Forms Of Dormancy

A number of terms have been used to describe types of dormancy (e.g., "quiescence," "rest"), what controls dormancy (e.g., "mechani cal dormancy," "physiological dormancy," "correlative inhibition"), and the duration of dormancy (e.g., "shallow dormancy," "deep dormancy"). These terms indicate the complexity of dormancy, but they can be confusing. Universal terminology, introduced by G. A. Lang and others (1985, 1987), provides a good model for explaining and understanding dormancy (Table D2.1). Three terms are used to describe the fundamental forms of dormancy. "Ecodormancy" is dormancy of growing tissues and organs imposed by factors in the surrounding environment. "Paradormancy" is used to explain dormancy imposed by growth factors outside of a tissue or organ (i.e., the control one part of a plant exerts over the growth of another part of the plant). "Endodormancy" is dormancy imposed by growth factors from inside a dormant plant structure.

Ecodormancy occurs when environmental conditions are not suitable for growth. All meristems and meristematic organs express this form of dormancy. Thermal ecodormancy occurs when the temperature for growth is too low or too high. Hydrational ecodormancy occurs when available soil moisture limits growth or flooding causes growth to cease. Atmospheric ecodormancy and photoecodormancy occur when a gas (e.g., oxygen) and light, respectively, are limiting. Once an optimum growth environment returns, growth resumes.

Paradormancy, also referred to as correlative dormancy, is regulated by physiological factors outside a tissue or organ. The best exTABLE D2.1. Definitions and examples of three fundamental forms of plant dormancy

Type of Dormancy

Definition

Examples

Ecodormancy

Paradormancy

Endodormancy

Regulated by environmental Temperature extremes factors Water stress

Nutrient deficiencies

Regulated by physiological factors outside the affected structure

Regulated by physiological factors inside the affected structure

Apical dominance Protective seed coverings

Chilling requirement Physiological seed dormancy Photoperiodic response

Source: Modified from Lang et al., 1985.

ample in fruit trees is the phenomenon known as apical dominance, in which an actively growing apex suppresses growth of subtending buds on the same shoot. Following removal of the apical meristem by pruning or pinching, paradormancy ends, allowing lateral and axillary buds to begin to grow.

Endodormancy is one of the more complex forms of dormancy. In response to changing environmental conditions (e.g., shorter days, lower average daily temperature, lower internal cell moisture content), a bud or seed enters a state of prolonged dormancy. Unlike eco-dormancy, growth will not automatically resume when there is an appropriate growing environment. Thus, it is often referred to as deep dormancy or rest. Endodormancy is regulated by physiological factors inside a plant structure. For instance, internal biological mechanisms sense passage of time based on temperature. After exposure to specific cool temperatures for specific periods of time, endodorman-cy is eliminated, or rest is satisfied, and growth may once again resume. During endodormancy, plant parts acclimate to the external changing environment and may gain or lose "hardiness," depending upon both the external conditions and the state of dormancy.

0 0

Post a comment