Seed Dormancy 

Seed Dormancy

This is the failure of viable seeds to germinate even when the germination conditions are provided. 

Seed viability: Is the ability of the seeds to germinate when all the conditions are available. For more notes read more notes about chemistry, home economics, physics,commerce

Causes of Seed Dormancy

Hardness of the seed coat: When the seed coat is hard, it prevents emerging out of the plumule and radicle.

The physiological immaturity of the embryo: The embryo may fail to secrete enough gibberellic acid used to stimulate secretion of hydrolytic enzymes needed to catalyze the hydrolysis of the stored food substances into simpler and soluble products to induce growth. 

Presence of growth inhibitors: When the concentration of germination inhibitors e.g. abscisic acid is high and higher than that of growth promoters, growth of the seed embryo is inhibited. 

Impermeability of the seed coat: When the seed coat is impermeable to oxygen and water, it prevents supply of oxygen and water to the embryo whereby little oxygen and water vapour diffuse across the micropyle to the embryo. The embryo may respire less aerobically and more anaerobically due to insufficient oxygen supply resulting into formation of little energy for growth inform of ATP.

Immaturity of the embryo: Seeds should be harvested when they have undergone the after ripening period. After ripening refers to the internal physiological changes the embryo or the seed must undergo before germination begins. This prevents premature seed germination which results into poorly adapted weak and non-health plants.

How seed dormancy can be broken

  • Mechanical removal or breakdown of the seed coat (i.e. scarification using knives, pins and paper?). 
  • Chemical break down using the gut enzymes and hydrochloric acid by soaking the seeds in the solution.

Germination inhibitors—By carrying out scarification or pre-chilling of the seeds which involves treating them under low temperatures (0 – 5oC) to reduce the concentration of growth inhibitors like abscisic acid and increase the level of growth promoters like gibberellic acid, and potassium nitrate can be used to reduce the thickness the seed testa.

It can be broken by providing the seed with growth promoters in order to raise their concentration above that of the growth inhibits.

Light: Enough light should be provided to positively photoblastic seeds to induce them to germinate.  To the negatively photoblastic seeds, light should be removed to induce them to germinate. Absence of light in positively photoblastic seeds prevents the embryo from growing and presence of light in negatively photoblastic seeds inhibits seed germination.

Drying out of the seeds: plant the seeds immediately after harvesting when they still possess adequate moisture in them.

Significance of Seed dormancy

It enables the seeds to survive adverse environmental conditions;, it prevents premature seed germination resulting into the development of well-developed and healthy plants;, it ensures dispersal of plant species;, it reduces competition by minimizing overcrowding of offsprings.

Seed Germination

This is the emergence and development of a seed embryo into a seedling capable of establishing itself into a new and independent plant under favourable conditions. Seed germination is also the onset of growth of an embryo into a mature plant which usually occurs at the end of seed dormancy.

Factors affecting seed germination

Oxygen: Because oxygen is a metabolite for respiration, its increase in concentration increases the rate of seed germination. When oxygen level is high, there is high rate of breakdown of soluble food substances at the growth centres (sites) and storage centres (sites) of the embryo to form energy in form of ATP used to run different anabolic growth processes at a high rate hence growth.

Low oxygen supply to the embryo results into low/ no seed germination because of little energy produced by the embryo. At early stages of seed germination the embryo dry mass may increase slowly because of little energy released by both aerobic and anaerobic respiration brought about by low oxygen supply to the embryo via the micropyle. At this moment the testa is still intact and relatively impermeable.

Optimum or suitable temperature: At optimum temperature the rate of seed germination is high because of optimum level of enzymes activity for fast hydrolysis of stored food substance into soluble forms hence fast growth of the seed embryo. When temperature is above optimum, the rate of seed germination reduces or even stop because of denaturation of enzymes for food hydrolysis and anabolic growth processes of the embryo.

When temperature is below optimum the rate of seed germination is lowered due to inactivation of enzymes which catalyse the hydrolysis of the stored food substances and also used for the anabolic growth reactions of the embryo. Different species of plants have varying ranges of optimum temperatures for seed germination whereby the temperate species have lower ranges of temperatures than the tropical plant species. 

  • Water: When the seeds are provided with enough water they germinate at a high rate because water is used to serve many roles which include: softening of the seed coat for the plumule and radicle to emerge out;, Activation of hydrolytic enzymes used to catalyse hydrolysis of stored food materials into soluble products;, Hydrolysis of stored  food materials into soluble products;, Activation of seed embryo to secrete gibberellic acid used to stimulate synthesis of hydrolytic enzymes;, Translocation of soluble food substances from storage sites to the growth sites of the embryo;, It results into cell elongation via due to uptake through the vacuole. For more notes read more notes about commerce, entrepreneur Notes, biology

Lack of enough water supply to seed, germination can occur at a low rate and total lack of water leads into failure of seed germination hence no growth.

Mechanism of Seed Germination

During early stages of seed germination the seed takes up water by imbibition (which is the absorption of water by the colloidal substances of the seed which include the proteins hemicellulose and pectic acid of the seed coat and also the stored starch) and osmosis. 

The absorbed water activates the hydrolytic enzymes to catalyse hydrolysis of the stored sugars into simple and soluble forms and also activates the embryo to secrete gibberellic acid which stimulates synthesis of hydrolytic enzymes which hydrolyse stored food substances (starch) into soluble products. As soluble products of stored food hydrolysis are formed, the solute concentration and osmotic pressure of the embryo increase which results into osmotic influx of water into the embryo. The soluble products of food hydrolysis or digestion are broken down using oxygen (respiration) into water, carbon dioxide and energy in form of ATP at optimum temperature. This reduces the dry mass of the seed.

The energy released is used during different growth processes which include protein synthesis, mitotic division, cell elongation and differentiation. Some food products are used for different growth processes e.g. amino acids are used for formation of the protoplasm, enzymes, proteins etc. Glucose is used for formation of cellulose cell walls. Fatty acids and glycerol are used for formation of lipids used for cell membrane formation.

As the embryo grows, its cell continues to division mitotically and increase in number and thereafter undergo elongation and increase in size. The seed coat softened by water eventually rupture for the plumule and radicle to emerge out. 

The enlarged cells differentiate into different tissues and organs including, the leaves which carry out photosynthesis for food accumulation. This increases the dry mass of the seedling (clear)

 

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