Seed dormancy and Germination
A Seed can be defined as a mature fertilized ovule that possesses an embryo, stored material in the form of cotyledons or endosperm and a protective seed coat. Seed dormancy (from the Latin word meaning to sleep) is the failure of an intact viable seed to complete germination under favorable conditions. A dormant seed does not have the capacity to germinate in a speciﬁed period of time under normal physical environmental factors that are otherwise favorable for its germination. Seed dormancy introduces a temporal delay in the germination process. It maximizes seedling survival by preventing germination under unfavorable conditions. Two types of seed dormancy have been recognized: coat-imposed dormancy and embryo dormancy.
The seed coat plays a vital role in the life cycle of plants by controlling the development of the embryo and determining seed dormancy and germination. Dormancy imposed on the embryo by the seed coat is known as a coat-imposed dormancy there are five basic mechanisms of coat-imposed dormancy. These include prevention of water uptake, interference with gas exchange, retention of inhibitors and inhibitor production. These limitations can be removed and germination can be accelerated by mechanically removing the seed coat, a process called scarification.
Dormancy in which the embryos themselves are dormant and is not due to any influence of the seed coat. Embryo dormancy is thought to be due to the presence of inhibitors, especially ABA, as well the absence of growth promoters, such as gibberellic acid. The loss of embryo dormancy is often associated with a sharp drop in the ratio of ABA to gibberellic acid.
Seed dormancy can also be distinguished on the basis of the timing of onset of dormancy-primary and secondary dormancy-rather than the cause of dormancy. Seeds that are released from the plant in a dormant state are said to exhibit primary dormancy. Contrary seeds that are released from the plant in a nondormant state, but that become dormant if the conditions for germination are unfavorable, exhibit secondary dormancy.
Various external factors terminate seed dormancy, and dormant seeds typically respond to more than one of two factors:
Many seeds lose their dormancy when their moisture content is reduced to a certain level by drying. A Period of dry Storage of freshly harvested, mature seeds is a method used to release dormancy and to promote germination (termed after-germination).The parameters that determine seed after-ripening are moisture and oil contents, seed covering structures, and temperature. After-ripening is prevented in very dry seeds; it requires seed moisture contents above a threshold value. This threshold moisture content is species-specific. After-ripening is also prevented during storage at very high air humidity. The molecular mechanisms of after-ripening are not known.
Pre-chilling: Low temperature can release fully hydrated seeds from dormancy. Some seeds can be induced to germinate by a period of pre-chilling (stratiﬁcation). It requires a period of low temperature (0-10°C) in order to germinate.
By definition, seed germination incorporates those events that commence with the uptake of water by the quiescent dry and terminate with the elongation of the embryonic axis. It is the resumption of growth quiescent dry seed. Seed germination depends on both internal and external conditions. The most important external factors include temperature, water, oxygen and sometimes light or darkness. Germination of many seeds is influenced by light. Seeds that are stimulated to germinate by light are described as positively photobiastic; seeds whose germination are inhibited by light are said to be negatively photoblasic. The germination qf positively photoblastic seeds (such as lettuce, Lactuca sativa, seed) is red light irradiation induces the germination of lettuce seeds, and far-red irradiation given after red light cancels the effect of red light. Phytochrome regulates lettuce seed germination via the control the endogenous level of gibberellin.
Based on the rage of the cotyledons, two kinds of seed germination occur – epigeal and hypogeal germination.
Epigeal germination is characteristic of bean and pine seeds and is considered evolutionarily more primitive than hypegeal germination. During germination, the cotyledons are raised above the ground where they continue to provide nutritive support to the growing points.
Hypogeal germination is characteristic of pea seeds and all grasses. During germination, the cotyledons or 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.