The two fundamental characteristics of all living organisms are growth and development. In broad sense the term development is applied to the whole series of changes which an organism goes through during its life cycle. Plant development involves both plant growth and differentiation. Growth is permanent quantitative changes in the size of a cell, organ or a whole organism while differentiation is qualitative differences between cells, tissues and organs. From the chemical point of view, growth is the transformation of simple, inorganic compounds into new chemical entities that form the organized protoplasm of the plant. However, a mere increase in size of a plant or a plant part is not necessarily the result of growth. On the cellular basis, the growth of a plant part is the increase in size caused by cell expansion when measured at constant positive cell turgor.
Affecting Factors of the plant Growth
There are several external and internal factors which influence plant growth.
External factors of plant growth
There have many factors which affect the plant growth. The main external factors which affect growth are as follows.
Temperature affects plant growth and developmental processes in several ways. Growth increases with the increase in temperature until it reaches a maximum at optimum temperature. The rate of growth decreases with further increase in temperature. Thus each species has an optimum temperature for growth though it may vary during the plant life. The growth of a species is typically adapted to its natural temperature environment.
There is also a minimum temperature below which growth does not take place. Very low temperature may cause death of the plant. The plant growth also does not take place beyond a certain maximum temperature, which is injurious for the plant and may kill the plant also.
Light affects growth depending on its intensity, quality and periodicity. It accelerates of plant growth associated with the production of leaf primordia, development of leaves, early division and cell elongation and cell differentiation in internodes. Plants grown in total absence of light are etiolated. Weak light promotes shortening of internodes and expansion of leaves. On the other hand, high light intensity increases rate of water loss and thus reduces the rate of growth. Different wavelengths of light also affect the plant growth. However, the overall development of a plant and increase in its dry weight take place more effectively in full spectrum of visible light than in any portion thereof. Ultraviolet radiations have a impeding effect on growth and often a destructive influence on seed plant. Blue-violet light enhances internodal growth, while green light reduces the expansion of leaves on the other side.
Water content in soil
Since plant growth depends on hydrostatic turgor pressure, water deficiency will delay or completely stop it. On the other hand, an excess of water may result in an abnormal type of growth. In water-logged conditions, development of leaves is poor and differentiation of tissues is delayed.
Air pollutants like ethylene, sulphur dioxide, fluorides, etc. also influence plant growth. Automobile exhaust fumes have a damaging effect on plant growth. Similarly, the widespread use of organic pesticides, herbicides, fungicides, etc. has a harmful effect on plant growth.
Internal factors in regards of plant growth
Growth regulators are responsible for a balanced and coordinated development of the plant body. They induce or suppress genes and regulate metabolites or nutrients. Genetic factors also play an important role in growth processes. As developmental process unfolds, particular genes become involve in the synthesis of enzymes which catalyze specific biochemical processes necessary for plant growth and differentiation.
What are the regulators of plant growth?
Nutrients and hormones control the plant growth and its development. Nutrients provide the plant necessary mineral ions and organic substances such as proteins, carbohydrates, etc. However, utilization of these substances for a balanced and coordinated development of the plant body is controlled by certain chemical messengers, called plant growth regulators. Thus growth regulators are organic substances other than nutrients which in minute quantities increase, decrease or modify the physiological processes in plants.
Growth substances include both synthetic chemicals which do not occur in plants and the chemicals that are synthesized by plants themselves. The latter are called hormones. A hormone is defined as an occurring organic substance that, at low concentration, exerts a profound influence on a physiological process and is, not a part of a major metabolic pathway.
There are five classes of plant hormones, viz. auxins, gibberellins, cytokinins, abscisic acid and ethylene, which have characteristic influence on plant growth and differentiation of plant cells. They exert their influence in germination, dormancy, flowering senescence and plant growth movements. So the hormones are called the growth regulators.