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In this article we will discuss about the mechanism involved in releasing bud dormancy.
Bud dormancy is unique to the perennials which remain dormant during the period of environmental stress. Most of the temperate trees require overwintering to release their buds from dormancy. Trees may require photoperiod to break their bud dormancy.
Temperature regulates bud dormancy and environmental stimuli perceived by the plant and their correlation of environmental signals with the built up of plant hormones and nucleic acids levels lead to the further control of bud dormancy.
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When a plant enters into dormancy, it does not involve only termination of active growth but represents a morphologically distinct state. Some of the important and distinct morphological states of the plant when it enters the dormant state are the suspended elongation of the axis, leaf and the enclosure of short end meristematic assemblage by distinctive scales.
The cessation of active growth is accompanied by very low rate of respiration and a low level of nucleic acids in bud scales reduce the efficiency of the respiratory functioning in the bud axis. In fact, respiration increases immediately after the removal of bud scales.
Buds at the most require as much as 14 months for the development of full leaf primordia components enclosed by scales and so when they are enclosed by scale they take several years to develop into a full leaf primordia which indicates a very slow rate of growth.
Buds enter dormancy by the repression of nucleic acid systems and their release from dormancy is accompanied by the derepression of this system. Changes in growth regulator systems gear nucleic acid systems. Bud dormancy also involves protein synthesis.
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Buds of this plant did not enter into a dormant phase when the inhibitors of nucleic acid synthesis were applied. This clearly suggests that dormancy is induced by the inducement of nucleic acid directed synthesis of proteins which in turn programme the imposement of dormancy.
Bud dormancy can be imposed by exposing the plants to short photoperiods typical of later summer or in many cases long photoperiods release them from dormancy. In most of the cases, plants shed their leaves before entering the dormant phase; so they are devoid of the most important photoperiodic perceptive organ; even then the photoperiodic response is exhibited in plants.
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In such cases the buds are capable of directly perceiving the photoperiodic stimuli as in Fagus sylvatica where the removal of bud scales considerably increased their sensitivity to photoperiods. Bud dormancy in plants is limited more by the dark period rather than the light in natural diurnal cycles.
Several examples are known where the buds respond to the release from dormancy even in short photoperiods if the light period is interrupted by the dark i.e., a night interruption treatment e.g., in Fagus. It therefore suggests that all the basic components of photoperiodic controls are present in the bud scales of certain trees and the light and dark periods in the diurnal cycle are involved in gearing dormancy.
The cold exposure of plants also releases them from dormancy and buds start sprouting. The natural exposure to low temperature is during the winter or during seasonal cool spells. The stimuli developed in response to low temperature are accumulated in the buds which normally is expressed as an effect on the quantitative increase in the growth of buds in response to the increasing chilling duration.
Cold-triggered release of bud dormancy is linked both with the metabolic turnover as well as physical actions. In some cases the dormancy can be broken by heat treatments and in some both cold as well as heat help in the dormancy breaking responses.
Extensive work has been done on the physiology of hormonal control of bud dormancy; Inhibitors play an important role in gearing bud dormancy. Potato buds remain dormant because of the presence of excessive inhibitors and the use of chemicals like ethylene, chlorohydrin which can overcome the inhibitory effect help in the release of dormancy.
The inhibitor concept in gearing dormancy has largely been supported by many examples where high accumulation of endogenous inhibitors could be recorded.
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Seasonal periodicity of the inhibitor content and their periodic distribution in different parts of the plant are reported in Acer pseudoplatanus in which inhibitors appear in the leaves and stem apices during summer which get accumulated in substantial quantity in the buds during the winter dormant season.
The release from dormancy in this plant is associated with the decline in the level of inhibitor during the growth season i.e. in the month of April. Further investigations have supported the view that all the environmental stimuli which induce dormancy do so by increasing the level of growth inhibitors.
Recently abscisic acid has been shown to be the principal inhibitor present in the dormant buds but there are some dormant species which do not accumulate abscisic acid during dormancy but do accumulate some other types of inhibitors like phenanthrene type of inhibitor in dormant yam bulbils, inhibitor β in dormant plants of Betula lutea, etc.
Gibberellins are believed to overcome the effect of inhibitor and release the plant from dormancy as is shown in dormant potatoes or woody plants.
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Besides GA, other growth hormones like cytokinins and ethylene are also involved in releasing dormancy in buds, corms, tubers and woody plants.
Besides the involvement of growth hormones in releasing the buds from dormancy, the evidences are known where the bud scales interfere with the growth of buds. It has been observed that bud scales compete with the meristematic tissues of the bud for their respiratory activity. Bud scales have relatively very high rate of respiration as compared to the enclosed meristematic tissues.
If bud scales are removed, the enclosed meristems record tremendous increase in their respiratory activity. This led to the hypothesis that the bud scales might participate in inducing dormancy by imposing respiratory limits on the enclosed meristems and that is why some times simply removing the scales releases dormancy in buds.
Besides this, scales are also known to have large amount of inhibitors and so the release of dormancy of buds by the removal of respiratory inhibition of enclosed meristems or the removal of inhibitors or the both from the scales, seems to be the possible explanation.
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Abbott (1970) suggested that bud scales act as the mobilizing centres and, therefore, their presence may deprive the enclosed meristems by getting the substrates for growth and, therefore, the senescence of bud scales remove the inhibition of mobilzation of the substrates to the enclosed meristem and so the dormancy is released. Much of the informations are still needed to ascertain the cause of bud dormancy.
