Dormancy

Dormancy

In the broadest sense, the state in which a living plant organ (seed, bud, tuber, bulb) fails to exhibit growth, even when environmental conditions are considered favorable. In a stricter context, dormancy pertains to a condition where the inhibition of growth is internally controlled by factors restricting water and nutrient absorption, gas exchange, cell division, and other metabolic processes necessary for growth. By utilizing the latter definition, dormancy can be distinguished from other terms such as rest and quiescence which reflect states of inhibited development due to an unfavorable environment.

Physically induced dormancy can be separated into two distinct classes, based on external conditions imposed by the environment (light, temperature, photoperiod) and restraints induced by structural morphology (seed-coat composition and embryo development).

The physical environment plays a key role in dormancy induction, maintenance, and release in several plant species.

1. Temperature. The onset of dormancy in many temperate-zone woody species coincides with decreasing temperature in the fall. However, it is the chilling temperature of the oncoming winter which is more crucial, particularly in regard to spring budbreak.

2. Light duration and quality. Possibly the single most important environmental variable affecting dormancy is day length or photoperiod. See Photoperiodism

3. Water and nutrient status. Dormancy is affected by the availability of water and nutrients as demonstrated by many grasses, desert species, and subtropical fruits which go into dormancy when confronted by drought or lack of soil fertility. See Plant mineral nutrition, Plant-water relations

4. Environmental interactions. Several of the factors previously discussed do not simply act independently, but combine to influence dormancy.

Examples of dormancy imposed by physical restrictions are most evident in the structural morphology of dormant seeds. These restrictions specifically pertain to the physical properties of the seed coat and developmental status of the embryo.

1. Seed-coat factors. The seed-coat material surrounding embryos of many plants consists of several layers of tissue, termed integuments, which are infiltrated with waxes and oils. In effect these waterproofing agents enable the seed coat to inhibit water absorption by the embryo. This results in a type of seed dormancy very characteristic of legume crops (clover and alfalfa). The environment itself can break this type of seed-coat dormancy through alternating temperature extremes of freezing and thawing. The extreme heat induced by forest fires is especially effective.

Seed-coat-induced dormancy can also result from mechanical resistance due to extremely hard, rigid integuments commonly found in conifer seeds and other tree species with hard nuts.

2. Embryonic factors. The morphological state of the embryo is yet another physical factor affecting dormancy. Often the embryo is in a rudimentary stage when the seed is shed from the maternal plant; dormancy will usually cease in these plants as the embryos reach an adequate state of maturation.

Studies dealing with dormancy have resulted in searches for endogenous plant hormones which regulate the process. Studies involving dormant buds of ash (Fraxinus americana) and birch (Betula pubescens) revealed the presence of high concentrations of a growth inhibitor or dormancy-inducing and -maintaining compound. This compound was later identified as abscisic acid. As buds of these trees began to grow and elongate, the levels of abscisic acid fell appreciably, supporting the role for abscisic acid in the regulation of dormancy. Abscisic acid is also important in the regulation of seed dormancy, as exemplified by seeds of ash in which abscisic acid levels are high during the phase of growth inhibition, but then decline rapidly during stratification, resulting in germination.

In conjunction with decreased levels of abscisic acid, the endogenous supply of many growth promoters, such as gibberellins, cytokinins, and auxins, have been reported to rise during budbreak in sycamore (Acer pseudoplatanus) as well as in Douglas fir (Pseudotsuga menziesii). Levels of these dormancy-releasing compounds also correlate well with the breaking of seed dormancy. The hormonal regulation of dormancy can best be perceived as a balance between dormancy inducers or maintainers and dormancy-releasing agents. See Auxin, Plant hormones

In addition to endogenous hormones, there are a variety of compounds that can break dormancy in plant species when they are applied exogenously. Many of these substances are synthetic derivatives or analogs of naturally occurring, dormancy-releasing agents.

The physical environment exerts a marked influence on dormancy. The plant, however, needs a receptor system to perceive changes in the environment so it can translate them into physiological responses which in most cases are under hormonal control. In the case of changing day length or photoperiod, phytochrome may serve as a receptor pigment. Phytochrome essentially favors the production of either abscisic acid (short days) or gibberellic acid (long days). Stress conditions, such as limited water or nutrient availability, favor the production of abscisic acid, whereas a period of chilling often promotes synthesis of gibberellic acid and other compounds generally considered as growth promoters. See Phytochrome

The mode of action of endogenous growth regulators can only be postulated at this time. Whatever the specific mechanism, it probably involves the regulation of gene action at the level of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), which subsequently controls protein synthesis. In this framework, abscisic acid is believed to repress the functioning of nucleic acids responsible for triggering enzyme and protein synthesis needed for growth. Gibberellic acid, on the other hand, promotes synthesis of enzymes essential for germination as in the case of α-amylase production that is crucial for barley seed growth. See Bud, Nucleic acid, Plant growth, Seed