Seed germination
Definition of Germination
Seed Physiologist
emergence of the radicle through the seed coat
Seed Analyst
the emergence and development from the seed embryo of those essential structures which for the kind of seed in question are indicative of the ability to produce a normal plant under favorable conditions.
Others
resumption of active growth by the embryo resulting in the rupture of the seed coat and the emergence of a young plant.
Morphology of Germination
two types of germination can occur. Based on the fate of the cotyledons
Epigeal Germination
the cotyledons are raised above the ground where they continue to provide nutritive support to the growing points.
during root establishment, the hypocotyl begins to elongate in an arch that breaks through the soil, this pulls the cotyledon and enclosed plumule through the ground and projects them into the air.
Plumule growth continues and the cotyledons wither and fall to the ground.
Hypogeal Germination
the cotyledons or comparable storage organs remain beneath the soil while the plumule pushes upward and emerges above ground
the epicotyl is the rapidly elongating structure.
coleoptile, a temporary sheath enclosing the plumule is associated with the hyogeal germination .
coleoptile provides protection and rigidity to the emerging plumule as it pushes through the soil and exposed to light.
Requirements for Germination
seed maturity
seeds of most species are able to germinate before they reach physiological maturity
maximum seed germination can only be obtained if the seed is allowed to dry down slowly as it matures.
Environmental Factors
Water
basic requirement
essential for enzyme activation, breakdown, translocation and use of reserve material.
field capacity- moisture is about optimum for germination in soil, however, germination will always varies with species.
permanent wilting point- germination can still occur.
precocious germination- germination following rains or high-humidity conditions.
high moisture levels may inhibit germination.
Air (O2/Co2)
respiration increases sharply during germination.
the influence of carbon dioxide is usually opposite that of O2.
Temperature
cardinal temperatures
minimum
optimum
maximum
germination may be proceeding at a slow rate
governed by the temperature at which denaturation of proteins is essential for germination to occur.
defined as the temperature giving the greatest percentage of germination within the shortest time.
Germinating at alternating temperatures
many species require daily fluctuating temperatures
diurnal periodicity is common and seems to be more prevalent in species that have no had a long history of domestication.
fluctuating temperatures during germination seems to be associated with dormancy but alternating temperatures may accelerate germination of nondormant seeds as well.
alternating temperatures can cause a change in the structure of components in the seed which in the original form could prevent germination.
alternating temperatures can create a shift in the inhibitor-promoter balance where the inhibitor is decreased during the low-temperature cycle and the promoter increased during the high-temperature phase.
Stratification of Pre-chilling
the practice of preconditioning imbibed seeds in cool, moist, conditions to promote germination.
adapted from the nursery industry in which propagating stocks have been stored between layers of moist sand and sawdust prior to planting in a field.
Chilling injury
injury can be avoided if imbibition occurs above 20C.
low temperatures create stress conditions in cell walls that causes increased cell leakage during imbibitional chilling.
Light
both light intensity and light quality influence germination.
the nature of light can be illustrated by visible light.
Light intensity
germination of seeds is dependent on the type of light needed.
different lux lights can affect different plant species.
Light Quality
the greatest promotion of light comes from the red area.
red light (660-700nm) encourages germination
far red light and above (700nm+) has an inhibiting effect
Day Length
several species exhibit a photoperiodical controlled germination response
the need for long days decrease progressively as temperature increase and at high temperatures, germination can proceed.
Factors that Influence Light Sensitivity
Age of the Seed
Period of Imbibition
Imbibition Temperature
Stratification
Germination Temperature
light influences the strongest immediately after harvest and diminishes with age of the seed and eventually disappears.
one of the reasons why differing accounts of light requirements for seeds from the same species exist in the literature.
light sensitivity can increase imbibition
increased temperature during imbibition hastens metabolic events associated with germination.
enhances water uptake, results in more rapid hydration and increased sensitivity to light.
Low temperature treatment could substitute for the red light requirement for germination.
weak red light, while sufficient to promote germination along.
the dependence of seed germination on ight is reduced when seeds experience alternating temperatures
stratification may cause seeds to become more sensitive to light in proportion to the duration of stratification.
Phytochrome
promotive response in the red region (600-700nm)
inhibition region far-red region (720-760nm)
Pfr influences GA synthesis, observation that gibberellins can substitute for light in breaking the dormancy of may light-sensitive seeds.
- Pfr stimulates the release of endogenous GA .
it has been shown that inhibitors of GA biosynthesis delay the light-induced germination of a number of species.
- Pfr selectively activates specific genes.
- Pfr alters membrane permeability. Phytochrome is localized in cell membranes.
- Kinase activity has been reported to increase.
phytochrome mediated control of germination has immense ecological significance.
this allows buried seeds to remain dormany until they are exposed to light.
Pattern of Seed Germination
major events of seed germination
imbibition
enzyme activation
initiation of embryo growth
rupture of the seed coat
emergence of the seedling
Imbibition
the early stages of imbibition or water uptake into a dry seed represent a crucial period for seed germination.
seeds are sensitive to rapid imbibition, chilling and anoxia
essential process initiation on seed germination
first key step that moves from a dry, quiescent, dormant organism to the resumption of embryo growth.
Water imbibition occurs dependent on three factors
- composition of the seed
- seed coat permeability
- water availability
Composition of the Seed
seeds typically possess extremely low water potentials
water potentials are attributed to their osmotic and matric characteristics.
chemical constituents also contribute to imbibition
the mucilage of various seeds increase imbibition as do the cellulose and pectins located in the cell walls.
starch molecules have little impact on imbibition.
Seed Coat Permeability
entry of water into seeds is greatly influenced by the nature of the seed coat (or pericarp)
water permeability is usually greatest at the micropylar area where the seed coat is ordinarily quite thin.
the hilum of many seeds also permits easy water entry
Hard seed Coat
special tissues around these natural openings that prevent water entry and contribute to hard seed coat dormancy
hard seededness has attributed to small elongated pores and a high density of waxy material embedded in the testa epidermis
hardheadedness has been attributed to the presence of lipids, tannins, and pectic substances in the seed coat.
genetically and environmentally controlled and is greatest when seed maturation occurs under high temperature, high humidity and dry conditions.
Water availability
the ability to imbibe water is dependent on cell water potential
based on three factors
- cell wall matric forces
cell walls and intracellular inclusions such as mitochondria, ribosomes and spherosomes are characterized by the presence if membranes
- Cell osmotic concentrations
greater the concentration of soluble compounds, the greater the attraction for water.
- Cell turgor pressure
results of the restraining force of the cell wall, tends to slow water absorption.