Growth hormones in plants are the chemicals that determine and regulate growth in plants. Plant growth hormones are defined as “Any of various hormones produced by plants that control or regulate germination, growth, metabolism, or other physiological activities.” These hormones are also known as phytohormones and are present in every cell of a plant. These plant growth substances are the determining factors in the appearance of plants. They determine the different features of a plant like height, color, shape, sex, survival and endurance. The hormones determine the normal or abnormal growth of plants. In plants, growth hormones determine every incident pertaining to the plant: when it flowers, when leaves form, the growth of stems and shoots, the development of fruits, the growth and maturing of the fruits, shedding leaves and ripening of fruits. How a plant responds to the different changes in climate and weather is determined by the plant growth hormones.
There are five major classes of plant growth hormones. These are:
Abscisic acid is found near leaves, stems and unripe fruit. It is a growth inhibitor responsible for stimulation of closing of stomata, inhibition of shoot growth and inducing seeds for synthesizing storage of proteins.
Auxins are growth stimulators found in the seed embryo, young leaves and apical buds meristem. Auxins are responsible for the stimulation of cell elongation, root initiation on stem cuttings and stimulation of flower parts.
Cytokinins are synthesized in roots and transported to other parts of the plant. They are growth stimulators responsible for leaf cell enlargement, stimulation of leaf expansion and shoot initiation.
Ethylene, a gaseous hydrocarbon, is a growth inhibitor responsible for release of dormancy state, leaf and fruit abscission, shoot and root growth. It is present in the nodes of stems, ripening fruits and senescent leaves and flowers.
Gibberellins are growth stimulators which are found in the meristems of apical buds, roots, young leaves and embryo. Gibberellins are responsible for the stimulation of stem elongation and can end seed dormancy in plants that need light to induce germination.
The different growth hormones work together to produce different manifestations of plants. Cytokinins and auxins are referred to as the two most important growth hormones of plants. Cytokinins and auxins are both necessary for the growth of shoots and roots. When cytokinins are high and auxin is low in a plant, shoots develop. If auxin is high and cytokinins are low, roots are formed. Auxins retard lateral bud development and cytokinins promote lateral bud development. Where auxins stimulate cell elongation; gibberellins stimulate both cell division and elongation. Concentrations of gibberellins and ethylene determine the sex of flowers on different plants.
Apart from the growth hormones in plants which are naturally produced within the plant, there are synthetic growth hormones for plants commonly referred to as Plant Growth Regulators. These man-made chemicals are mainly used to manipulate and regulate the growth and production of cultivated plants like fruits and vegetables on farms. They are very useful in tissue culture, cuttings, grafting and plant propagation; which is the process of creating new plants from different sources like seeds, cuttings and bulbs. They are also used to control the growth of weeds on farms.
Plant growth hormones determine how different plants grow and produce and exist in whatever environmental conditions they are in. Each different plant has the capability to produce the hormones in accordance to its particular needs.
