Growth response of Agropyron smithii individuals to increased summer water availability
by John Joseph Newbauer
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in
Biological Sciences
Montana State University
© Copyright by John Joseph Newbauer (1985)
Abstract:
Four irrigation treatments -- 0, 6, 12, and approximately 25 mm/week -- were applied to a
homogeneous stand of Agropyron smithii to determine the response of individual plants to differences
in water availability. Sigmoid growth curves were observed. From these and records of green leaf area,
unit leaf rates for major growth stages were calculated. Across treatments these averaged. 78, 32, 4, and
7 mg dm^-2 day^-1 for early-rapid-growth, peak-of-green, early-quiescent-period and season's end,
respectively. Aboveground production of individual plants read from growth curves averaged 156, 151,
150, and 307 mg in dry 1979 and 301, 209, 431, and 343 mg in wet 1978 in the 0, 6, 12, and 25 mm
treatments respectively. Replacement of unit leaf rates with estimated photosynthetic rates suggested
that belowgrowth. production was at least half and probably not much more than twice aboveground
production. Production seems to be controlled both by water stress—-with slowing of growth at -5 bars
and halting at -20 bars—-and by another seasonally correlated factor, perhaps daylength. GROWTH RESPONSE OF Agropyron smithii INDIVIDUALS TO
INCREASED SUMMER WATER AVAILABILITY
by
JOHN JOSEPH N E WBAUER,
III
A thesis submitted in partial fulfillment
of the requirements for the degree
of
Master of Science
in
Biological Sciences
MONTANA STATE UNIVERSITY
Bozeman, Montana
March 1985
COPYRIGHT
by
John Joseph Newbauer , III
1985
All Rights Reserved
ii
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John Joseph Newb a u e r , III
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iv
VITA
John Joseph N e w b a u e r , III was born November 10, 1943
in Washington,
D .C .
His father is John J . Newb a u e r , Jr.
and his mother, Eleanor W . N e w b a u e r . Following graduation
from Miller School of Albermarle, Miller School, Virginia,
in 1963,
he moved to W h i t e f ish, Montana, where he met and
married his wife,
Sydney L . Newbauer in January 19.65.
He
received
a Bachelor
of Science degree in Zoology
from
Montana State University in June 1973.
He has worked as a
Range
Biologist
for the Montana
Department
of Natural
Resources
and Conservation on the High Plains
Experiment
in Miles
City,
Montana since 1974.
He began
Graduate
studies at Montana State University,
in the Department of
Biology, winter 1980.
V
ACKNOWLEDGMENTS
Sincere
appreciation
is
individuals
who
made this
study
thanks
due
to Dr.
are
guid a n c e ,
T.
extended
the
possible.
Weaver for
many
Particular
providing
field
constructive criticism during the course of the
investigations and editorial a i d .
extended
to
to
cooperation,
Dr.
J.
Pickett
Special thanks are also
for
his
efforts
and
without which completion of this thesis would
not have been poss i b l e .
I
also
thank
my
continual encouragement
wife,
Sydney,
for
throughout the study.
patience
and
vi
' V
'
TABLE OF CONTENTS
Page
LIST OF T A B L E S .........................................
LIST OF F I G U R E S ...............................
. . .
vii
viii
A B S T R A C T ................................ -Ix
INTRODUCTION...........................................
■ LITERATURE
R E V I E W .....................................
I
2
\
Effects of water stress on
plant performance.
2
................................
6
Site and plot d escription ..................
Growth measurement and a n a l y s i s ................
Leaf weight and area determinations...........
Water potential measurements ..................
6
7
9
IO
MATERIALS AND METHODS
RESULTS AND D I S C USSION ...........
Aboveground growth rates .......................
Unit leaf ra t e s ..................................
Estimation of p r o d u c t i o n .......................
Belowground production ....................
Factors controlling production and g r o w t h . . .
Growth response to water supplements . . . . .
CONCLUSIONS
11
11
13
17
23
26
33
. . .......................... ..............
37
LITERATURE C I T E D .......................................
39
vi i
LIST OF TABLES
Page
Table
I.
Calculation of unit
leaf r a t e ..............
16
Table 2.
Unit leaf rates of Agropyron smithii
Table 3.
Calculation of seasonal production,
a demonstration of methods ................
19
Table 4.
Production (mg/plant) of individuals of
Agropyron smithii grown under four
irrigation regimes .........................
20
Belowground production (mg/plant)
calculated using minimum or maximum
estimates of photosynthetic r a t e s .........
24
Belowground production (% of total)
calculated using mimimum or maximum
estimates of photosynthetic ra t e s .........
25
Total aboveground yields,
on culm
length (mm) and
biomass
(mg) bases,
of average Agropyron smithii plants
from plots subjected to four water
regimes.
Each datum is reexpressed
(in parentheses) as a percentage of
the control. ................... .............
34
Table 5
Table 6
Table 7 .
. . .
14
viii
LIST OF FIGURES
Page
Figure
I.
Figure 2.
Figure 3.
Cumulative growth, green leaf length,
soil water status, and plant water
status of Agropyron smith!!
plants subjected to four water
treatments in three years ..............
12
Relationships of growth rate to water
availability for Agropyron smithii
plants subjected to four water treatments
for three y e a r s .........................
28
The effect of water potential and season
on unit leaf r a t e ............................ 31
ix
ABSTRACT
Four
irrigation
treatments
—
0,
6,
12,
and
approximately
25 mm/week -- were applied to a homogeneous
stand
of Agropyron smithii to determine the
response
of
individual
plants
to differences in water
availability.
Sigmoid
growth
curves were
obse r v e d .
From
these
and
records
of
green leaf a r e a , unit leaf rates
for
major
growth
stages were calculated.
Across treatments
these
averaged. 78,
32,
4, and 7 mg.dm
.day
for early-rapidgrowth, p eak-of-gre e n , earTy-quiescent-period and season's
end,
respectively.
Aboveground production of individual
plants read from growth curves averaged 156, 151, 15 0, and
307 mg in dry 1979 and 301,
209,
431,
and 343 mg in wet
1978 in the 0,
6 , 12, and 25 mm treatments respectively.
Replacement
of
unit
leaf
rates
with
estimated
photosynthetic rates suggested that belowgrowth. production
was
at least, half and probably not much more
than
twice
aboveground production.
Production seems to be controlled
both
by
water stress--with slowing, of growth at -5
bars
and
halting
at
-20
bars— and
by
another
seasonally
correlated factor, perhaps d a ylength.
I
INTRODUCTION
Agropyron
smith!!
ranges from Alberta
and
British
Columbia
is
south to Texas and Arizona (Hitchcock 1950)
and
2
a dominant grass in 437,000 km
of the North
American
Great Plains
the
(Kuchler 1964).
northern
and
early pasture,
importance
central Great Plains and is
h a y , and erosion control
I
the
used
for
(USDA 1972).
Its
east of the Mississippi has been increased
agricultural planting
management
It is a major range grass in
(Hitchcock 1950).
Interest In
by
its
is therefore high.
have examined the effects of water availability on
performance of Agropyron smith!! because the
might modify water availability by a variety of
including
cloud
water
pitting,
by
(Valentine
seeding
1980).
(Hess 1974) or
water
spr e a d i n g ,
Detailed
manager
processes
reallocation
or
measurements
of
irrigation
of
Agropyron
smithii growth allowed me to describe the seasonal pattern
of
above- and below-ground growth,
the control
of
growth by water availability and perhaps photoperiod,
the
might
growth responses to treatments bracketing
be
achieved
by
irrigation
or
cloud
those
or at least 25 mm per week.
and
that
seeding:
repeated supplementation of rainfall to 6 mm per week,
mm per week,
that
12
2
LITERATURE REVIEW:
EFFECTS OF WATER STRESS ON PLANT PERFORMANCE
The
Great
growth of native forage species
Plains
is often limited by
lack
in the
of
northern
water.
The
growth process is the most sensitive of plant processes to
moisture stress
(Acevedo et al.
Fischer and Hagan 1965).
1971;
1979;
Haiao 1973;
Among the first effects of plant
water stress are reduction in shoot and leaf growth (Hsiao
1973).
Brown
(1977)
and
others have reported
continuing decline in plant water potential
substantial
reduction
in
respiration
that
a
causes
a
(Henckel
carbon translocation (Hsiao 1973; Henckel 1964),
1964),
cytokinin
titer
(I tai and Vaadia 1971),
and an increase in' abscisic
acid
level
When water
(Ackerson 1980) .
severe,
proline accumulation occurs
1978)
and
CO^
assimilation
stress
(Haglund 1980;
ceases
(Hsiao
becomes
Stewart
1973).
Senescence induced by drought may become visible at severe
water
stresses.
severe stress,
to
If water is added to the
after
growth may r e s u m e , but it will never return
the rates of unstressed plants
Hsiao 1973;
system
(Noggle and Fritz 1976;
Fischer and Hagan 1965; Acevedo et al.
1971).
3
Translocation of photosynthates continues under water
stress
(Wardlaw
portions
1977).
the plant
The
(roots
for
of
1974) and is primarily
storage
(Sosebee
and
moderate
when
leaf -- or
Wiebe
to
1971;
recall
conditions
Moser
organs
resources
i m p rove.
During
stress photosynthate accumulates
in other portions of the
accumulation
short
moisture
water
below-ground
of resources in perennating
and crowns) allows the plant
growth
to
occurs,
in
plant -- a n d ,
growth may be more vigorous
since
for
a
time after a period of water deficiency than before
(Boyer
1970) .
Translocation
to
storage
depots
generally most active in the fall and these will be
on
the
is
drawn
during either fall regrowth or in initiation of spring
growth
(Trlica 1977).
Carbohydrate
t
reserves
therefore
show a significant decline during spring g r o w t h ,
with the
extent and duration of the decline differing among species
(Trlica
1977).
Phleum
prat e n s e ,
initiation
found
For
example,
using
of stem elongation,
and
inflorescences
carbohydrates;
C
Balasko and Smith (1973)
at
contained
and at anthesis,
transport
relative
report
should
found that
at
most
of
the
these
stems and roots contained
The results of
be observable as changes
masses of plant parts.
that,
in
flowering,
73% of the labeled soluble carbohydrates.
such
techniques
mobile carbohydrates were
primarily in roots and leaves;
stems
14
Sims and Singh
in a variety of grasslands,
crown
in
the
(1978a)
biomass
4
decreased
in the early season period of
increased
thereafter,
and
initial
peaked after the
growth,
live
shoot
biomass p e a k e d .
Generally,
environmental
moisture
factor
semi-arid
regions
sensitive
to
elongation
but
is
the
(Moore 1977).
reductions
in
P
than
of
shoots
accumulation
1964).
and
of
The
leaf
1971)
in
The photosynthate produced
initiation
2 ) increase in the dry
elongation
of
internodes,
carbohydrate reserves (Begg
carbon
is
declines
in a prioritized manner to I)
and development of l e aves,
in
less
Acevedo et al
it is severely inhibited with continued
(Boyer 1970).
rates
Photosynthesis is
(Fischer and Hagan 1965;
transferred
dominant
affecting photosynthetic
initial
water potential
is
stress
allocation
pattern
weight
and
and
3)
Wright
changes
phenological development and with root and rhizome
with
growth
(Moser 1977).
Any
expected
influence
to
competitive
addition
plant
be
delay the onset and reduce the intensity
of
among
plants
(Harper
various environmental factors,
and time of year may affect the growth
juvenile
size,
to
might
stress
that slows the growth rate
plant
is
capable of exponential
1977).
age
rate.
increases
but as it matures the growth rate declines
1975).
juvenile
of
In
the
The
in
(Leopold
Dahl.and Hyder (1977) suggest that the end of the
stage can only be defined,by the development
of
5
spikelet buds and that the differentiation to reproductive
status
apex.
is
preceded by a rapid elongation
of
the
shoot
6
MATERIALS AND METHODS
Site
and
Plot
homogeneous
alluvial
Description.
stand
The
study
of Agropyron smith!!,
site
the
soil
montmorillonitic•
as
The
a
Brollic
texture
is
® 3Ca horizon at 41 cm;
through
the
horizon
at
horizon
between 84 and 152 cm.
84 cm;
a
on
an
located
fan of Kobar Silty Clay Loam soil.
classified
was
The USDA/SCS
camborthid
a
silty
fine,
clay
loam
loam through the
and very fine sandy loam
in
the
The site has a slope
approximately 2% and a northerly a s p e c t .
of
It is located on
the U .S . Livestock and Range Research Station, Miles City,
Montana
(latitude 4 6 ° 2 1 *15" N ,
detailed
description
of
longitude 105°55'00"W).
the
study
site
and
A
the
experimental plots is given by Weaver et al (1981).
Water
treatments
control— no
rainfall
on four experimental
plots
augmentation throughout
the
were:
study
^
wk
—
period;
six
millimeter— a
minimum
6
of
mm
.
guarantee; w e t — plots in which the soil moisture at 25 and
75 cm was maintained between O and -2 bars.
stress
condition
25 mm .
wk
could
generally
be
The low water
maintained
with
-I
additions.
Irrigations occurring between I
May and I September are summarized in Figure 2 (p. 28).
7
Irrigation
water was applied with sprinklers located
approximately
50
applications
minimize
cm above
were
made
ground
between
evaporation
distribution.
the
and
surface.
0300 and
wind
Water
0900
effects
HDT
on
Irrigation was halted when winds
to
water
exceeded
_^
13
cm .
sec
to avoid an irregular distribution pattern
and unacceptable overspray.
Drop
by
Mason
sizes were determined using a method
(1971).
described
Shallow rectangular pans approximately
31 x 61 cm with a layer of uncompacted flour on the bottom
were e x p osed,
seconds
and
determine
at canopy height,
the
to sprinkling for
dough balls produced were
measured
the drop size distribution and mean drop
The sprinklers
tested
10-20
(N=50) delivered drops in. the
of 0.5 to 3.0 mm with a mean of 1.8+ 0.1 mm.
to
size.
range
These sizes
are within the limits expected from summer rain showers in
the northern Great Plains
Growth
Measurement
and
(Edmond H o l r o y d , 1978).
Analysis.
measured in three treatments
wet)
(control,
Plant
growth
was
six millimeter,
in 1977 and in all four treatments in
1978-79.
In
each plot 20 individuals were marked approximately 0.75
a p a r t , in a line across the center of each plot.
arid
m
Numbered
metal reference tags were tied to the base of each plant.
The growing leaves of Agropyron smithii were measured
at
weekly
September
intervals
from
of each y e a r .
approximately
I
On the first sampling
May
to
date
I
of
8
each
year
all except the lowermost leaf
thereafter
were
measured;
only the upper elongating leaves of the
were m e a s u r e d .
plant
Measurements originated at the tip of the
leaf and terminated at the ligule of the next leaf
This
method
entire
of me as ur emment included the growth
leaf (blade and sheath) and therefore,
above-ground
nearest
growth.
significance
x
variance (ANOVA) was used to
of
differences
1200,
The
Rate
4 treatments
1978,
1979
data
data
sets
used
for
the
were from the control and
transformation
assumptions
was
(P>0.20)
applied
of A N O V A .
normal distributions well
Mensing
examination
wet
of
treatments,
on the assumption that if
met in the remaining plots.
distributed
test
distributions
normality criterion was met in these plots
normally
and 1979
on selected data sets (Os11e and
the most diverse plots studied,
be
for
The Kolmogorov-Smirnov goodness-of-fit
distributions
also
test
and 4 treatments x 12 dates in
and 960 observations in 1977,
normality
1975).
the
treatments.
employed to assess deviations of
from
total
Each cell contained 20 individuals for a total
respectively:
the
among
of 3 treatments x 13 dates in 1977,
were used.
was
the
the
All measurements were made to
of
15 dates in 1978,
of 780,
of
I mm.
Analysis
matrices
below.
to
The data were
so
better
a
would
not
logarithmic
satisfy
This yielded data sets
(P <0.01,
it
that
the
fit
Dixon and Massey 1969).
9
The
transformation used was log (x+1)
to
eliminate
zero
values in the rate matrix.
Leaf Weight and Area Determinations.
collected
Agropyron
sections equivalent
(i e , a phytomer).
Leaves from randomly
smithii plants were
The units isolated included the sheath,
of each leaf which
cut away at the ligule of the leaf
weight
mg.
The
dry
of each section was determined to the nearest
0.1
Leaves
were
below.
then soaked in detergent
facilitate unrolling the b lades,
on clear laminating film.
recorded
Length,
fully
as developing,
solution
area,
and condition of
The condition
fully developed and
green,
developed with brown tip > 5 mm but < 75% of
leaf surace,
or dead
The
damaged
to
blotted d r y , and mounted
individually mounted leaves were recorded.
was
into
to those used in growth determinations
b l a d e , and associated stem (=Internode)
was
dissected
total
(leaf area reduced because of
loss)
(75% or more of the leaf surface brown).
area of individual leaves was
calculated
using
the method of trapezoidal area approximation (Beyer 1979),
on
a
Tektronix
4051
minicomputer
Tektronix 4956 digitizing
to
the nearest 0.01 mm
closest I mm.
2
tablet.
interfaced
with
a
The areas were recorded
and lengths were recorded to
the
10
Water
Potential M e asurements.
Soil water potential
( 'f s )
was determined in the field using gypsum blocks (Taylor et
al
1961) buried at three locations in each plot at depths
of 10,
mean
25,
values
mean
and 75 cm.
The blocks were read weekly
for each depth were determined to
and
obtain
a
for each plot and depth at each measurement date.
Plant
water
potential
(j) ^
was
measured
with
a
Scholander-type
pressure chamber (Scholander et al
using procedures
recommended by Ritchie and Hinkley (1978)
except
as noted b e l o w .
Measurements were
within
one hour of sunrise,
made
1965)
weekly,
on five plants selected from
predetermined locations randomly placed within each of the
treatments.
treatment.
The
Dawn
^
presented are the means
measurements
were
used
for
each
because
should be close to its maximum at that time (Slayter 1967;
Ritchie and Hinkley 1978).
dissecting
scope,
Plants were observed through a
with magnification capabilities of
30x, within seconds of being c u t .
sufficient
point
magnification
for
Generally,
7-
IOx provided
determination of
the
end
Pressure was increased at less than I bar . sec
until water was expressed from the cut e n d .
-I
11
RESULTS AND DISCUSSION
Above
Ground
Growth Rates.
Agropyron
smithil
(Fig.
with
I)
This
In all treatments and
plants show a sigmoidal
growth
growth
pattern has been observed
(e.g.
Golubev 1971;
Evans
1972;
and Milthorpe and Moorby 1974) .
each
temperature)
period)
spring
Erickson 1976;
when
become suitable.
a r e a , and m u s t , therefore,
rapid
and
leaf
conditions
index
of
begins when
Canopy
the
rapid
could
be
due
to
May
or
growth
earlier.
The quiescent phase (third
of
under
Figure I suggests that
begins by 15
exhaustion
an
conditions
generation is rapid during this period but
suddenly at its end.
root
photosynthetic
capabilities
conditions,
(Sims and Singh 1978a; Hunt 1970).
phase
water,
Since growth rate is
the plant's photosynthetic
growth
is
photosynthetic
period indicates the season of optimum growing
rapid
most
Growth
(light,
formation are adequate for high
environmental
in
During the lag phase (first
growth phase (second period)
(Noggle and Fritz 1976).
the
a
be determined by food reserves.
rates
prevailing
by
Larcher 1975;
plant growth is limited by a small
The
curve
and finally a quiescent or stationary
plants
initiated
years
an early season lag phase followed
rapid growth p h a s e ,
phase.
.
water
or
ceases
period)
nutrients,
f
12
180
~0m m , 1977
Kw= 2 I mg/cm
. K0 = 17 Sm m vcm
- TREATMENT. YEAR
Kw(WE)GHT CONSTANT)
K0(AREA CONSTANT)
"6 m m . 1977
Kw= 2 5 mg/cm
. K0= I75m m z/cm
I 20
PLANT LENGTH (CM)
AG
B PRESENT GREEN
LEAF (cm)
C MAXIMUM SOIL
D DAWN PLANT
^ I--I
V
V
25mm, 1978
6 mm, 1978
Kw= 3 I mg/cm
K0 = 19 6m m z/tm
8 0 r O m m , 1979
Kw= 2 Gmg/cm
, Kq= I7 9 m m 2/cm
MAY JUN J U L A U G
Figure
^ o -°A
” A CUMULATIVE
LEAF (cm)
- -OA
I
25mm. 1977
Kw =3 Smg/cm
K0 = 19 9m m z/cm
I.
6 mm, 1979
Kw= 2 8m g/cm
K0 = 18 Smm2Zcm
MAY JUN JU L AUG
12mm, 1979
Kw=2 SmgZcm
K0 = 16 Amm2Zcm
MAY JUN JUL AUG
25mm, 1979
Kw= 2 7 mgZcm
K0 = 181 mm2/tm
MAY JUN JU L AUG
Cumulative growth (cm,
solid line), green leaf
length
(cm,
broken line),
soil water
status
(A),
and
plant water status (B) of
Agropyron
smith!i
plants
subjected
to
four
water
t reatments (supplementation of natural rainfall
to 0,
6 , 12,
and 25 mm/week) in three
years
( 1977-1979 ) .
Growth
and
green leaf
expanse
expressed
in mm (as measured) can be converted
to weight
or
area
by
application
of
the
constants provided (K^ and K ).
The shading in
bar
'A ' indicates the lowest soil water stress
present in the upper 75 cm of the soil (clear =
0-5 negative bars, dots = 5-10 bars, hatching =
10-15 bars,
and solid * 15+ bars.
Shading in
bar
'B 1 summarizes weekly measurements of dawn
plant
water
potential (clear = 0-2
negative
bars,
dots - 2-10 bars, hatching - 10-20 bars,
and
solid
- over 20 bars).
Unit
leaf
rate
calculations
(Tables I and 2) are based on the
leaf areas indicated by triangles on the
green
leaf
curve and time periods between circles on
the cumulative leaf c u r v e .
13
accumulation
of
m aterials),
toxic substances
increased
photosynthesis,
or
to
(inhibitors
respiration
diversion of
or
waste
relative
to
photosynthate
from
vegetative growth to seed or storage reserves.
Unit
per
Leaf Rates
unit
of green surface,
calculated
leaf
by
area.
process
calculations
rate'
doesn't
unit
Aboveground
leaf
production
rate
(ULR),
dividing the rate of production
by
The necessary data appear in Fig.
is
preferred
(mg.dm" 2 .day 1 ).
demonstrated
in Table I.
with
formulae
over the once popular phrase
'net
green
I and the
and
The term 'unit leaf
is
sample
rate
is
assimilation
(NAR) because it clearly relates to leaf growth and
suggest
any
relationship
(Evans 1972 and Thomas 1980).
to
total
production
14
Table I.
Calculation of unit leaf rate.
Unit leaf rate = rate of above ground production/per
of photosynthetic area (= U L R ) .
= ( W/ T)
unit
(1/A) where;
T = a specified time period
W = change in weight in mg during that
time period
=
(change
in leaf length read
from
Fig. I, upper c u r v e ) X ( K^, a weight
conversion constant (mg/cm) reported
in F i g . I ).
2
A = ' area in dm
= (green length at the time considered
from Fig. I , lower curve) X £Ka , an
area conversion constant [mm /cm]
reported in Fig. I) X (1/10
dm /mm ).
Examples:
Unit leaf rate for the early rapid growth phase of control
1977 is calculated as;
ULR = 60 mg,
=
dm .da
(570 mm - 3 60 mm) (0.21 mg/mm)
(day 158 - day 144)
X
(300 mm)
I.
(1.75 m m ^ / m m ) (0.0001 dm^/mm^)
Similarly
unit
leaf rate for the peak of green phase
control 1977 is calculated as:
ULR = 39 mg
= (570 mm - 360 mm) (0.21 mg/mm)
dm .da
(day 158 - day 144)
X
(465 mm)
I ____________
__
(1.75 m m 2 /mm) (0.0001 d 2 /mm2‘
)
of
15
At
a given season ULR varies relatively little among
treatments
(Table
early
and the variation shows no consistent
2).
Production averaged 78 mg.dm
season,
m g .dm
-2
area,
.day
-I
32
these
inconsistently
-2
.day
approximately
and 7 mg.dm
Because
mg.dm
-2
.day
rates
among
-I
at peak
two weeks
pattern
day ^ in
green
past
the
area,
peak
4
green
-I
. at the end of August (Table 2).
differ
relatively
treatments,
I
little
deduce
and
that
the
principal effect of irrigation has been on the area of the
producing unit
(leaf surface)
rather than on its condition
(photosynthetic capacity).
Since
ULR is primarily a function of
(Potter and Jones 1977),
photosynthetic
photosynthesis
I speculate that the maximum net
rate of Agropyron smithii is equal
to
or
greater
than the ULR observed early in the log phase
(78
-2
-I
mg.dm
.day
).
Equality would occur if I) storage depot
subsidies
to shoot growth have ceased by the time the log
phase of growth has b e g u n ,
2 ) roots present in the
early
season support the sparse canopy adequately so there is no
transport
downward
to support root growth,
and 3 )
stem
respiration is a constant proportion of total respiration.
The
(32 mg.dm
decline in ULR later in the log phase of
.day
) and the low production rate per unit of
leaf surface observed late in the season (7 m g .dm
probably
understate
these seasons.
growth
-2
.day
the actual photosynthetic rates
-I
),
for
The presumed understatement of production
16
Table 2.
Unit Leaf Rates of Agropyron smith!i .
- 2 , -I
ULR (mg.diii .day
aboveground)
Early
Rapid
Growth
Control
1977
1978
1979
6 mm
1977
1978
1979
12 mm
1977
1978
1979
At
Peak
Green
After
Peak
Green
At
S e a s o n 's
End
60
87
53
39
26
31
3
I
I
7
3
13
69
106
64
43
29
40
9
2
4
8
5
6
——
——
31
29
3
6
9
7
27*
24
28
5
4
3
6
6
78
32
4
7
8
2
I
I
H O
46
Wet
1977
1978
1979
Mean
Standard
Error
107
84
69
—
Measurements not taken
*
Green material never p e a k e d , but continued to increase
to the last measurement period in this year -
17
is due to increased allocation of photosynthate
areas.
Reallocation
is
probably
the
to storage
major
factor
6„
determining the declines recorded in the control,
12 mm plots where the canopy never c l o s e d .
It is
and
likely
that declines in wet plot ULR are due both to reallocation
and
to competition for light -- ie reduced photosynthesis
-- after canopy c l o sure.
The
very
lowest ULR (4 mg.dm
—2
.day
—1
) was
observed
just after the
end of the log phase when growth rates were
low
leaf areas and available water were
but green
Because
unit
it
seems very unlikely that
to
rates
speculate
preceeding
-2
lower
that,
photosynthetic
mg.dm
photosynthesis
of leaf area should plunge at mid-season —
rates
.day
-I
should
were,
at a
)
but
leaf
and,
minimum,
-2
at a m a x i m u m ,
.day
-I
).
roots.
and
Wiebe
the
and
the
not observed in above-ground
7
early
production,
c rowns,
This is consistent with the report
1973) that,
net
between
equal to
I
The excess photosynthesis
appear as growth or storage in
and
—
ra t e s ,
succeeding unit leaf rates (ie 32
season ULR (78 m g .dm
predicted,
unit
per
from high
end-of-season rates
despite
rates
and
than
high.
in Agropyron smithii,
rhizomes,
(Sosebee
almost
all
translocation during the quiescent phase is to the roots.
Estimation of production.
two methods
methods
The following paragraphs review
for estimating aboveground production and
for
calculating
total
production.
Table
two
3
18
summarizes
these methods and Table 4 compares the
results
of all methods.
Leaf measurements made between mid-May and the end of
August
for
permit us to calculate net aboveground
any
sub-period by either
production
of
two
unit
of length (Ky ,
summarized
in
Second,
production
Fig.
for
Fig.
Such estimates
any
very
short
period
of course,
Fig.
are
aboveground
m g .dm
I lower curve).
the product of leaf
Unit
extension
from the slope of the upper curve of Fig.
and the weight of a unit of length
I).
in
I) by the weight
is the product of unit leaf rate (ULR,
rate is,
divided
First,
Table 4.
rate (mm.day ^ ,
I)
Fig.
I).
day ■*") and leaf surface (mm.
leaf
methods.
can be estimated by multiplying the change
plant length during any period (mm.
per
production
(mg.mm
Fig.
I)
by the green leaf area producing it (lower cu r v e ,
For
longer periods,
one can
estimate
total
production by plotting instantaneous production,
estimated
in
integrate
this
matter,
under the c u r v e .
over a series of periods and
Identical estimates must result if
same data base is u s e d .
than
Either estimate is more accurate
those made by harvest methods because material
before harvest
the
lost
to either grazing or senescence is included
(Sims and Singh 1975).
the
Aboveground
production might occur before
measurement
season.
We
doubt
that
or
post
after
season
19
Table
3.
Calculation
of
seasonal
demonstration of methods.
production,
a
Method I, read it directly from production graphs of Figure I.
Seasonal production = P (mg/plant/season) =
change in leaf length (■»"»fllull-'“ initial'
from
I, upper curve) X K (a constant converting mm to mg,
(presented) in FigurewI).
Method 2, calculate it from graphs in Figure I.
Instantaneous production (mg/plant/day) = ULR x green leaf area
2
ULR (mg/dm /day) is read from Table 2 or calculated from
Fig. I by methods illustrated in (Table I).
green leaf area = gregn leaf length (mm, from lower
Fig. I) X K q (mm /mm, presented in Fig. I).
Seasonal production =
days
instantaneous production
curve,
(mg/plant/da) x
This was estimated by plotting instantaneous rates over time
and integrating under the curve by cutting out the area
and comparing its weight with weights of known areas.
Three sets of rates were considered.
If we assume ULR = those actually observed (Fig. I and Table
I), production calculated from method I equals that
calculated by method 2 .
If we assume (see text) that the initial ULR persists until
the peak of green leaf area and end-of-season ULR
applies for the remaining time, the seasonal production
by method 2 exceeds that calculated by method I
significantly.
Table 4 summarizes the differences in
production for the rapid growth and quiescent periods;
.we believe the unobserved production contributes to
below ground growth Table 5.
If
we assume (see text) that the initial ULR persists until
the peak of green leaf area and that ULR after that
time
is
actually
ten
times
end-of-season-ULR
(approximately initial growth rates) production is
further increased, but without violation of possible
root growth rates.
Table 4
Parame ter
3
Season
Production (mg/plant) of^individuals of Agropyrbn smithii grown under
four irrigation regimes.
2
Aboveground
production
P
E
' Q
T
Minimum total
production
P
E
T
Q
Maximum Total
producti o n
P
E
Q
T
Treatment and year
O mm
1977
1978
1979
76
78
70
44
202
74
19
16
12
139
301
156
76
78
70
55
523
100
31
16
34
162
61
20
76
78
70
55
523
100
312
160
343
443
761
513
6 mm
1977
1978
1979
86
88
67
53
112
46
49
9
38
188
209
151
86
88
67
62
351
81
49
30
38
197
469
186
80
88
67
62
351
81
485
301
380
633
740
528
12 mm
1977
1978
1979
94
71
302
53
35
26
431
150
1977
1978
1979
42
69
85
357
241
202
11
33
20
510
343
307
PO
Wet
—
_
-
O
_
94
71
621
73
142 1324
69
588
85 323
35
26
750
170
11 1477
33 690
29 437
94
71
621
73
350
260
1065
404
142 1324
588
69
85 323
HO
250
292
1576
907
700
Irrigation regimes were an unirrigated control (O mm),
a plot guaranteed 6 mm per week
(6
mm),
a plot guaranteed 12 mm per week (12 ram), and a plot in which soil water potentials
were maintained above -2 bars with irrigiations of 25 mm per week or more.
Aboveground production was estimated by multiplying leaf length (mm) produced in the period
by the weight per mm.
Total production was estimated by integrating under a curve of leaf
production
rate
created by multiplying observed green leaf areas by either a minimum
or
maximum estimate of the photosyynthetic rate.
3
Production was
estimated for the period before measurements began in May (preseason = P )
for the period of exponential growth before the peak-of-green (E ), for the quiescent period
after peak-of-green (Q) and for the total season (T).
21
production was significant because the soils dried and the
plants
turned
observable
brown
as
measurements
production
green
were
is
Estimates
(Fig.
I )•
plant
Preseason
material
initiated.
easily
growth
present
Preseason
estimated
by
was
the
are presentated in Table 4.
when
aboveground
first
Our
method.
data
cannot
provide the rate estimates needed for the second method.
Belowground
smithii
production . must
grasslands
aboveground masses
since
occur
in
belowground
(Weaver et al 1981).
Agropyron
masses
exceed
Consideration of
the second method of estimating production outlined
and
the
unit
estimating
leaf
this
rate discussion suggests
belowground
production:
a
above
way
one
of
should
integrate across time the product of green leaf area (Fig.
I,
lower curve) and its production r a t e .
production
(ULR)
the
production
calculated
as
For aboveground
rate was the unit
above.
. For
total
leaf
rate
production
a
production rate including belowground transport is n eeded.
Two
possibilities,
labeled
maximal
and
minimal,
are
outlined and applied below.
A
estimate of total production was made
by
summing production in three subseasons 5 the preseason,
the
period
after
minimal
before
maximum green leaf a r e a ,
maximum green leaf area.
assumed
calculated
to
be
entirely
and
the
period
Preseason production
aboveground
and
is
therefore
as the product of aboveground growth (mm)
and
22
weight
per
unit
production
area
is
(Fig.
of
length
(K^=mg/mm).
Early
calculated as the integral of
I)
and
green
a maximal unit leaf rate
calculated from the curves in Figure I.
season
(Table
observed drop in ULR during the log growth
due
to increasing transport to belowground o r g a n s .
leaf
phase
(Fig.
I) and the ULR at
results appear in Table 4.
production
is
season's
is
Late
production is calculated as the integral of
area
2)
This assumes that
the
season
leaf
green
end.
The
Our minimal estimate of total-
about
150%
of
measured
aboveground
suspect
that
the
proceeding
procedure
we
that
production.
While
we
underestimates
following
total
procedure
total production.
production,
believe
will give us a maximal
We assume,
as above,
the
estimate
that
of
preseason
production is entirely aboveground (for lack of data to do
otherwise) and therefore underestimate total production if
actual carbon transport is downward and overestimate it if
carbon
transport
calculation
season
maximal
upward.
We
assume
early season production
estimate
which
for
differs,
then,
only
in
on
late
the
I) it is generally
that most late season photosynthate is stored
Wiebe 1973);
that
early
correct.
we estimated as ten times
two reasons:
again
based
ULR and actual green leaf, areas is
production
estimate
of
is
Our
season
minimal
believed
(Sosebee and
and 2) early season unit leaf rates are about
23
ten
times late season unit
photosynthetic
rates
leaf rates
(Table
2)
do not actually fall,
so,
90%
of
photosynthate would be transported below g r o u n d .
our estimate maximal bec a u s e ,
green
leaf areas,
if
the
We call
while it is based on actual
estimated photosynthetic rates may
be
high
since water and nutrient resources per unit of
leaf
area
are probably less available late in the summer
than
in the spring.
Table
4
Maximal production estimates presented in
are about 300% of aboveground
production
rates
measured by method I.
Belowground
r h i zome ,
pr o d u c t i o n .
and
aboveground
crown)
production
downward
absence
flows
During
calculated
observed,
the
therefore
ie
an
implies
8
estimate
SE
and
production
that there
and
will
To the extent that
be
net
log phase transport
for
is
of
an
carbon
minimal
downward
and
maximal
belowground production
of 43 +6 SE
%
of
is
total
is
net
Quiescent phase transport averages
% of total net
92+1
both
similar
average
production (Table 6).
+
total
belowground production may actually be
similarly
estimates;
(root,
In the preseason our assumptions
of root growth.
negative.
19
from
transport
are u p w a r d ,
production
is easily calculated by substracting
presented in Table 5.
no
Belowground
production!
in
SE % in the maximal
latter seems more reasonable since 98,
the roots are belowground in our 0,
96,
6mm,
the
minimal
estimate.
The
95, and 56% of
12mm,
and
wet
24
Table
5.
Parameter
Season
I
Belowground production
(mg/plant)
calculated
using
minimum
or maximum
estimates
of
p hotosynthetic rates.
2
Production, minimum
P
E
T
Q
Production , maximum
P
E
T
Q
.mg/plant1
Treatment and year
0 mm
1977
1978
1979
0
0
0
11
316
26
12
0
22
23
316 .
48
0
0
0
11
316
26
293
144
331
304
460
357
6 mm
1977
1978
1979
0
0
0
9
239
35
0
21
0
9
260
35
0
0
0
9
239
35
436
242
342
445
531
377
12 mm
1977
1978
1979
—
0
0
—
319
20
—
0
0
_
319
20
0
0
319
20
315
234
634
254
1977
1978
1979
0
0
0
997
347
101
.0
0
9
967
347
130
0
0
0
997
347
101
99 1066
217 564
272 393
Wet
Belowground production = total production (Table 4) minus above
ground production (Table 4).
Production was estimated for the period before measurements
began in May (preseason = P), for the period of exponential
growth before the peak-of-green (E)s for the quiescent period
after peak-of-greenn (Q), and for the total season (T).
Irrigation regernes were an unirrigated control (0 mm), a plot
guaranteed 6 mm per week (6 mm), a plot guaranteed 12 mm per
week (12
mm) and a plot in which soil water potentials were
maintained above - 2 bars with irrigations of 25 mm per week or
more.
25
Table 6.
Belowground production
using
minimum
or
p h o t osynthetic r a t e s .
Parameter
Season
I
(% of total)
calculated
maximum
estimates
of
Production , minimum
P
E
T
Q
Production, maximum
P
E
T
Q
% of Total2
Treatment and year
0 mm
1977
1978
1979
0
0
0
20
60
26
38
0
65
14
51
24
0
0
0
20
60
26
94
90
96
69
60
70
6 mm
1977
1978
1979
0
0
0
15
68
43
0
70
0
5
55
19
0
0
0
15
68
43
90
97
90
70
72
71
12 mm
1977
1978
1979
_
_
0
0
51
27
0
0
43
12
0
0
51
27
90
90
60
63
1977
1978
1979
0
0
0
73
59
6
0
0
8
65
50
6
0
0
0
73
59
6
90
87
I
68
42
3
Wet
.
Belowground production = total production (Table 4) minus above
ground
production (Table 4).
Belowground production
is
expressed as a percentage of total production reported in Table
4.
Production was estimated for the period before measurements
began in May (preseason = P), for the period of exponential
growth before the peak-of-green (E), for the quiescent period
after peak-of-green (Q), and for the total season (T).
Irrigation regimes were an unirrigated control (0 mm), a plot
guaranteed 6 mm per week (6 mm), a plot guaranteed 12 mm per
week (12
mm) and a plot in which soil water potentials were
maintained above - 2 bars with irrigations of 25 mm per week or
more.
26
treatments
across
respectively
season
(minimal
total
one
(Weaver.et
calculates
al
1981).
Summing
that between 3 3 + 6
SE
estimate) and 64 + 3 SE % (maximal estimate)
net
production
is deposited
in
the
%
of
belowground
compart m e n t .
Factors
controlling
observations
availability
production
support
is
one
the
major
and
growth.
: Three
that
water
conclusion
d e t e r m i n a n t . of
production.
F i r s t , increased water availability slows the rate of loss
of green leaf areas and results in maintenance of a larger
photosynthetic surface
in
the
rapid
(Fig.
growth
phase
I) which might function late
or
after
occurring during the quiescent phase.
leaf
any
rainstorms
The amount of green
material on the average plant peaked before the
end
of the rapid growth phase of growth and was maintained for
two
1979,
to three weeks
before it began to
and
wet
plots.
dry
small populations,
less rapid in the 6 mm,
The same is true in dry 1977
super-adequate water supplies,
Even
In
the decline in green leaf material was most rapid in
the dry plot and progressively
mm,
decline.
due to heavy watering
12
when
and
resulted in no decline in the wet plot.
in the wet summer of 1978 when declines were similar
in the control,
6 mm,
and 12 mm treatments,
the drop was
least in the wet treatment.
Secondly,
in
relatively
leaves began to die (Fig.
I),
dry years (1977 and
1979)
and growth stopped earlier
27
in
the
(Fig.
a
dry control treatment
2).
dual
than in the
wet
treatment
Extending the period in which growth occurs has
positive effect -- f i r s t ,
productive period and s e c o n d ,
on the length
of
the
on any returns on additions
to photosynthetic surface due to increased production,
to a 'compound interest e f f e c t * .
treatme n t ,
In contrast to the
ie
wet
the 6 mm and 12 mm treatments had little or no
effect on the length of the active growth period (Fig. 2).
I
tentatively conclude that the 6 mm and 12 mm treatments
produced
little
additional plant material
despite
the
additional water available.
Thirdly,
in all seasons,
growth rates declined with
increasing plant water stress (Fig.
3).
Maximum
above­
ground growth occurred when plant water stresses were less
than
-5
bars and cessation of growth occurred
water stresses
higher
near -20 bars.
stresses
(Fig.
3) was
at
plant
The scatter associated with
probably caused by
small
spurts of growth after rainstorms. While such growth would
have
occurred
in nonrstress moments it would
associated in our records with low plant water
(indicative
of
high stresses)
have
been
potentials
read at the beginning
and
end of the measurement p e r i o d .
While growth always ceased when water was
exhausted,
the fact that it always slowed markedly in mid-July,
though
factors,
water
was
not
perhaps season,
limiting,
suggests
also limit growth.
that
even
other,
The reader
28
Figure 2
*
t illers in m m / d a y .
29
6 m m , 1977
O m m 1 1977
I5
-D
-D
GROWTH RATE (mm/day)
5
6 mm, 1978
r
20
----- a—
Ieeeeeeeeej
—
—
— -£2
MM
^ I I
Kg » I 9 6 m m 2/m m
6 m m , 1979
20
Ar
Al'
j m g /m m
K a = I SS m m 2Zmm
„ _ - jm g /m m
K a = I 7 9 m m 2/m m
_D
5
E
I
MAY
JUN
JUL
AUG
_____ r
i I i I
I
I
I
r
MAY
JUN
JUL
AUG
30
TREATMENT, YEAR
2 5 m m , 1977
A WATER ADDED (m m ) r a in □
ir r ig a tio n
B SOIL WATER POTENTIAL (MPn)
C PLANT WATER POTENTIAL (MPa)
* L2
25
0
-i I j. .Lm
HiiL I I
1
K w = O 3 5 m g /m m
K a = I 9 9 m m z/m m
WEIGHT CONSTANT (m g /m m )
AR E A CONSTANT (m m 2/m m )
I5
D GROWTH OF INITIAL PLA N TS (m m /da)
GROWTH RATE (mm/day)
5
E. T ILLE R GROWTH (m m /d a )
-D
~
I
2 5 m m , 1978
r
20
ArIiL
L I If kJ I
B
LI I I I
I--H
I-H
Kw = O 2 2 m g/m m
K a = I 5 5 m m 2/m m
I
I
2 5 m m . 1979
a
r
A F20 ; ,I
B
C
32
Kw = 0 2 5 m gg //m
mm
r
6 4 m m vm m
D
5
E
Figure 2.
T “r— 7
AUG
Continued.
l—i
Ik I I
I I
I............ =L-T=
KZ
Kw = 0 2 7 m g /rn m
K a = I 81 m m vm m
31
PLANT WATER POTENTIAL CMPaD
Figure
3.
The effect
of water potential and
season
on
unit leaf r a t e . The dashed lines are hand fit-
32
should
verify from Figure 2 that growth slowed in the wet
plot
in
kept
plant
mid-July in all years despite moist
water
Comparison
cessation
a
of
potentials between
the
wet
and
control
O
soils
and
plots
that
-5
bars.
shows
that
of growth is probably not due to achievement of
maximum
plant
exhaustion
of
concurrently
size,
to
nutrients,
internal
since
in
shading,
1978
in dry plots and in wet plots
larger plants,
it
or
to
occurred
(which,
with
had more internal shading and had consumed
more nutrients).
I
hypothesize,
above-ground g r o w t h ,
at
mid-summer
—
therefore,
that
the potential
and therefore production,
perhaps
due
preparation for fall and winter.
to
for
is reduced
daylength-induced
Three lines of evidence
support
the daylength-control speculation.
First,
above­
ground
growth ceases when no other factor seems to
limit
it.
Second,
the fact that growth rates associated with a
given water stress fall as the season progresses (Fig.
suggests
to
that photosynthate is being diverted from
storage
1970).
(Trlica 1977,
I
Moser 1977,
and Brown & Blaser
photosynthetic
capacity of healthy green leaves
with
Thirdly,
variety
Bouteloua
comata,
growth
do not accept.the alternative hypothesis
season.
of range grasses,
gracilis,
all
3)
that
declines
under experimental conditions
including
Andropogon
Agropyron
scoparius,
a
smithii,
and
I
grew more rapidly under energetically
Stipa
equal
33
short
night
conditions than under long night
conditions
(Weaver and Forcella 1983).
While
the
midsummer,
capacity
for
growth
was
reduced
drought-stressed plants apparently retain some
capacity to respond to late summer showers.
both
in
Plants
dry control plots and 6 mm plots grew after
1977
and
extension
greater
1979 (Fig.
rates
throughout
170,
mm
from
showers
plots
and
400
leaf
percent
than those of the control plots in
respectively.
late August 1977,
Though
mid-
and early
growth
continued
the summer in 12 mm and wet plots,
no effects
of summer showers
could be detected.
Late season
growth
in the wetter plots may have been masked by the
irrigation
state
In the 6
217,
early August 1977,
1979,
responses
2).
were 150,
(PCO.Ol)
July 1977,
July
at
treatments,
(e.g.
or prevented by
carbohydrate
balance)
a
physiological
induced
by
regular
watering.
Growth
response
increased
with
J^o water
increasing
extension
availability.
treatment
culm was 107 %, the average 12 mm treatment culm
in
(100 %),
the
6
mm
while the average 25 mm treatment culm was 207
dry 1977 and 1979 and 135 % in wet
Table 7).
average
When
with
%
control
water
Leaf
compared
was 106 %,
the
s u pplements.
I;
/
Statistical analysis — ANOVA after logarithmic
transformation
1978
(Fig.
-- of the growth data summarized in Figure
2 show that plants in the control,
6 mm,
and 12 mm plots
34
Table
7.
Total aboveground yields,
on culm length (mm)
and biomass (mg) b a s e s , of average
Agropyron
smith!!
plants
from plots subjected to
four
water r e g i m e s .
Each datum is reexpressed (in
parentheses) as a percentage of the co n t r o l .
Treatments
Control
6 mm
12 mm
Wet
Total length production (mm)
■ -•—
1977
606
(100%)
735
(111%)
1978
1245
(100%)
1340
(106%)
1365
1979
585
(100%)
600
(102%)
600
1455
(220%)
(110%)
1680
(135%)
(102%)
1140
(195%)
510
(367%)
Total mass production (mg)
1977
139
(100%)
188 (135%)
1978
301
(100%)
209
(6 9%)
431
(143%)
343
(114%)
1979
156
(100%)
151
(97%)
150
(96%)
307
(197%)
— —
35
grow
less than those in the wet plots ( P < 0 .0001)
years.
in
in
all
Plants from the control plots grew less than those
the
6 mm and 12 mm treatments in all years
Although
growth rates were not different
12 mm treatments in wet 1978,
(P<0.02).
in the 6 mm
and
they did differ in dry 1979
(P<0.0001).
When
weights
with
leaf
(K^)
length
is multiplied
specific
there is no clear pattern of yield
increasing water availability
Despite
by
increases
leaf
increase
(Table 4 or Table
in growth described a b o v e ,
we
7).
cannot
therefore confidently conclude that per-plant yields
increased
yields
by
6 mm or 12 mm
would
increased
i n c rease,
irrigation
however,
(see b e l o w ) .
if
were
regimes.
plant
Total
densities
Per-plant yield increases due to
the wet treatment are obv i o u s .
Tillers
Although
1977,
were
present in every plot in
it
is assumed that since tillers appeared
three
treatments under study,
present in the 12 mm plot.
Tillers,
defined as lateral vegetative shoots,
and
growing
intravaginally
growth
while
year.
measurements were not taken in the 12 mm plot in
remaining
axil,
every
—
less
upward
(Moser
which
than
1977
they were
as used
leaf
Thomas
was measured only in 1978
that
seasonal pattern (Fig.
of parent plants
2).
the
also
here,
are
arising from a leaf
within the
and
in
sheath,
1980).
and
had
Tiller
1979
a
ie
—
similar
Tiller growth declined
more
36
r a p idly,
however,
in response to drought stress than did
the growth of parent plants.
The reader will note (Table 7) that production of the
average
due,
but
culm was less in 1979 than in 1977.
in part,
is
to the fact that 1979 was drier than 1977,
probably
undoubtedly
due
are
(discussed
here)
elsewhere)
and
population
of
)
increased
with increasing
competition,
plant
a
product
of
individual
population size
and
(to
be
expected
(and
observed)
- that this
paper
'water
of
res ponses
discussed
tO
the
increases
in
understates
the
in so doing will conclude — - due
density
densities
especially in the wet plots.
benefits of added water.
\
to
will realize that the effects
reader
supplement s
omis sion
also
associated
which were observed,
The
This may be
37
CONCLUSIONS
The
results
discussed
above
suggest
five
major
conclus i o n s :
1)
The cumulative growth of individual plants (gm/plant)
followed the usual sigmoidal c u r v e .
2)
Green
curve
leaf
which
peaked
area generally exhibited
peaked about the time the
and
sometimes before significant
a
bellshaped
aboveground
plant
mass
or
soil
area
was
water stress developed.
3)
Aboveground
production
largest early in the season,
peak
of green leaf a r e a ,
fell to a low soon after the
and rose slightly in
summer.
The
reserves
from the previous season.
leaf
first
per unit of leaf
growth
the
late
built
^ith
The decline in
unit
must have
been
rates through the log growth phase to the post-peak-
of-green-stage
increasing
was
amounts
belowground
probably
due
to
of photosyntate to
storage.
allocation
root
growth
growth;
observation
that
Seasonal
changes
and
By growth analysis it is estimated
that one to two thirds of net photosynthate is devoted
belowground
of
this
.55-95%
is not inconsistent
of the
plant
is
with
to
the
belowground.
in allocation of photosynthate
may
be
38
triggered
by
exhaustion
of
water
and/or
changes
in
daylength.
4)
On a plant length basis,
6-7%
after
mm/week)
plant
per-plant growth increased
light shower treatments
(guarantees,
and 33-107% when soils were kept
weight
basis,
however,
of
moist.
aboveground
6-12
On
yields
a
were
significantly increased only by the wet treatment.
5)
Yield
per-plant
effects
effects
that
effects of water treatments are a product
(discussed
here)
and
(the subject of another study).
water
supplements
ever
reduced
plant
It is
plant
Until the density effects are multiplied in,
conclude
that
the effects reported here
magnitude of water treatment effects.
!
of
density
doubtful
densities.
one can only
understate
the
39
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WftlN '"
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Growth response of
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N378
N422
cop. 2
C A V ^ O H D 40