Seed Dormancy PDF

Seed Dormancy Dormancy What is dormancy? Dormancy is a period in an organism’s life cycle when growth, development, and physical activity are temporarily stopped. This minimizes the metabolic activity and therefore helps an organism to conserve energy.  Seed Dormanc...

Seed Dormancy Dormancy What is dormancy? Dormancy is a period in an organism’s life cycle when growth, development, and physical activity are temporarily stopped. This minimizes the metabolic activity and therefore helps an organism to conserve energy.  Seed Dormancy  A dormant seed is not a 'failure'.  The inability of certain seeds to germinate readily even when they are provided with all conditions required for germination.  Dormancy may be due to conditions associated either with the seeds itself or with existing environmental factors such as temperature and moisture.  Types of dormancy  Seed coat dormancy  Dormancy due to rudimentary embryos  Dormancy due to chemical inhibitors  Dormancy due to internal factors  Double dormancy etc.. Seed coat dormancy  It is due to the seed coats or other tissues covering the embryo which are hard & are impermeable to soil & oxygen which prevents germination. E.g. Malvaceae.  Also the seed coats are apparently permeable to water & gases but they are so hard to resist the embryo expansion. E.g. Pits of stone fruits, shells of nuts. Dormancy due to rudimentary embryos  Some plants shed their fruits before the embryo within the seed has attained the maturity stage to germinate.  Such embryos require several weeks to several months after harvest to attain its full maturity so that it can germinate. E.g.. Pinus sp., palms. Dormancy due to chemical inhibitors  In certain species specific chemical substances that prevent germination occur in the seed coats, endosperm or the embryo.  They are reduced or eliminated by leaching with water or adsorption of water. Dormancy due to internal factors  It is due to physiologically dormant embryos.  In this case, the dormant embryos do not resume active growth even though all environmental conditions are favourable, unless the seeds are subject to moist, chilling treatments.  E.g. Freshly harvested seeds of apple, grapes, peach & apricot. Double dormancy Seeds of some species exhibit both seed coat dormancy & embryo dormancy. E.g. Cercis occidentals Methods to overcome dormancy  Mechanical scarification  Soaking in water  Acid treatment  Cold stratification  Dry storage  Treatment with chemicals Mechanical scarification  It includes breaking or scratching the seed coats mechanically to modify the hard or impervious seed coats.  This can be done easily by revolving the seeds in a drum lined with sand paper. Soaking in water  Generally seeds will be soaked in hot water for a few seconds & then soaked for 24 hours in cold water which make the seed coat to get soften & wash off the incubators. E.g. Wattle seeds.  In some case, the seeds are soaked in running cold water for a period of 8-12 hours which help in removing the inhibitors. Acid treatment  Soaking the seeds for a few seconds (15-60 seconds) in conc. Hydrochloric acid or sulphuric acid modifies the hard or impermeable seed covering.  At the end of treatment period, the seeds are washed to remove the remnant acid. E.g. Mucuna bracteata Cold stratification  During stratification, seeds are exposed to abundant moisture, ample oxygen & a relatively cool temperature.  It consists of placing the seeds in a moist medium of sand, peat or vermiculite & holding at a temperature slightly above freezing.  This time varies between 1 to 4 months depending upon the type of seeds.  This permits the physiological changes within the embryo to occur. During this process, the level of endogenous growth promoting substances (e.g.. Gibberellins & cytokinins) increase while the level of growth inhibiting hormones (e.g.. Abscissic acid) decreases, thus removing the block & permitting germination. E.g. Peaches Dry storage  It promotes the after ripening in certain seeds which are dormant when freshly harvested.  Freshly harvested seeds of many annuals & herbaceous plants fail to germinate until after a period of dry storage.  Such post-harvest dormancy may last few days to several months. Treatment with chemicals  Soaking in potassium nitrate (0.2%), gibberellic acid (200 to 500 ppm) or thiourea (0.2%) solution prior to sowing has been found to stimulate germination of different kinds of seeds.  For instances, soaking of seeds in gibberellic acid stimulates the germination of many citrus species viz. Trifoliate orange, Rangpur lime, Sweet orange, Sour orange etc..