14%).
Boron
1982
precipitation,
(mean
hourly
temperature,
11.9°
sprays applied in the fall of 1981 had no influence on final fruit
set
or yield in 1982 (Table 3.1).
between
the
Fruit counts made on 15 dates
bloom and harvest showed no difference in set earlier
season (App.
breakage
3.).
occuring
Yield per tree was high with
due to the weight of the crop.
some
Boron
in
limb
sprays
increased B levels in fruit in May (Table 3.1).
Weather during the 3 week bloom period in 1983 was relatively
cool and wet (mean hourly temperature, 7.7 C, total precipitation,
2.16 cm).
Fruit set was low throughout the orchard (2.8-3.8%) and
the
on
crop
Trees
many trees was too light to
economically
harvest.
sprayed with B or B plus Ca in the fall of 1982 had
higher
fruit set and shorter pistils than trees receiving no spray or
Ca
alone (Table 3.2). The effect of B on fruit set was first seen
in
late
May,
Calcium
Pollen
and
resulted
sprays
tube
unaffected
present
on
anthesls,
did not
growth
by
B
in a 32% increase in
influence
through
set
fruit set or
the styles of
applications (Table 3.3).
harvest.
pistil
intact
Pollen
the stigmas of only 25-30% of flowers
and
at
1
length.
flowers
was
tubes
were
week
after
had grown less than half the distance to the ovary
after 15 days.
Boron
applied
concentrations
alone or in combination with Ca
in flower buds and spurs in January,
increased
flowers
B
and
25
Table 3.1.
Effect of 500 ppm B spray applied in autumn, 1981 on
fruit set, yield and fruit B concentrations of 'Italian'
prune.
Fruit
B (ppm)
5/22/82
Yield
(kg/tree)
Fruit
set
Rootstock
Treatment
peach
control
55
243
12.4
peach
+ B
73
181
8.3
plum
control
42
607
14.4
plum
+ B
69
752
13.9
!
F-test significantly different at 5% level, (*).
(%)
Table 3.2.
Effect of 500 ppm B and 0.5 M Ca sprays applied in autumn, 1982
on bud and flower dry weight, pistil length and fruit set of
•Italian* prune in 1983.
Bud
weight
(mg)
Flower
weight
(mg)
control
4.1
24.6
+Ca
4.3
+ B
+ Ca, + B
Treatment
Pistil
length
(mm)
Fruit Set i(%)
Aug. 12
May 9
May 28
12.3 az
10.6
3.9 a
2.8 a
25.6
12.4 a
11.1
4.1 a
2.8 a
4.1
23.7
12.1
b
11.0
5.1
b
3.6
b
4.1
24.7
12.1
b
12.7
5.4
b
3.8
b
^eans separated within columns by Fisher's LSD, 5% level.
Table 3.3.
Effect of 500 ppm B spray applied in autumn, 1982 on pollen
tube growth in 'Italian' prune flowers in 1983.
Days after anthesis
*
4
Treatment
7
"
10
'i
15
19
% Stigmas with
Longest mean pollen
% Ovaries with
visible pollen tubes
tube length2 (mm)
visible pollen tubes
Control
25
30
0.6
3.6
100
+ B
25
25
1.1
3.8
95
!
Average style length, 9.0 mm.
to
28
pistils
at
anthesis
(April)
and fruit
in
June
(Table
Calcium sprays had no effect on B concentrations (Table
Ca
concentrations in buds,
shown).
trees
Flowers
accumulated
forced
the
spurs,
forced
3.4),
or
flowers or pistils (data
not
on branches from B treated
and
control
same amount of B (Table 3.5)
as
flowers
blooming on intact trees (Table 3.4).
in
3.4).
Boron concentrations (ppm)
flowers from B treated and control trees
were
higher
than those in flowers on trees.
The
highest
branches
rate of simulated rain (90 cm)
applied
to
cut
prior to bloom reduced the B content (ug) of flowers
by
16% but had no significant effect on the B concentrations (ppm) in
flowers
compared to controls (Table 3.6).
The 45 cm rate had no
effect on B concentration or content of flowers.
Branches forced to bloom in 29% RH transpired 2.6 times
water
than those in 86% RH (Table 3.7).
flowers
in
blooming in low RH was 13% greater than flowers
high
differ.
(12%),
The B content
RH,
but
Flowers
K
(17%),
(data not shown).
the B concentration in flowers (ppm)
in low RH also contained significantly
Mg
more
(ug)
of
blooming
did
more
(24%) and Ca (25%) than flowers in high
not
P
RH
Table 3.4.
Effect of 500 ppm B and 0.5 M Ca sprays applied in autumn, 1982
on B levels in 'Italian' prune trees in 1983.
January
April
Bud
B (ppm)
Spur
B (ppm)
control
35 az
24 a
+ Ca
36 a
22 a
+ B
56
b
33
b
72
b
1.71
b
75
b
60
b
+ Ca, + B
55
b
31
b
69
b
1.70
b
79
b
59
b
Treatment
Flower
B (ppm)
June
Flower
B (ug)
Pistil
B (ppm)
Fruit
B (ppm)
49 a
1.21 a
60 a
43 a
49 a
1.24 a
54 a
41 a
z
Means separated within columns by Fisher's LSD, 5% level.
1J3
Table 3.5.
Boron levels in excised branches from B treated (500 ppm B in
autumn, 1982) and control 'Italian' prune trees forced to bloom in
February.
Full Bloom i(February 19)
Dormancy (February 2)
Treatment
control
Bud
weight
(mg)
Bud
B
(ppm)
Bud
B
(ug)
4.1z
39
.16
*
+ B
4.3
58
Spur
B
(ppm)
Flower
weight
(mg)
Flower
B
(ppm)
Flower
B
(ug)
30
17.2
69
1.19
19.5
*
.26
Spur
B
(ppm)
*
30.1
*
33
18.5
87
*
1.63
!
F-test significantly different at 5% level, (*),
o
31
Table 3.6.
Effects of simulated rain applied during the prebloom
period to excised 'Italian' prune branches on flower B
levels and dry weight.
treatment
(cm)
dry weight
(mg)
Flower
B
(ppm)
Flower
B
(ug)
0
25.0Z a
69 ab
1.74 a
45
23.5
a
77 a
1.81 a
90
23.4
a
63
b
Cleans separated within columns by Fisher's LSD, 5% level.
1.46
b
32
Table 3.7.
Effects of low and high relative humidity (RH) during
the prebloom period on transpiration and flower B levels
of excised 'Italian' prune branches.
RH
(%)
Transpiration/
branch (ml)
Flower dry
weight (mg)
Flower
B
(ppm)
Flower
B
(ug)
29
94z
25.4
53.5
1.36
86
36
23.7
50.5
1.20
2
Paired t-test significantly different at 5% level (*).
33
Discussion
The
lack
of a fruit set response in 1982 may be related
the heavy overall set and full crop that year.
did
not
respond
overall
to B sprays in earlier
fruit
set
work
also
had
with peach rootstocks,
trees
(C.V.,
52%)
been detected.
fruit
levels
in
lower
variable
on
between
a small treatment effect would not
have
When initial fruit set is high, other factors such
competition
final
and
it was extremely
higher
(4-ll%)(4).
in the present study was somewhat
trees
as
Two orchards which
set (11-12%) than orchards which did respond
Although
to
between fruit for assimilates (6) may
set at harvest.
trees,
as
The 1981 boron sprays
evidenced
by
the
limit
the
increased
significantly
B
higher
concentrations in fruit in May.
The
smaller
30%
increase
in fruit set induced by
B
in
1983
was
than the response seen in other work using similar fall B
sprays (4,5).
indicating
The improvement in set was first seen in late May,
that B affected a process of fruit set or early
development and not mid summer fruit drop.
fruit
The poor overall
set
in 1983 was probably due to the cool temperatures during bloom and
slow pollen tube growth.
Thompson and Liu (23) found pollen tubes
had reached 44% of ovules in 'Italian' prune flowers 21 days after
anthesis
during
a cool year.
Fruit set was low because
were already degenerating by this time.
In contrast, pollen tubes
were present in 39% of ovules 5 days after anthesis during a
year.
In the present study,
third
of
stigmas
7
ovules
warm
pollen had germinated on less than a
days after
anthesis.
By
15
days
after
34
anthesis,
the
longest pollen tube in each style was 3.6-3.8
mm,
less than half the length of the average style (9.0 mm).
The
mechanism by which B increased set in 1983 could not
be
determined. Although fruit set seemed to be limited by slow pollen
tube growth,
et al.
B did not hasten the growth of pollen tubes.
Callan
(4) studied the rate of pollen tube growth at 10 C in hand
pollinated excised flowers from B treated and control trees, using
all combinations of pollen and flowers.
the
They also concluded that
increase in set was not due to B enhanced pollen tube growth.
Although
ovule
shorter pistils reduce the distance from stigma
and could
enhance fruit set when the rate of
growth
is limiting,
pistil
length
fruit
set.
since
B
tube
it is not known whether the 2% reduction
on
Boron
pollen
to
in
B treated trees in 1983 was enough
to
may influence set through
mechanism,
reportedly
another
125%
in
relatively warm springs when pollen tube growth would not seem
to
be
by
limiting
bringing
(4).
stigmas
transfer.
A
increased fruit set by as much
as
affect
Shorter styles may also improve fruit set
closer
to
anthers
to
permit
better
pollen
close correlation between the pistil/stamen
length
ratio and fruit set has been seen in 'Italian' prune (10).
The
might
be
relatively small increase in fruit set due to B in
associated
with the low B
concentrations
in
1983
control
flowers (49 ppm) and the relatively small increase due to B sprays
(69-72 ppm).
fruit set,
ppm
In a similar study in which fall B greatly
enhanced
the B concentration in flowers of control trees was 87
compared
to 165 to 186 ppm in those from sprayed trees
(4).
35
The increase in fruit B concentrations due to B sprays was similar
in 1982 (Table 3.1)
and 1983
(Table 3.4),
suggesting
that
the
concentrations in flowers may have been similar in both years. The
total
B content of flowers may not be as important as the
of soluble or available B.
Two B fractions have been
amount
identified
in plant tissue, an insoluble fraction which remains constant, and
a
soluble fraction which declines to near zero when plants
to
develop deficiency symptoms (19).
begin
The insoluble fraction may
refer to B bound to cell wall polysaccharides (3,21).
Although in
the present study B sprays increased the total B concentration
in
flowers,
of
they
may
not
have appreciably changed
the
level
soluble B.
Since flowers forced indoors on excised branches
accumulated
as much B (Table 3.5) as flowers developing on intact trees (Table
3.4),
the
adjacent
B
content
branch
branches
may
of prune flowers is
tissues.
also
affect
development the next spring.
quantities
of
Factors
the
likely
which affect
amount
derived
B
available
for
in
flower
large
B and reduce concentrations in wood and bark
explain why 'blossom blast* of pears,
severe
reserves
Apricot fruit may accumulate
Low soil moisture reduces B absorption by plants (11),
most
from
(9).
which
may
a B deficiency disorder, is
following excessively dry summers (12).
If
the
B
requirements of flowers are supplied by reserves in branches, then
the
yearly
branch
1982,
variation
in number of flowers per
unit
length
may also influence the concentration of B in flowers.
8 to 14% set resulted in a full crop,
while 25 to 30%
of
In
set
36
was required for a full crop of 'Italian' prunes in New York (14).
This
implies that the number of flowers per unit length of branch
may vary considerably.
Boron
rain
was not readily leached from flower buds by
applied
affect.
reduced
during
prior
Misting
to bloom,
apple
as only the 90 cm
the
rate
trees with 122 cm water to
B concentrations in flowers by
30%
simulated
had
delay
(7).
any
bloom
Precipitation
2 months prior to bloom would rarely exceed 45 cm
in
western Oregon, so it is doubtful that significant quantities of B
are
leached from flower buds in the field.
concentrations
may
Tissue with
contain a larger proportion in
high
soluble
B
form
(19), so that more leaching might be expected from buds containing
higher B concentrations.
A
large
transpiration
flowers.
difference
rate
Similar
had
in RH and a several fold
a
slight effect on
differences
barley seedlings by 70% (16).
variation
in
the
B
content
of
in RH changed the
B
content
of
Average RH prior to bloom would not
be expected to vary from 29% to 86%
from year to year under field
conditions, and would not seem to be an important factor affecting
B
had
accumulation in flowers.
a
Relative humidity and
transpiration
much stronger effect on the accumulation of Ca and
Mg
in
flowers.
Why
fall B sprays raised flower B concentrations
relatively
little and had a variable effect on fruit set is not clear.
Scott
and
grape
Schrader
leaves
occurs
(18) reported that remobilization of B
only
when
B
from
concentrations in those leaves
are
37
above
a
branches
certain level.
to
supply
Remobilization of B reserves
flowers
concentrations are low.
may also
be
limited
in
prune
when
tissue
The fruit set response to fall B
sprays
varies, and may be influenced in a given year by the amount of set
and B levels in flowers.
38
Literature cited
1.
Agarwala, S. C, P. N. Sharma, C. Chatterjee and C. P. Sharma.
1981. Development and enzymatic changes during
pollen
development in boron deficient maize plants. J. Plant Nutr.
3:329-336.
2.
Batjer, L. P. and B. L. Rogers. 1953. "Blossom blast" of
pears: an incipient boron deficiency. Proc. Amer. Soc. Hort.
Sci. 62:119-122.
3.
Bowen, J. E. and P. Nissen. 1976. Boron uptake by excised
barley roots 1. Uptake into the free space. Plant Physiol.
57:353-357.
4.
Callan, N. W., M. M. Thompson, M. H. Chaplin, R. L. Stebbins
and M. N. Westwood. 1978. Fruit set of 'Italian' prune following fall foliar and spring boron sprays. J. Amer. Soc. Hort.
Sci. 103:253-257.
5.
Chaplin, M. H., R. L. Stebbins and M. N. Westwood. 1977.
Effect of fall-applied boron sprays on fruit set of 'Italian*
prune. HortScience 12:500-501.
6.
Crane, J. C. and M. M. Nelson. 1970. Apricot fruit growth and
abscission as affected by maleic hydrazide-induced seed abortion. J. Amer. Soc. Hort. Sci. 95:302-306.
7.
Crassweller, R. M., D. C. Ferree and E. J. Stang. 1981.
Effects of overtree misting for bloom delay on pollination,
fruit set and nutrient element concentrations of 'Golden
Delicious' apple tree. J. Amer. Soc. Hort. Sci. 106:53-56.
8.
Dickinson, D. B. 1978. Influence of borate and pentaerythritol concentrations on germination and tube growth of Lilium
longiflorum pollen. J. Amer. Soc. Hort. Sci. 103:413-416.
9.
Eaton, F. M. 1941. Quality of irrigation waters of the
Hollister area of California with specific reference to boron
content and its effect an apricots and prunes. USDA Technical
Bull. No. 746.
10.
Grzyb, Z. S. and S. W. Zagaja. 1973. The influence of various
rootstocks on the morphological changes in Italian prune
flowers. Acta Hort. 48:7-11.
11.
Hobbs, J. A. and B. R. Bertramson. 1949. Boron uptake by
plants as influenced by soil moisture. Soil Sci. Soc. Amer.
Proc. 14:257-261.
12.
Johnson, F., D. F. Allmendinger, V. L. Miller and D. Policy.
39
1954. Fall application of boron sprays as a control for
blossum blast and twig dieback of pears. Phytopathology
45:110-114.
13.
Kamali, A. R. and N. F. Childers. 1970. Growth and fruiting
of peach in sand culture as affected by boron and a fritted
form of trace elements. J. Amer. Soc. Hort. Sci. 95:652-656.
14.
MacDaniels, L. H. 1942. Notes on the pollination of Italian
prune. Proc. Amer. Soc. Hort. Sci. 10:84-86.
15.
Montgomery, F. H. 1951. The effect of boron on the growth and
seed production of alsike clover Trifolium hybridum L.
Can. J. Bot. 29:597-605.
16.
Oertli, J. J. 1963. The influence of certain enviromental
conditions on water and nutrient uptake and nutrient distribution in barley seedlings with special reference to
boron. Adv. Front. Plant Sci. 6:55-85.
17.
Reeve, E. and J. W. Shive. 1944. Potassium-boron and
calcium-boron relationships in plant nutrition. Soil Sci.
57:1-14.
18.
Scott, L. E. and A. L. Schrader. 1947. Effect of alternating
conditions of boron nutrition apon growth and boron content
of grape vines in sand culture. Plant Physiol. 22:526-537.
19.
Skok, J. and W. J. Mcllrath. 1958. Distribution of boron in
cells of dicotyledonous plants in relation to growth. Plant
Physiol. 33:428-431.
20.
Stanley, R. G. and F. A. Loewus. 1964. Boron and myo-inositol
in pollen pectin biosynthesis, p. 128-136. In H. F.
Linskens (ed.) Pollen physiology and fertilization. NorthHolland Publishing Company, Amsterdam.
21.
Tanaka, H. 1967. Boron adsorption by excised sunflower root.
Soil Sci. Plant Nutr. 13:77-82.
22.
Thompson, A. H. and L. P. Batjer. 1950. The effect of boron
in the germination medium on pollen germination and pollen
tube growth for several deciduous tree fruits. Proc. Amer.
Soc. Hort. Sci. 55:227-229.
23.
Thompson, M. M. and L. J. Liu. 1973. Temperature, fruit set
and embryo sac development in 'Italian' prune. J. Amer. Soc.
Hort. Sci. 98:193-197.
24.
Woodbridge, C. G., A. Venegas and P. C. Crandall. 1971. The
boron content of developing pear, apple and cherry flower
buds. J. Amer. Soc. Hort. Sci. 96:613-615.
40
CHAPTER 4
XYLEM DIFFERENTIATION AND BORON ACCUMULATION
IN 'ITALIAN' PRUNE FLOWER BUDS
Abstract
Because B levels in prune flowers have been associated with
the amount of fruit set, the accumulation of B in buds was studied
for
2
months prior to bloom to determine when B enters buds
the importance of the xylem as the supply route.
bud
anatomy
and
azosulfamide
axes
of
when
buds
the
distribution
of
the
Observations of
water
soluble
in excised prune twigs indicated that xylem
flower buds differentiates and becomes
begin to swell,
5 weeks before
and
in
functional
bloom.
dye
the
only
Discontinuous
xylem connection prior to bloom appeared to impede B movement into
dormant
before
buds.
In
intact trees,
bud swelling,
B accumulated in
buds
slowly
but rapidly as buds accumulated dry
matter
from swelling until bloom. The concentration of B in xylem exudate
was
unchanged
from January 19 until
bloom
(April
1),
whereas
marked increases in the concentrations of P, K, Ca and S were seen
beginning
5
remobilization
weeks
of
before
bloom.
This
suggests
B in branches prior to bloom is
than other elements.
that
more
the
limited
Transpiration from buds was calculated from
the rate of Ca accumulation in buds and Ca concentrations in xylem
exudate,
calculated
by
assuming
all Ca entered buds via the
xylem.
Using
daily transpiration rates and concentrations in
xylem
exudate, it was estimated that only 26% of the B entering buds was
supplied by the xylem.
41
Introduction
'Italian*
not
have
prune trees containing adequate leaf B levels
sufficient B in flowers for optimum fruit
may
set.
to 'Italian* prune trees with sufficient
Boron
sprays
applied
levels
(35 ppm) increased flower B levels from 78 to 165 ppm
enhanced fruit set (7).
leaf
and
Boron applications have increased set on
'Italian' trees with sufficient leaf B levels in other studies
well
(9,
B
Chapter 3).
Chaplin et al.
as
(9) suggested that the
B
requirement for optimum fruit set in prune may not normally be met
by the general B nutrition of the tree.
Factors
affecting
the
movement of B to prune
flowers
and
possible reasons for an inadequate B supply have not been studied.
In
Chapter 3,
it was shown that prune flowers contain
nearly
6
times as much B as dormant flower buds, and that nearly all of the
B which accumulates in flowers is derived from researves in nearby
branch
tissue.
contain
much
Flowers
of pear,
apple and sweet
cherry
also
more B than is stored in dormant flower buds
(24).
Boron accumulates slowly in these buds as they begin to swell
and
rapidly as bloom approaches.
The
in
transporting
passively
such
extent to which the xylem and phloem systems participate
as
typically
B
to
flower
buds
is
unknown.
within the transpiration stream to
leaves
(16).
limited (11),
Remobilization
transpiring
in
the
moves
organs
phloem
is
although evidence of phloem mobility has
been seen in cotton and turnip
clover and peanut (8),
of B
Boron
(17),
and apple (23).
broccoli (4),
subterranean
The ability of B to move
42
in the phloem may be dependent on its concentration in tissue.
grape, evidence of
when
leaf
In
B transport out of mature leaves was seen only
concentrations were above a certain
level
(20).
In
apricot and prune trees, large quantities of B accumulate in fruit
and
bark
when
trees are exposed to
excessive
soil
B
levels,
suggesting that considerable remobilization of B in the phloem may
occur when tissue concentrations are high (10).
An
explain
understanding of how and when B enters prune flowers
why
flowers normally receive insufficient B for
may
optimum
fruit set.
This work was conducted to determine the importance of
the
system in supplying B to prune flower
xylem
bloom.
Because
buds
prior
to
xylem differentiates in the axes of peach flower
buds when they begin to swell in the spring (1), we also wanted to
know when xylem differentiates in prune flower buds and whether
discontinuous
flower buds.
vascular
connection
may impede
B
movement
a
into
43
Materials and Methods
Xylem
differentiation
connections in
and B^ movement into dormant
buds.
axes of 'Italian' prune flower buds
Xylem
were
studied
anatomically by collecting flower buds from January until bloom in
April.
Buds
were
vacuum
infiltrated
(chromic
acid-acetic acid-formalin),
tertiary
butyl
sectioning
stained
alcohol
with
a
series
rotary
with
GRAF
dehydrated in an
and
imbedded
microtome
in
(14).
fixative
ethanol-
paraffin
for
Sections
were
with safranin-fast green.
Dye
movement into prune flower buds was studied using 15
to
30 cm long twigs collected every week from mid January until green
tip
stage in mid March.
The base of twigs was immersed in a
1%
solution of azosulfamide and kept indoors at 200C for 3 to 4 days.
Excised
buds
were bisected longitudinally and examined
under
a
dissecting microscope to determine the extent of dye movement.
Boron
uptake by prune twigs and movement into
was studied by removing 8 twigs,
buds
15 to 20 cm long, from each of 3
'Italian* prune trees on January 19.
3
dormant
Twigs were also removed from
common chokecherry (Prunus virginiana L.) trees for comparison.
The base of twigs was placed in either distilled water or 200
B
solutions and kept indoors at 200C for 7 days.
Sections
the base of the twigs just above the level of the solution,
ppm
from
spurs
and dormant flower buds were then collected and analyzed for B.
Nutrient concentrations in xylem exudate and accumulation in buds.
Boron
accumulation
concentration
in
in
xylem
'Italian'
prune
flower
exudate were studied in a
buds
10
year
and
old
44
western
other
Oregon
Levels
nutrients in buds and xylem exudate were also measured
comparison.
each
in
early
concentrations.
April
and
analyzed
for
element
Xylem exudate was sampled from 6 adjacent
bloom.
trees
Exudate
was extracted under suction (5) from 4 branches,
20 cm long,
tree,
Exudate
yielding
8 to 12 ml of extract per tree.
evaporated
to
samples
were
tissue
for
until
nutrient
10 to 20 day intervals from January 19 until
were
of
Samples of 50 to 100 flower buds were collected from
of 8 trees at 10 to 15 day intervals from January 28
bloom
at
orchard for 2 months prior to bloom.
dryness and ashed.
analyzed
for
All xylem
nutrient
per
samples
exudate
elements
and
with
an
inductively coupled plasma emission spectrometer.
Flower
'Italian'
bud
transpiration rates were measured
full bloom.
1) green tip;
2) first white; 3)
Bark surface within the chamber
with parafilm to prevent transpiration through the
At each growth stage,
1
single
A section of twig containing flower buds was inserted
into the porometer chamber.
with
a
prune tree with a LI-COR LI-1600 steady state porometer
at 3 stages of bud development:
covered
on
to
3
buds.
was
bark.
transpiration was measured on 20 twigs each
The same twigs and
buds
were
used
measurements during each growth stage to reduce variability.
for
45
Results
Xylem
differentiation
vessel
they
began
to
swell
5 weeks
before
bloom.
xylem
presence of cytoplasm.
collected
cells
buds
when
Elongated
elements (Figure 4.1),
since
secondary wall thickening and stained dark green,
(April
Xylem
cells
in the axes of buds prior to swelling did not appear
functional
buds
buds.
elements were first seen in the axes of flower
observed
be
and B^ movement into dormant
1).
no
from
Secondary
longer
they
to
lacked
indicating
the
Xylem vessels were observed in the axes of
early bud swelling (March
2)
until
wall thickening was clearly
stained dark green,
indicating
bloom
visible
and
cytoplasm
was
absent (Figure 4.2).
The
distribution
indicated
when
that
of
and
collected
within
in
the xylem in the axes of buds
buds began to swell.
twigs
azosulfamide dye
3 days,
twigs
became
to bud swelling,
When
prune
twigs
were
dye moved to the base of
buds
but did not enter flower primordia even
days in dye solutions.
also
functional
Dye moved into the cut ends of
to the top of twigs within 3 days.
prior
cut
after
5
When twigs were collected after buds began
to swell, dye moved into flower primordia within 3 days.
Boron solutions moved readily through the cut ends of dormant
prune but not chokecherry twigs (Table 4.1).
of
prune
sections
twigs
in B solutions
increased
B
from the base of twigs and in spurs,
the concentrations in dormant buds.
sections,
Placing the cut ends
spurs,
concentrations
in
but did not change
Boron concentrations in
base
and dormant buds of chokecherry were unaffected
46
Fig. 4.1. Radial section of procambial tissue in the axis of dormant 'Italian' prune flower bud collected in January. Elongating
cells were cytoplasmic with no secondary wall thickening (X1000).
47
Fig. 4.2. Radial section of the axis of 'Italian' prune flower bud
collected at the green tip stage (March 10). Helical secondary wall
thickening of the primary xylem elements present. (X1000).
48
Table 4.1. Boron distribution in excised twigs of 'Italian'
prune (Prunus domestica L.)and chokecherry (Prunus
virginiana L.) placed in distilled water and B solutions.
B (ppm)
'Italian prune'
Treatment
base
distilled
water
23
spur
bud
base
spur
bud
24
65
24
36
81
63
28
36
75
z
*
200 ppm B
Chokecherry
34
*
36
z
F-test significantly different at 5% level, (*).
49
by B solutions.
Nutrient concentrations in xylem exudate and accumulation in buds.
Boron
accumulated
during
buds
slowly
in buds during
March as bloom approached (Fig.
February
4.3).
but
rapidly
The B content
of
increased over 6 fold from February 1 until bloom (April 1).
Boron
concentrations in xylem exudate averaged 0.26 ppm
and
did
January
and
not change significantly from January 19 until bloom.
The
Ca
February,
Calcium
content
of
buds
changed
little
in
but increased in March as bloom approached (Fig.
content
of buds increased 40% from January until
The concentration in xylem exudate began to increase in
and reached a peak 10 days before bloom.
4.4).
bloom.
February,
Bud dry weight began to
increase 5 weeks before bloom, with the most rapid accumulation of
dry matter occurring just before bloom (Fig. 4.4).
Flower
transpired
repectively.
buds at green tip,
an average
of 0.5,
first white and full bloom
1.1
and
1.3
ug
stages
water/bud/sec,
50
l-2h ■ XYLEM EXUDATE B
B/BUD
B/BUD SUPPLIED BY XYLEM
•Or
(CALCULATED)
QQ
0.6
0.4
0.2
3/1.
40
2/1
0
20
60
J*
BLOOM
DAY
Fig. 4.3. Boron accumulation in 'Italian' prune flower buds and
concentrations in xylem exudate from January 19 (Day 0) to bloom
(Day 72).
There was no change in B concentration in xylem
exudate so the grand mean was plotted.
B/bud supplied by the
xylem was calculated from bud transpiration rates (Fig. 4.4) and
B concentration in xylem exudate.
51
4
0a
x
TRANSPIRATION (CALCULATED)
• BUD WEIGHT
§ 03
CD
\
X
.02
E
01
I
<
2/1
v-
0
DAY
BLOOM
Fig. 4.4. Ca accumulation in 'Italian' prune flower buds and Ca concentration in xylem exudate from January 19 (Day 0) to bloom (Day 72)
Transpiration rates calculated from the change in Ca content of buds
divided by the Ca concentration in xylem exudate.
52
Discussion
Xylem
in
the
axes of prune flower buds
differentiated
and
became functional when buds began to swell 5 weeks before bloom. A
similar pattern was observed in peach flower buds (1).
To test if
an earlier bloom date may change the time of xylem differentiation
relative to bloom,
February
7
and
distribution
axes
of
possible
27.
within
buds
regardless
prune branches were forced to bloom indoors on
Observations
bud
anatomy
functional
when
buds
begin
the date of bloom (data not shown).
through
and
branches again indicated that xylem
becomes
of
of
dye
in
to
It
the
swell,
was
anatomical observations to determine if
not
phloem
was present at this time.
Boron
systems
of
and
dye
solutions moved
of cut prune twigs,
readily
through
the
suggesting that a significant amount
transpiration may occur through the bark of twigs.
that
B solutions moved readily into spurs,
(Table
4.1),
xylem
The
fact
but not dormant
buds
suggests that the discontinuous xylem connection in
the axes of dormant buds impedes the flow of water and B into buds
prior to swelling. This may explain why very little B was observed
to move into prune buds prior to bud swelling.
were
included
in
this
study
for
Chokecherry
comparison
because
twigs
dormant
chokecherry buds appear to have a continuous xylem connection (2).
Boron
solutions
(Table 4.1),
species.
preserve
had no effect on B levels in
chokecherry
twigs
indicating solutions did not move into twigs of this
Although the base of all twigs was recut under water to
a
continuous water column in the twig,
a broken
water
53
column could explain the lack of B movement into these twigs.
is not clear why B solutions moved into prune but not
It
chokecherry
twigs.
Mean
than
B
concentration in xylem exudate (0.26 ppm) was
concentrations
reported
for
annual
lower
plants.
Boron
concentrations in the sap exuding from the base of detopped tomato
plants
ranged from 1.0-5.2 ppm (6).
manner
from
Raven (19),
weight
sunflower
Sap collected in a
plants contained 0.68-0.82
similar
ppm
B
(13).
using reported values of B concentration per unit dry
of
plant
tops and volume of water
transpired
per
unit
increase in dry weight, calculated the expected B concentration in
the
xylem
stream
to
be
0.01-0.6
ppm.
The
range
in
Ca
concentrations in xylem exudate observed in this study (30-80 ppm)
compares well with concentrations reported in the xylem exudate of
Lupinus
albus
L.
(60-90 ppm)
(12),
Quercus
rubra
L.
(17-18
ppm)(22), and apple (110-120 ppm) and pear (120-180 ppm)(15).
While
change
concentration of B in the xylem exudate
significantly over the sampling period (Fig.
increase
before
the
in
the concentration of Ca
did
not
4.3), a 170%
was seen beginning 5
bloom (Fig. 4.4), and similar increases were seen
weeks
in
the
concentrations of S (200%), K (320%) and P (570%) beginning at the
same time (App.
P
and
1).
Marked increases in the concentrations of N,
Mg in the xylem exudate of apple trees occur during
bloom
(5),
and substantial increases in the concentrations of Cu and Fe
were
reported
suggested
in
earlier
pear
trees just
(3,5),
increases
prior
in
the
to
bloom
(3).
concentrations
As
of
54
mineral elements in the xylem as growth resumes in the spring
may
reflect
and
nutrients
released
to
which
the
were absorbed the previous
xylem from storage tissue.
The
season
fact
that
B
concentrations in the xylem changed little prior to bloom suggests
that B is not as easily remobilized as P,
K,
Ca and S.
Mcllrath
(21)
declines
to near zero as plants become increasingly B
If
branches
have shown that the soluble fraction of
contain higher B concentrations
greater proportion of B may be in a
in
Skok and
tissue
B
deficient.
the
spring,
a
form available for release to
the xylem stream or other transport systems.
An
understanding
transpiration
supply to buds.
buds
as
the
stream
of
how
much
B
enters
buds
in
may help to identify factors that
limit
Although B and Ca began to accumulate in
xylem
differentiated
5
weeks
before
the
B
flower
bloom,
B
accumulated at a much greater rate.
Boron content increased 600%
(Fig.
4.4) from the end of January
4.3) and Ca content 40% (Fig.
until bloom (April 1).
from
7
The amount of S, P and K in buds increased
to 14 fold over the same period.
Although there
was
a
close correlation between the time of xylem differentiation in the
axes
of
buds
and
nutrient accumulation,
accumulate dry matter at this time.
was
also
functional
at
buds
also
began
to
This suggests that the phloem
this time but
could
not
be
observed
anatomically.
The amount of B entering buds in the transpiration stream can
be
determined from the B concentrations in xylem exudate and
transpiration
rates
of
buds.
An
estimate
of
the
the
water
55
transpired/bud/day can be made by assuming that all Ca enters buds
via
the
xylem.
There is general agreement that
long
distance
transport of Ca occurs predominantly in the xylem, and that phloem
mobility
rates
of Ca is extremely limited
(18).
Daily
transpiration
(ml H»0/bud/day) plotted in Figure 4.4 were calculated from
the
regression
0.672(Day)+131,
exudate
(x =
functions
for
ug
Ca/bud
(x
2
0.019(Day) -
=
2
r =.867) and Ca concentration (ppm) in the
-0.00204(Day)3+0.215(Day)2-4.78(Day)+39,
r2=.804).
calculated
The
increment
increase in the Ca content of buds was
for
each
from
day
January 29
until
bloom.
Knowing
concentration in xylem exudate on each day (Fig 4.4),
of
xylem
xylem
the
the
Ca
volume
exudate needed to supply the daily increase in
bud
Ca
content was calculated.
The
transpiration
increased
until
sharply
bloom
increased
considerably
measured
of buds estimated in
5 weeks before bloom,
(Fig.
as
rates
buds
4.4).
Measured
opened
and
and continued
transpiration
approached
manner
to
rise
rates
also
but
were
bloom,
higher than calculated values.
transpiration
this
At full bloom,
rate was 1.3 ug H_0/bud/sec
or
the
0.11
ml
H_0/bud/day, compared to a calculated rate of .034 ml H-0/bud/day.
Transpiration
the
air
pressure
rate
temperature was close to its daily maximum.
gradient between the bud and air and
were
transpiration
lower.
measurements were made in the early afternoon
The
likely
maximal
at
this
time,
of buds was kept dry
vapor
the
transpiration
and
the
rate over a 24 hr period was probably
surface
The
when
during
average
considerably
transpiration
56
measurements,
which could also have resulted in an overestimation
of transpiration rates.
during
Measurable precipitation fell on 19
the 5 weeks prior to bloom (total precipitation,
and flower buds were often wet during this time.
on
the
surface
of
buds
could
reduce
days
3.4 cm),
A film of water
transpiration
rates
substantially.
The
used
calculated daily transpiration values (Fig. 4.4) can
be
to estimate the amount of B delivered to buds via the xylem.
Multiplying the daily transpiration rates (ml H_0/bud/day) by
the
B
the
concentration
amount
of
period
from
xylem
B supplied to buds each day via the xylem.
accumulating
and
January 29 until bloom,
totaled
indicated
in xylem exudate (0.26 ppm)(Fig. 4.3) gives
0.28
in
ug,
buds
this
the
by
the
B supplied to buds
which was only 26% of
over
Over
time.
the
Similar
that the xylem supplied 17% of P,
1.07
ug
B
calculations
25% of K,
28% of Mg
71% of S accumulating in buds over the same period (App.
1).
In Lupinus angustifolius L., the xylem contributes 5% of P, 21% of
K,
and 55% of Mg acctimulatlng in fruit (12).
delivered
be
The proportion of B
to prune flowers which entered via the xylem (26%)
conservatively
high.
Calculated transpiration
values
may
were
based on the assumption that 100% of Ca entered buds by the xylem.
If
a
small percentage of Ca entered buds
actual
via
transpiration rates would be lower,
the
phloem,
the
and the percentage of
the total B content of flowers supplied by the xylem would be even
smaller.
An
angustifolius
estimated
L.
12%
fruit
of the Ca
is
supplied
accumulating
by
the
in
phloem
Lupinus
(12)
57
It
rate
was
of
shown in Chapter 3 that reducing
forced
flowers reduced the Ca supply
severely than B supply.
(29%),
25%
the
to
transpiration
flowers
more
Flowers blooming in low relative humidity
where transpiration was high,
accumulated 13% more B
and
more Ca than flowers blooming in 86% relative humidity.
fact
that B supply to flowers is less affected
rate
than
Ca
by
The
transpiration
is further evidence that the primary
route
of
B
supply to flowers is not the xylem.
This
to
bud
work indicates that very little B moves into buds prior
swelling.
This may be due to
the
discontinuous
xylem
connection
in the axes of buds which impedes passive movement
of
water
B into buds.
in
buds
and
The limited dry matter accumulation
indicates that they are weak carbon sinks at his
suggests
that
symplastic
flow to buds would
also
time,
be
and
limited.
Boron began to accumulate in buds as they increased in dry weight.
Nearly
between
85% of the B content of prune flowers arrived in
5
weeks
bud swelling and bloom, and only one fourth of this B was
supplied by the transpiration stream.
58
Literature Cited
1.
Ashworth, E. N. 1982. Properties of peach flower buds which
facilitate supercooling. Plant Physiol. 70:1475-1479.
2.
Ashworth, E. N. 1984. Xylem development in Prunus flower
buds and the relationship to deep supercooling. Plant
Physiol. 74:862-865.
3.
Bennett, J. P. and J. Oserkowski. 1933. Copper and iron in
the tracheal sap of deciduous trees. Amer. J. Bot. 20:
632-637.
4.
Benson, N. R., E. S. Degman and I. C. Chmelir. 1961. Translocation and re-use of boron in broccoli. Plant Physiol.
36:296-301.
5.
Bollard, E. G. 1953. The use of tracheal sap in the study of
apple-tree nutrition. J. Exp. Bot. 4:363-368.
6.
Brown, J. C. and W. E. Jones. 1971. Differential transport
of boron in tomato (Lycoperslcon esculentum Mill.).
Physiol. Plant. 25:279-282.
7.
Callan, N. W., M. M. Thompson, M. H. Chaplin, R. L. Stebbins
and M. N. Westwood. 1978. Fruit set of 'Italian' prune
following fall foliar and spring boron sprays. J Amer. Soc.
Hort. Sci. 103:253-257.
8.
Campbell, L. C, M. H. Miller and J. F. Loneragan. 1975.
Translocation of boron to plant fruits. Aust. J. Plant
Physiol. 2:481-487.
9.
Chaplin, M. H., R. L. Stebbins and M. N. Westwood. 1977.
Effect of fall-applied boron sprays on fruit set of 'Italian'
prune. HortScience 12:500-501.
10.
Eaton, F. M. 1941. Quality of the irrigation waters of the
Hollister area of California with specific reference to boron
content and its effect on apricots and prunes. USDA Technical
Bui. No. 746.
11.
Eaton, F. M. 1944. Deficiency, toxicity and accumulation of
boron in plants. J. Agr. Res. 69:237-277.
12.
Hocking, P. J., J. S. Pate, C. A. Atkins and P. J. Sharkey.
1978. Diurnal patterns of transport and accumulation of
minerals in fruiting plants of Lupinus angustifolius L.
Ann. Bot. 42:1277-1290.
59
13.
Husa, J. G. and W. J. Mcllrath. 1965. Absorption and translocation of boron by sunflower plants. Bot. Gaz. 126:186-194.
14.
Johansen, D. A. 1940. Plant Microtechnique. McGraw Hill, New
York.
15.
Jones, 0. P. and R. W. Rowe. 1968. Sampling the transpiration
stream of woody plants. Nature 219:403.
16.
Kohl, H. C. and J. J. Oertli. 1961. Distribution of boron in
leaves. Plant Physiol. 36:420-424.
17.
Mcllrath, W. J. 1965. Mobility of boron in several dicotyledonous species. Bot. Gaz. 126:27-30.
18.
Marschner, H. 1983. General introduction to the mineral
nutrition of plants. In A Lauchii and R. L. Bieleski, eds.
Encyclopedia of Plant Physiology, Vol. 15A. Springer Verlog,
Berlin.
19.
Raven, J. A. 1980. Short and long-distance transport of boric
acid in plants. New Phytol. 84:231-249.
20.
Scott, L. E. and A. L. Schrader. 1947. Effect of alternating
conditions of boron nutrition upon growth and boron content
of grape vines in sand culture. Plant Physiol. 22:526-537.
21.
Skok, J. and W. J. Mcllrath. 1958. Distribution of boron in
cells of dicotyledonous plants in relation to growth. Plant
Physiol. 33:428-431.
22.
van Die, J. and P. C. M. Willemse. 1975. Mineral and organic
nutrients in sieve tube exudate and xylem vessel sap of
Quercus rubra L. Acta Bot. Neerl. 24:237-239.
23.
van Goor, B. J. and P. van Lune. 1980. Redistribution of potassium, boron, iron, magnesium and calcium in apple trees
determined by an indirect method. Physiol. Plant. 48:21-26.
24.
Woodbridge, C. G., A. Venegas and P. C. Crandall. 1971. The
boron content of developing pear, apple and cherry flower
buds. J. Amer. Soc. Hort. Sci. 96:613-615.
60
CHAPTER 5
GENERAL DISCUSSION AND CONCLUSIONS
This
have
study
supports earlier work indicating fall
a variable effect on fruit set in prune.
B
sprays
The cause of
the
variable response remains unclear.
The lack of response in 1983 may have
heavy overall set and full crop.
increased
competition
nutrients.
The
been
full
related
crop
likely
between fruits for assimilates
Even if B enhanced initial set,
to
the
caused
and
other
the additional fruit
may have aborted soon thereafter. Two orchards not responding to B
sprays in earlier work also had high overall set (29).
increase
in
set
in
difficult to explain.
1983,
when the overall
set
The small
was
low,
is
Although overall set appeared to be limited
by the cool temperatures and slow pollen tube growth, no effect of
B
was
seen on the rate of pollen tube growth.
increase
small
in
pollen tube growth would be needed to
increase
difficult
have
How much
of
explain
in set (2.8 to 3.7%) is not known.
It
is
an
this
also
to judge whether a 2% difference in pistil length could
reduced the time required for pollen tubes to
reach
ovules
enough to affect set.
Many
pollen
factors
number,
longevity,
and
can limit fruit
pollen
viability,
post-fertilization
set,
including
pollen
tube
competition
pollination,
growth,
between
Fruit set and the factors affecting it vary extensively from
to year.
ovule
fruit.
year
A more controled experimental system may be required to
61
study the subtle influence of B.
evaluated
systems
fertilization
excised
twigs
growth.
They
in
for
plums.
could
Jefferies et al.
studying
pollen
(68) recently
tube
growth
and
They found that flowers developing
be used to
accurately
study
pollen
on
tube
also grafted short scion sections to seedlings
in
the fall, and found that flowers developing on the scions the next
spring
were
effective tools for studying pollen tube
well as fertilization and early embryo development.
these
growth
as
Systems like
could be used to control yearly environmental variation and
possibly
identify the mechanism through which B influences
set in prunes.
fruit
Boron levels in flowers could be closely regulated
to determine the optimum levels for fruit set.
An explanation for the limited fruit set response in 1983 may
be
the
low B concentration in control flowers (49 ppm)
increase due to sprays (to
71
ppm).
and
the
relatively
small
Similar
treatments
in an earlier study increased B levels in flowers from
87 ppm to 165-185 ppm,
and induced a much larger increase in
(44-125%)(29).
concentrations in leaves sampled in August
Boron
before treatments were similar in both studies.
This suggests
concentrations in flowers fluctuate widely from year to year,
set
B
and
that August leaf analysis, the standard measure of tree nutrition,
does
not accurately predict how much B will accumulate in flowers
the following spring.
Flower B concentrations may have to be above a critical level
before
a
fruit set response is seen.
The results of
Skok
and
Mcllrath (10) indicate that only a portion of the B in tissue
may
62
be
available.
is
bound
Others believe that a large fraction of tissue
to cell walls (23,116,120).
saturate
the
binding
sites
It may
be
on cell walls before
B
necessary
to
enough
B
is
buds
as
available to influence set.
Xylem
they
vessels differentiated in the axes of flower
began to swell.
time.
Boron
but
not
Little B accumulated in buds prior to this
and dye solutions moved readily through cut
into dormant buds,
twigs,
indicating that discontinuous
xylem
connections impede B movement or that transpiration from buds
insufficient
to
draw solutions into buds.
accumulated
little
dry
matter
at
The fact
this
time
was
that
buds
suggests
that
symplastic movement into buds is also limited
The
begin
dry
first xylem vessels appear in the axes of buds
to swell 5 weeks before bloom.
matter
flowers
and B at this time.
arrives
presence
of
both
the
Over 80% of the
between bud swelling and
phloem
anatomically,
Buds begin
tissue
in
buds
to
B
bloom.
could
not
as
they
accumulate
content
Although
be
of
the
determined
rapid accumulation of dry matter suggests that
xylem and phloem are present in the axes of buds during this
period.
Calculated
gradually
bloom.
8
daily
transpiration
before bud swelling,
rates
of
but rapidly from
buds
swelling
until
Using the B concentrations in the xylem exudate during the
weeks
before bloom,
it was estimated that the xylem
only a fourth of the B accumulating in buds over this
fact
increased
that
varying the transpiration rate
by
supplied
time.
altering
The
relative
63
humidity had
also
only
a small effect on B
accumulation
suggests the xylem supply is minor.
which
may
such as
in
flowers
Environmemtal
factors
alter transpiration rates of buds from year
relative humidity and rainfall,
to
year,
should not have a
large
effect on flower B content.
Flowers
as
flowers
flowers
on intact trees,
may
important
amount
developing on excised branches accumulated as much B
indicating that the
B
normally be derived from nearby branch
content
of
tissues.
An
factor determining the B content of flowers may be
of mobile B in branch tissues prior to bloom.
fraction
walls
The
the
mobile
of B in branches may be that which is not bound to
or
readily
complexed
in cells.
The fact that
out of prune leaves (App.
2),
applied
while
endogenous B is usually very restricted in
B
cell
moves
remobilization
plants,
of
suggests that
the factors limiting mobility in prune may be different from those
in
other species.
such
as
reduce
a
Factors during the previous growing
heavy crop or excessively dry
the
amount
of B reserves in
soil
branches.
season,
conditions,
The
may
number
of
flowers per unit length of branch also varies widely from year
to
year, and could affect the B available for each flower.
Six general conclusions can be drawn from this work:
1)
The
fruit
set response to B sprays may
be
influenced
differences in the overall set and B levels in flowers
year to year.
by
from
There was no evidence that the fruit set res-
ponse was due to B enhanced pollen tube growth.
2)
Most
of
the
B
accumulating
in
flowers is supplied from
64
researves
in
nearby
branche
tissue,
indicating
that
B
movement from roots to shoots in the spring may not influence
flower B levels.
3)
There is no continuous xylem connection in the axes
of
buds
prior to bud swelling, indicating that the xylem supply of
B
to dormant buds is negligible.
4)
Over 80% of the B content of flowers arrives during
between bud swelling and bloom,
supplied by
the
xylem.
5
weeks
and only a fourth of this is
The majority of B appears to
enter
buds symplastically.
5)
Applied B moves
readily out of
leaves
independent of
leaf
Heavy prebloom rain or differences in relative humidity
have
senescence (App. A.2).
6)
little effect on flower B levels.
65
Bibliography
1.
Agarwala, S. C, P. N. Sharma, C. Chatterjee and C. P.
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Nutr. 3:329-336.
2.
Albert, L. S. 1965. Ribonucleic acid content, boron deficiency symptoms and elongation of tomato root tips. Plant
Physiol. 40:649-652.
3.
Alexander, T. R. 1942. Anatomical and physiological responses of squash to various levels of boron supply. Bot. Gaz.
103:475-491.
4.
Anderson, D. E. and K. Ohki. 1972. Cotton growth response
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5.
Antles, L. C. 1951. Eighteen years of commercial pollen research. Hoosier Hort. 33:68-74.
6.
Ashworth, E. N. 1982. Properties of peach flower buds which
facilitate supercooling. Plant Physiol. 70:1475-1479.
7.
Ashworth, E. N. 1984. Xylem development in Prunus flower buds
and the relationship to deep supercooling. Plant Physiol.
74:862-865.
8.
Batjer, L. P., B. L. Rogers and A. H. Thompson. 1953. "Blossom blast" of pears: an incipient boron deficiency. Proc.
Amer. Soc. Hort. Sci. 62:119-122.
9.
Batjer, L. P. and A. H. Thompson. 1949. Effect of boric
acid sprays applied during bloom upon the set of pear
fruits. Proc. Amer. Soc. Hort. Sci. 53:141-142.
10.
Bennett, J. P. and J. Oserkowski. 1933. Copper and iron in
the tracheal sap of deciduous trees. Amer. J. Bot. 20:632637.
11.
Benson, N. R., E. S. Degman and I. C. Chmelis. 1961. Translocation and re-use of boron in broccoli. Plant Physiol. 36:
296-301.
12.
Berger, K. C. 1950. Boron in soils and crops. Adv. Agron. 1:
321-351.
13.
Berger, K. C. and P.P. Pratt. 1963. Fertilizer technology
and usage. Soil Science Society of America, Madison,
Wisconsin.
66
14.
Bingham, F. T. 1973. Boron in cultivated soils and irrigation waters. In: D. A. Robb and S. W. Pierpoint (eds.) Trace
elements in the environment. (Advances in Chemistry Series,
Vol. 123) Academic Press, London.
15.
Bingham, F. T., A. Elseewi and J. J. Oertli. 1970. Characteristics of boron absorption by excised barley roots. Soil
Sci. Soc. Amer. Proc. 34:613-617.
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APPENDICES
76
150
400
300
• XYLEM EXUDATE K
■ K/ BUD
▼ K/BUD SUPPLIED BY XYLEM
4/1
DAY
BLOOM
Fig. A.l. Accumulation of K, P and S in 'Italian' prune flower buds
and concentrations in xylem exudate from January 19 (Day 0) to bloom
(Day 72). Xylem supply to buds calculated from bud transpiration
rates (Fig. 4.4) and concentrations in xylem exudate.
77
80
CONTROL o
500 ppm B. 7/29 •
SOOppm B, 9/18 ■
8/1
9/1
10/1
DATE
Fig. A.2. Changes in boron concentrations in 'Italian* prune leaves
treated with 500 ppm B solutions on July 29 or September 19. Treatments were made by immersing isolated groups of 4 to 10 leaves in B
solutions.
Vertical bars represent LSD, 5% level.
78
80
o CONTROL
• +BORON
60
LiJ
if) 40
20
-•—•
-O
5/1
7/1
6/1
O
8/1
DATE
Fig. A.3. Effect of 500 ppm B sprays applied in autumn, 1981 on
fruit set of "Italian prune in 1982.
rootstocks combined.
ments on any date.
Data for plum and peach
No significant difference between treat-