Dormancy and Seed Germination
Almost all plants experience a period of suspended growth. The suspension of growth may be due to exogenous control, such as a change in environmental conditions, and is referred to as quiescence. However, many seeds may not grow even in favourable conditions. This is termed as rest or dormancy, and is under endogenous control. Thus, while quiescence is the condition of a seed when it is unable to germinate because favourable environmental conditions are not present, dormancy is the condition of the seed when it fails to germinate even though favourable environmental conditions are present. The dormancy in seeds may be due to rudimentary embryos , impermeable seed coats, mechanically resistant seed coats, physiologically immature embryos and even by the presence of germination inhibitors, like abscisic acid, phenolic acids, short-chain fatty acids and coumarin.
The dormancy of seeds can be broken or its duration can be reduced for early germination of seeds. This is possible by mechanical or chemical scarification of the seed coat, stratification of seeds (i.e. subjecting the moist seeds in the presence of oxygen for variable periods to low or high temperatures) or changing the environmental conditions, such as temperature, light and pressure.
Seed germination is the first step towards plant growth. Seeds often wait for favourable signals and inputs from the environment to germinate. After the dormancy period is over or it is broken, and necessary conditions for growth are present, the dormant embryo resumes metabolic activities and growth. The process is called seed germination.
The availability of water and oxygen is the essential condition for seed germination. The imbibition or uptake of water by the seed, is the first step. It causes the seed to swell, rupturing the seed coat to enable the radicle to emerge. The stored polysaccharides and proteins get hydrolysed to resume metabolic activities. The metabolic activities require oxygen for breaking down the reserve food and to release energy for various activities. In some plants, like certain varieties of lettuce and tobacco, light plays an important role in germination. The red region of the visible spectrum is most effective for the onset of seed germination.
The emergence of radicle from one end to the embryonic axis, which gives rise to the root system, is usually considered as a symbol of seed germination. At the other end of the embryonic axis is the plumule, which forms the shoot (stem and leaves). The growth of the radicle and the plumule involves cell extension and cell division, besides initiation of several biochemical processes. There is a rapid increase in the rate of respiration during germination. Some plants growing in marshy land, show a special type of germination known as vivipary as in Rhizophora, Sonneratia and Heritiera. In vivipary, the seeds germinate inside the fruit, while still attached to the parent plant. The weight of the germinating seed increases, and thus the seedling separates and falls down vertically into the mud. The lateral roots develop quickly for proper anchorage.