DORMANCY MUSHAYABASA T.

DORMANCY: Regulation of germination Seed dormancy is a condition in which viable plant seeds fail to germinate under optimal environmental conditions. This is known as primary dormancy. Seed dormancy prevents immediate germination and regulates the time, conditions and place that germination will occur.

DORMANCY In nature various types of primary dormancy have evolved to aid in the survival of species by programming the germination time for suitable times in the annual cycle. Living, non dormant, viable seeds germinate when soil temperatures and moisture conditions are suited for cellular processes and division;

DORMANCY Seeds with primary dormancy do not germinate even under ideal germinating conditions. Secondary dormancy is a further survival mechanism resulting or induced by unfavourable external or environmental conditions.

ADVANTAGES OF DORMANCY Dormancy enables the storage, transportation and handling of seeds of vegetables, flowers or grains. Dormancy allows synchronized germination e.g ensuring that summer germinating seedlings are at the proper stage of development prior to the commencement of winter.

DORMANCY Types of dormancy include: Primary dormancy: exogenous, endogenous and combinational. Secondary dormancy: thermodormancy and conditional. Seeds have primary dormancy when they are shed from the plant while secondary dormancy is when seeds were previously not dormant but renter dormancy due to external unfavourable conditions.

PRIMARY DORMANCY Exogenous dormancy is a result of factors imposed form outside the embryo for example the seed coat and or the fruit parts. This material surrounding the embryo may inhibit imbibition of water, or limit oxygen to the embryo, or prevent inhibitor leaching or conversely supply inhibitors to the embryo.

PRIMARY DORMANCY Therefore exogenous dormancy may be physical or chemical. Physical dormancy: seeds with such dormancy fail to germinated due to seed coats that are impermeable to water. E.g the Malvaceae family- okra. Chemical dormancy: chemicals that are present in the flesh of fruits and seed coverings may inhibit germination . E.g the Brassica family- mustard.

PRIMARY DORMANCY Endogenous dormancy (physiological) results from characteristics of the embryo that prevent germination . The basis of this type of dormancy is that the embryo lacks the growth potential. Growth potential is the force exerted by the radicle to penetrate the seed coat.

PRIMARY DORMANCY In this case dormancy can be broken by weakening the seed coat and increasing growth potential in the embryo. Photodormancy : seeds requiring light or the absence of light (darkness) are referred to as photodormant or photoblastic.

PRIMARY DORMANCY Combinational dormancy is a result of the combination of exo and endogenous dormancy. The seed coat may be hard coupled by physiological dormancy (lack of growth potential).

SECONDARY DORMANCY (1) Ecodormancy is a reduced growth response due to an external stimulus such as drought or cold. Removal of the stimulus results in a resumption of growth. (2) Paradormancy is a reduced growth response induced by a biochemical signal that is transported to a target tissue. Removal of the signal results in a resumption of growth.

SECONDARY OF DORMANCY A good example of paradormancy is apical dominance, where auxins transported from an apical shoot suppresses the growth of lateral buds. (3) Thermodormancy: for some species like lettuce (Lactuca), celery (Apium) germination at high temperatures (> 25 0C) can induce thermodormancy. Differentiate between thermodormancy and thermo inhibition.

SECONDARY OF DORMANCY Thermal inhibition on the other is what seed experience when the temperature exceeds the maximum temperature for germination. Seed experiencing thermodormancy will not germinate when the temperature returns to optimum, where as thermo inhibited seeds will germinate when temperatures are lowered.

FUNCTIONS OF DORMANCY One of the functions of dormancy is to prevent a seed from germinating before it is surrounded by a favorable environment. In some trees and shrubs, seed dormancy is difficult to break, even when the environment is ideal. Various treatments are performed on the seed to break dormancy and begin germination.

BREAKING SEED DORMANCY Dormancy in seeds is broken in two ways: By altering the restraint of seed covering and By increasing the embryo’s growth potential. Methods of breaking seed dormancy include: Scarification, stratification/vernalisation and hormonal.

Methods of breaking seed dormancy 1. Seed Scarification Seed scarification involves breaking, scratching, or softening the seed coat so that water can enter to stimulate the seed germination process. There are several methods of scarifying seeds. In acid scarification, seeds are put in a glass container and covered with concentrated sulfuric acid.

Methods of breaking seed dormancy The seeds are gently stirred and allowed to soak from 10 minutes to several hours, depending on the hardness of the seed coat. When the seed coat becomes loose, the seeds can be removed, washed, and planted.

Methods of breaking seed dormancy Another scarification method is mechanical. Seeds are filed with a metal file, rubbed with sandpaper, or cracked with a hammer to weaken the seed coat. Hot water scarification involves putting the seed into hot water at a temperature of 75o- 100o C.

Methods of breaking seed dormancy 2. Seed Stratification/ Vernalisation Seeds of some trees and shrubs from the temperate zone do not germinate unless they are exposed to chilling temperatures. E.g peach and apple. This can be accomplished artificially by a practice called stratification.

Methods of breaking seed dormancy The following procedure is usually successful: 1. Put sand or vermiculite in a clay pot to about 2-3 cm from the top. Place the seeds on top of the medium and cover with 1 cm of sand or vermiculite. 2. Wet the medium thoroughly and allow excess water to drain through the holes in the pot. Place the pot containing the moist medium and seeds in a plastic bag and seal.

Methods of breaking seed dormancy 3. Place the bag in a refrigerator. Periodically check to see that the medium is moist, but not wet. Additional water will probably not be necessary. 4. After 10 to 12 weeks, remove the bag from the refrigerator. Take the pot out and set it in a warm place in the house. Water often enough to keep the medium moist.

Methods of breaking dormancy 5. When the young plants are about 5-7 cm tall, transplant them into pots to grow until time for planting at their permanent sites.

Hormonal control of dormancy The two most important hormones involved in seed dormancy and breaking seed dormancy are abscisic acid (ABA) and gibberillin (GA). Evidence of hormonal involvement comes from correlations between hormone concentrations with stages of development among other factors.

Hormonal control of dormancy Abscisic Acid (ABA) Its major role is preventing “precocious germination” of the embryo. ABA levels peak during the late stages of seed development and is largely involved in the induction of primary dormancy. E.g in peach seed ABA is high in the seed coat and the cotyledons in freshly harvested fruit seeds.

Hormonal control of dormancy However, after about 30 days of stratification ABA levels are almost at zero, however dormancy will only break after a further 8 weeks of chilling. Among other effects ABA action has a negative regulation of gibberillin levels. ABA regulation of active gibberillin levels has a direct impact on dormancy and consequently a seed’s ability to germinate.

Hormonal control of dormancy Gibbeellins are important in the prevention as well as promotion of seed dormancy. Gas stimulate germination by inducing acids the weaken the endosperm and testa/ seed coat surrounding the radicle. This therefore increases growth potential. GA synthesis and perception are affected by many environmental signals.

Hormonal control of dormancy These signal include light, temperature (including statification) and nitrate levels. GA occur in relatively high concentrations in developing seeds becoming lower in mature dormant seeds. Dormancy release treatments induce GA biosynthesis as well as GA sensitivity

Hormonal control of dormancy E.g during stratification, Gas are synthesized at the chilling temperatures or are converted to an available form. Ethylene gas, a naturally occuring hormone may be linked with overcoming dormancy as a majority of seeds were observed to have high levels of the gas present at the stage of radicle emergence.

Hormonal control of dormancy Cytokinin application can offset the effects of ABA and “rescue” seeds from thermodormancy. Cytokinins are also thought to allow the functioning of GA while being antagonistic to ABA.