Factors Affecting Seed
Germination
Mark A. Bennett
Seed Biology Program
Dept. of Horticulture and Crop Science
Ohio State University
Columbus, OH 43210-1086
bennett.18@osu.edu
1
Internal Factors Affecting
Seed Germination
Seed vitality
„ Genotype
„ Seed maturation
„ Seed dormancy
„
2
Germinability During Seed Development
„
„
Developing alfalfa
seeds (A) do not
germinate when
excised from the
pod until late
development
(maturation drying)
stages
Tomato seeds (B)
take about 60 DAP
to reach full
maturity, but
germinate at 90%
levels by about 40
DAP
alfalfa
tomato
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and
Germination.
ABA and Seed Development
Arabidopsis:
ABA content of seed
commonly increases
during development;
declines during late
maturation (drying).
„ For seeds developing
within fleshy fruits,
ABA content + low
water potential of fruit
tissues (osmotic
stress) both prevent
precocious
germination.
„
ABA
insensitive
ABA deficient
Wild type
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
3
Vivipary (vp) Mutants and
Seed Development
„
„
Even if wild-type
kernels of vp maize
are sprayed with
fluridone (ABA +
carotenoid synthesis
inhibitor) during
seed development,
precocious
(viviparous)
germination occurs
Mutants can be
ABA-insensitive or
ABA deficient
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
Acquisition of Desiccation Tolerance
„
„
Developing
castor bean
(Ricinus
communis)
seeds removed
from pod and
placed on water
do not germinate
until about 50
DAP
If excised seeds
are first
desiccated,
germination is
seen by 25 DAP
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
4
Soluble Sugars and Desiccation Tolerance
In maturing seeds
of many species
conc. of sucrose
(disaccharide) &
the
oligosaccharides
raffinose,
stachyose and
verbascose (RFO’s)
increase.
„ Monosaccharides
(glucose,
mannose, fructose,
galatose)
predominate in
seeds at the
desiccation-intol.
stage.
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and
Germination.
Proteins & Seed Development
LEA (late
embryogenesis
abundant) proteins
occur as 2 distinct
classes; one class
increases at
midpoint of cotton
seed development
(25-35 DAP),
coincident with ABA
peaks
„The second LEA
class increases at
maturation drying
(45-50 DAP)
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
5
Seed Dormancy
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
„
„
Seeds are dispersed from the parent plant with different degrees of
dormancy (polymorphism, heteromorphy, or heteroblasty)
This seed variation is often seen in color, size, and coat thickness
Seed Dormancy Types
From Bewley, J.D. and M. Black. 1994. Seeds – Physiology of Development and Germination.
6
‘Relative Dormancy’ Concept
Dormancy
expression strongly
linked to
temperature
„ The critical temp
frequently shifts
with seed age or
various treatment
(cytokinins, nitrate,
GA’s)
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
From Bewley, J.D. and M. Black. 1994. Seeds – Physiology of Development and
Germination.
7
From Bewley, J.D. and M. Black. 1994. Seeds – Physiology of Development and Germination.
Environmental Factors and
Seed Germination
Water
„ Temperature
„ Oxygen
„ Light
„ Smoke
„
8
Water Potential Differences –
Seed vs. Soil
The rate of water
transfer from soil
to seed declines
with time.
„ Capillary and
vapor movement
of water near
seed influenced
by soil
compaction, soil
types.
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
Seed Water Uptake
„
„
Seed-soil contact
varies with seed
size, shape.
Small seeds
(+mucilaginous
types), and smoothcoated seeds most
efficient in H2O
uptake, increased
soil contact; larger
surface area/volume
ratio
9
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
Seed Water Uptake
Permeability of
seed to H2O is
important for
rate of uptake.
„ Soybean
embryos with
higher initial
moisture
contents (25%;
40%) imbibe
faster than
embryo w/ lower
water contents.
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
10
Soaking Injury and Seed
Solute Leakage
With imbibition,
rapid leakage of
solutes (sugars,
organic acids,
ions, amino
acids, proteins)
into the
surrounding
medium occurs.
„ These solutes
may stimulate
growth of soil
microbes (fungi,
bacteria).
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and
Germination.
Soaking Injury (continued)
„
„
Seeds of many legumes
(esp. Phaseolus sp., Pisum
sp., Glycine max) germinate
very poorly when hydrated
without their testas.
Some seeds (e.g. soybeans)
also leak proteinase
inhibitors and lectins upon
early imbibition – may
protect against microbes,
insects.
11
Temperature and Germination
Temperature
affects
capacity for
germination,
and rate of
germination.
„ Temperature
minimas,
maximas and
optimal ranges
vary by
species.
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
12
Temperature Effects on
Dormancy, Germination
All the unchilled
seeds are dormant
at 20oC and above
„
„
The chilled
Delphinium seeds
have no dormancy;
temperature range
for germination
extends above 30oC
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and
Germination.
Temperature and Germination
Rates
Maximum rate
typically occurs
over a narrow
range (1-2oC)
„ This
heterogenous
response to
temperature
distributes
germination in
time
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
13
Germination Test Durations
and Temperature Optima
„
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
Highest
germination
rates around
25oC (per 2d
values) for
Brussels
sprouts.
Very good
eventual
germination
from about
10oC to 30oC,
per 14d values.
Oxygen and Germination
Embryo:
„
Storage tissue:
Patterns of O2 consumption by the embryo (A)
differs late in germination from that by storage
tissues (B)
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and
Germination.
14
Oxygen Availability in Soil
„
„
Found to vary from 19%
to 21% in a wide range of
conditions (time of year,
tillage, soil depth, soil
type) – Sells, 1965
Exception may be when
seeds are encased in soil
particle aggregates,
sealing them off from gas
exchange with the soil
environment -(Pareja et
al., 1985)
Oxygen Solubility in Water
„
„
O2 solubility increases with decreasing
temperatures (twice as much 02
available to seeds at 5oC than at 40oC)
Literature on O2 role in seed dormancy
is sometimes conflicting, inconclusive
because of these solubility differences.
15
Seed Respiration – Anaerobic Conditions
Alcohol
dehydrogenase
(ADH) activity
parallels declines
in seed ethanol
levels.
„ Periods of natural
anaerobiosis in
germinating seeds
can last from a
few hours to
several days.
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination.
Variable Seed Coat
Permeability to Oxygen
From Bewley, J.D. and M. Black. 1994. Seeds – Physiology of Development and Germination.
16
Seed Coats also Linked to Germination Inhibitors
From Bewley, J.D. and M. Black. 1994. Seeds – Physiology of Development and
Germination.
From Kigel, J. and G. Galili. 1995. Seed Development and Germination.
17
O2 Concentrations and Tomato Seed
Germination
From Kigel, J. and G. Galili. 1995.
Seed Development and Germination.
Temperature Effects on O2
Requirements
From Kigel, J. and G.
Galili. 1995. Seed
Development and
Germination.
18
Effects of Light on Seed
Germination
„
„
„
Light can promote or inhibit
germination
Sensitivity to light is important to seed
banks and other ecological responses,
providing a mechanism for optimal
timing of seedling establishment
The photoreceptor for most types of
seed responses is phytochrome
Species that Produce
Photosensitive Seeds
From Black, Bewley and Halmer, 2006. The Encyclopedia of Seeds
19
Light and Germination
„
„
„
Levels of photodormancy vary not only
among genotypes, but also among
seed lots of the same cultivar (lettuceWurr et al., 1986)
Photo conversions of Pr to Pfr usu. take
place when seed is hydrated above 8%
Critical periods for light environment
effects on seed germination or
storability occurred after physiological
maturity (PM) in lettuce (Contreras et
al., 2008
Lettuce - Normal
Seedlings after AA
20
White light also inhibits
germination of some species
„
„
„
„
Illumination over many hours of germination
period generally required
Blue region of light spectrum (thru
cryptochrome) likely responsible
This type of photo inhibition is magnified for
seeds under water stress
Tomato seed studies -> ABA biosynthesis;
blue light increases sensitivity to ABA.
(M. Black, 2006)
Influences of smoke on
germination
Stimulatory effects of smoke on >100 Fynbos (heathland)
species in South Africa, Australia, N. America
„ Butenolide (3-methyl-2H-furo[2,3-c] pyran-2-one)
conforms to the necessary attributes of smoke from fires in
natural settings:
1) stable at high temps (melting pt. is ~119oC)
2) water-soluble
3) active at wide range of concentrations (1 ppm to ppt)
4) capable of promoting germination of a wide range of firefollowing species [Flematti, et al 2004 – Science vol. 305]
„
21
Chemical promotion of germination– endogenous hormones
From Bewley, J.D. and M. Black. 1994. Seeds – Physiology of Development and Germination.
Composition and location
of Auxin
Major auxin in
developing seeds is
IAA – indole-3acetic acid
„IAA is most
abundant in young
plant tissues
(immature seeds,
fruits)
„IAA commonly
conjugated to
sugars, sugar
alcohols, and amino
acids (asp, glu, ala)
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination
22
Gibberellins (GA’s)
„
„
„
First GA identified (GA3 – gibberellic acid) was
isolated from fungus Gibberella fujikuroi in the
1930’s
The first plant gibberellin (GA1) was isolated in
1957; since then over 125 naturally occurring GAlike compounds have been isolated from higher
and lower plants (mosses, algae)
GA’s play a key role in degradation of storage
reserves -> e.g. barley aleurone system
GA’s (continued)
„
„
„
Studies with mutants of Arabidopsis and tomato
basis for ‘hormone balance theory’; germination
timing coordinated by action of GA’s and ABA
Some types of seed dormancy also linked to
changes in GA, ABA contents or sensitivity
Germination of photoblastic seeds (light
promoted or inhibited) such as lettuce,
Arabidopsis also linked to red light effect on
synthesis of GA
23
Abscisic acid - ABA
Found in seeds of
many species;
promotes maturation
and dormancy,
synthesis of certain
storage proteins, and
inhibits germination
„ ABA generally rises in
concentration during
seed development,
reaches 1 or 2 peaks,
then (usually)
declines rapidly at the
time of seed
drydown.
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and
Germination
Cytokinins (CK)
CK’s have
diverse effects
on plants;
notable in
promoting cell
division
o
„ Synthesized 1 in
root tips and
transported thru
xylem to aerial
plant parts
„ CK’s also occur
in developing
embryos (e.g.
zeatin from
maize kernels)
„
From Bewley, J.D. and M. Black. 1994.
Seeds – Physiology of Development and Germination
24
Ethylene (C2H4) and
germination
„
„
„
„
Eth is made in most plant tissues; also induced by
various stresses (e.g. wounding, drought, flooding)
Eth may be induced by auxin, which can be further
enhanced by CK’s or ABA; eth may also selfregulate synthesis (+ or -)
Eth is biosynthesized from ACC (aaminocyclopropane-1-carboxylic acid), which is
synthesized from SAM (S-adenosyl-L-methionine)
Exogenous Eth-generating compounds (e.g.
ethephon) can be used in seed enhancements
(Encycl of Seeds; 2006)
From Bewley, J.D. and M. Black. 1994. Seeds – Physiology of Development and Germination
25
Hydrogen peroxide (H2O2) and
germination
„
„
„
Early literature – H2O2 as a fungicidal seed
treatment stimulated germination of certain
seeds
Mechanism(s) unknown; H2O2 may
accelerate early respiration phase of
germination?
H2O2 test for seed analysts – results in more
rapid root protrusion from seed (SCST/Seed
Technol. Training manual; pgs. 10:14-15)
Other chemical promoters of
germination
„
„
„
Potassium nitrate (KNO3) – commonly
used in germ testing; linked to relieving
light dormancy by increasing sensitivity
to light in many species
Respiratory, other metabolic inhibitors
(e.g. cyanide)
Anaesthetics (e.g. chloroform, acetone,
ethanol)
26
Other chemicals that stimulate
germ/break dormancy
From Black, Bewley and Halmer, 2006. The Encyclopedia of Seeds
Chemicals which inhibit
germination
From Black, Bewley and Halmer, 2006. The Encyclopedia of Seeds
27