seed germination Flashcards
Definition of Seed Germination
Definition of Seed Germination
Various definitions of seed germination have been proposed, and it is important to understand their distinctions
To a seed physiologist, germination is defined as “the emergence of the radicle through the seed coat”
Such a definition says nothing about other essential structures such as the epicotyl or hypocotyl that become the above ground parts of a successful seedling
To the seed analyst, germination is “the emergence and development from the seed embryo of those essential structures which are indicative of the ability to produce a normal plant under favorable conditions”
Others consider germination to be the “resumption of active growth by the embryo resulting in the rupture of the seed coat and emergence of a young plant”
This definition presumes that the seed has been in a state of quiescence, or rest, after its formation and development
During this period of rest, the seed is in a relatively inactive state and has a low rate of metabolism
It can remain in that state until environmental conditions trigger the resumption of active growth
Morphology of Seed Germination
Based on the fate of the cotyledons, two kinds
of seed germination occur
These two types are illustrated by the germination of bean and pea seeds
Although these seeds are similar in structure and are in the same taxonomic family, their germination patterns are quite different
Epigeal Germination
Epigeal Germination
Epigeal germination is characteristic of bean seeds and is considered evolutionarily more primitive than hypogeal germination
During 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, pulling the cotyledon and the enclosed plumule though the ground and projecting them into the air
Afterwards, the cotyledons open, plumule growth continues and the cotyledons wither and fall to the ground
Hypogeal Germination
Hypogeal germination is characteristic of pea seeds, all grasses such as rice and corn, and many other species
During germination, the cotyledons or comparable storage organs remain beneath the soil while the plumule pushes upward and emerges above the ground
In hypogeal germination, the epicotyl is the rapidly elongating structure
Regardless of their above-ground or below-ground locations, the cotyledons or comparable storage organs continue to provide nutritive support to the growing points throughout germination
Requirements for Germination
Water
Water is a basic requirement for germination
It is essential for enzyme activation, breakdown, translocation, and use of reserve storage material
In their resting state, seeds are characteristically low in moisture and relatively inactive metabolically
That is, they are in a state of quiescence
Thus, quiescent seeds are able to maintain a minimum level of metabolic activity that assures their long-term survival in the soil and during storage
Most seeds have a critical moisture content for germination to occur
For example, this value in corn is 30%, wheat 40% and soybeans 50%
Once that critical seed moisture content is attained in the seed, sufficient water is present to initiate germination
Gases
Air is composed of about 20% oxygen, 0.03% carbon dioxide, and about 78% nitrogen gas
If one provides different proportions of each of these gases under experimental conditions, it soon becomes clear that oxygen is required for germination of most species
Carbon dioxide concentrations higher than 0.03% retard germination, while nitrogen gas has no influence
Oxygen is required by the germinating seed formetabolism and for aerobic respiration
Temperature
Seed germination is a complex process involving many individual reactions and phases, each of which is affected by temperature
The effect on germination can be expressed in terms of minimium, optimum, and maximum temperatures at which germination will occur
The minimum temperature is sometimes difficult to define since germination may actually be happening but at such a slow rate that determination of germination is often made before actual germination is completed. The optimum temperature may be defined as the temperature giving the greatest percentage of germination in the shortest time
The optimum temperature for most seeds is between 15 and 30oC
The maximum temperature for most species is between 30 and 40oC
Process of germination
Process of germination
Most seeds undergo a specific sequence of events during germination
Prior to germination, seeds are in a “maintenance” phase that is often characterized as dormancy, or some other conditions hindering the transition to germination
At some point, the seed becomes sensitive to the presence of “trigger” agents such as light or temperature alterations shifting the balance of inhibitors to favor promoters such as gibberellins. The major sequence of events leading to germination is imbibition, enzyme activation, initiation of embryo growth, rupture of the seed coat and emergence of the seedling
Imbibition. The early stages of imbibition or water uptake into a dry seed represent a crucial period for seed germination
It is the first key event that moves the seed from a dry, quiescent, dormant organism to the resumption of embryo growth
The extent to which water imbibition occurs is dependent on three factors: (1) composition of the seed, (2) seed coat permeability, and (3) water availability
The principal component of seeds responsible for the imbibition of water is protein – thus high protein containing seeds will imbibe more water than starch or oil containing seeds
Seed coat permeability. Entry of water into seeds is greatly influenced by the nature of the seed coat
Water permeability is usually greatest at the mycropylar area where the seed coat is ordinarily quite thin
The hilum of many seeds also permits easy water entry
Enzyme Activation
Enzyme Activation : on hydration of seeds, various enzyme systems are activated and cellular activity increases
Activation of enzymes may be from enzymes formed during seed maturation or newly synthesized enzymes
Enzymes formed during maturation may be of 2 kinds – those requiring hydration for activation or those requiring action of hormones or other enzymes
Initiation of embryo
Initiation of embryo growth
Generally, the only substances taken up by the seedling during these early stages are water and oxygen
Because the seedling is underground and not photosynthetically active, there is an overall loss in dry weight
As the seedlings initiate photosynthesis and have the capability to fix CO2 into sugars, an increase in seedling dry weight occurs
SEED PRIMING
Seed Priming is defined as controlling the hydration level within seeds so that the metabolic activity necessary for germination can occur but radicle emergence is prevented.
Different physiological activities within the seed occurat different moisture levels.
The last physiological activity in the germination process is radicle emergence.
The initiation of radicle emergence requires a high seed water content. By limiting seed water content, all the metabolic steps necessary for germination can occur without the irreversible act of radicle emergence.
After drying, primed seeds can be stored until time of sowing.
Benefits of Priming
To decrease the time necessary for germination and emergence
To improve the stand uniformity, aiding in production management and increasing the chance for uniformity at harvest
To increase the rate of germination at any particular temperature. Priming can reduce germination times in the field by approximately 50% upon subsequent rehydration.
Priming can eliminate or greatly reduce the amount of seed-borne fungi and bacteria. Priming has been commercially used to reduce the amount of seed-borne fungi and bacteria, such asXanthomonas campestrisinBrassicaseeds.
Methods of seed priming
Several different priming methods have been reported to be used commercially including liquid priming and solid matrix priming.
With the SMP®method, water uptake is controlled by suspending seed in a defined medium (or matrix) of solids (organic and/or inorganic) of known water holding properties.The seed and matrix compete for available water, coming to equilibrium at precisely the right point for priming to occur.
The techniques are proprietary (patented by companies)
On-farm seed priming
Overnight soaking of seeds before sowing
ARTIFICIAL SEED
Artificial or synthetic or manufactured seed is encapsulated plant propagule (somatic embryo/shoot bud) in a suitable matrix, containing substances like nutrients, growth regulators, herbicides, insecticides, fungicides and mycorrhizae which will allow and help it to grow into a complete plantlet.
The concept of artificial seed has been applied, commercially and successfully in crops like Orchids, Alfalfa, Cotton etc.
