Growth in plants depends on various internal and external factors. Followings are some of the important ones.
1. Nutrient supply
Green plants require several mineral ions and other essential elements for normal growth and development. These nutrients come from the soil for manufacturing of food. Growth ceases when the nutrient supply becomes limiting.
Water is a medium for all chemical reactions and physiological processes. Water is essential for photosynthesis. Early growth which is due to the turgid conditions of the cells requires plenty of water. Under conditions of water scarcity, plant growth is severely impaired. On the other hand, excess of water in the soil may cause waterlogging, resulting anaerobic conditions in the roots whose growth is severely affected.
Oxygen is necessary for cellular respiration in the plants. Food materials are broken down in the process of respiration and energy is released in the form of ATP molecules. This is the utilizable form of energy for the living cells. This energy is used for various activities of the cell and directly take part in the growth processes.
Plants are influenced by variation of temperature of the soil in which they grow. Most plant species prefer temperature in the range of 20°C to 40°C.
Light influences many physiological and growth processes of plants. Plants which can grow well under bright, direct sunlight and grow poorly in shady conditions are called photophilic plants. On the other hand, the plants capable of growing best under low light conditions arc photophobic plants.
Light has great morphological effect on the leaves and stems of the plants. Plants growing. in darkness are characterzied by long, succulent weak stems. The leaves become underdeveloped, pale, yellowish, chorophyll deficient.
This is called etiolating variations in the intensity, quality and duration of light variously affect seed germination and flowering in different plants. The effect of light on reproductive growth is called Photoperiodism.
6. Growth regulators
These are, otherwise, known as hormones. These have pronounced effect on the growth of the plants.
B. Growth regulators (Hormones)
Growth regulator or hormone is an organic substance produced within a plant and which at low concentrations promote, inhibit or modify the growth process. The effect is usually observed at a site other than its place of origin.
The occurrence of plant hormones or phytohormones was first suggested by Julia Von Sachs. Besides, the naturally occuring hormones, several synthetic compounds have also phytohormone like activities. All these are categorised under growth regulators.
Different types of phytohormones are auxins, gibberellins and cytokinins which promote growth. Abscissic acid and ethylene either inhibit or suppress growth. I. Auxins – nature and physiological effects
Auxins are one of the most important groups of plant hormones because of their variety of roles on the plants. The famous biologist Charles Darwin (1880) suggested the probability of some growth substance at the tips of the plants. F.W. Went (1928) first collected auxin from the coleoptile tip of Oat (Avena sativa).
He proposed this growth substance as auxin. Commonly occurring natural auxin is indole 3 – acetic acid (IAA). Several synthetic chemical substances like Napthyl acetic acid (XAA), Indolebutyric acid (IBA), Phenyl acetic acid (PAA) and 2, 4, dichlorophenoxy acetic acid (2,4D) have auxin like activities.
Natural auxin (IAA) is synthesized in the plants from an amino acid called tryptophan. It is produced in the apical meristems and is translocated to other parts. Its movement is polar and occurs only away from the apex to other zones even if the position of the organ is inverted.