Plant growth is influenced by a number of factors both external and internal.
External factors are those factors present in the environment that affect the growth of the plants directly or indirectly. These factors are:
Light is necessary for the process of photosynthesis. Besides photosynthesis, light is also essential for seed germination, growth of seedling, differentiation of various tissues and organs, and reproduction.
When plants grow in dark, they become tall, yellowish and weak, and the Heredity leaves are very small.
Some plants grow in cold climate and some in hot climate. The optimum temperature required for growth of plants ranges between 28-30°C, but it may occur in the temperature range of 4-45°C. All metabolic activities of plants are directly affected by variation of temperature. A very low temperature causes injuries to the plant due to chilling and freezing, and very high temperature stops its growth.
Plant absorbs water by its roots, uses it in photosynthesis and other biochemical processes and some of it is lost through transpiration. For proper growth of plants a particular quantity of water is required. Both deficiency and excess of water retards the growth of plants.
All metabolic processes require inorganic nutrients. Plant growth is adversely affected by the deficiency of nutrients.
In addition to the external factors, there are some substances produced in the plant body itself, which affects the growth of the plant. These are called plant hormones or phytohormones or growth hormones.
A phytohormone is an organic substance produced in a small quantity in one part of plant body and capable of moving to other parts to influence the growth of that part.
The growth of the plants can also be influenced by certain synthetic chemicals resembling plant hormones both in structure and functions. These are called growth regulators. They are not produced by plants naturally.
Growth regulators are chemical substances, other than naturally produced hormones, which promote, inhibit or modify growth and development in plants. The naturally produced growth hormones are broadly grouped under five major classes. They are:
Auxin is a growth promoter, generally produced by the growing apex of stem and root of the plants. It helps in the elongation of shoot and root tips behind apical meristem.
Gibberellin or Gibberellic Acid (GA) was initially isolated from a fungus Gibberella fujikuroi. In plants, it is produced in embryos, roots, and young leaves and it enhances growth.
They were extracted from coconut milk. Cytokinins are synthesized in root apex, endosperm of seeds, and young fruits where cell division takes place continuously.
Ethylene is a gaseous hormone. It is found in ripening fruits, young flowers and young leaves.
Abscissic acid also known as Dormin is a naturally occurring growth inhibitor found in wide variety of plants. It is synthesized in leaves.