Effects of Plant Growth Regulators on Development
Like animals plants require input of energy from the outside environment to sustain growth and development. But unlike animals, plants can draw energy directly from sunlight and by using water and a few essential mineral nutrients from soil, can synthesize the molecules that constitute their structure and function (autotrophic). There are parasites, semi-parasites, saprophytes and insectivorous plants which are exceptional to this generation There are certain physical factors like light, temperature, gravity and touch which trigger one or other physiological processes like flowering, germination, chlorophyll development etc. or modify the pattern and direction of growth e.g. phototropism, geotropism and hydrotropism responses. There are also certain chemical substances endogenously present in plants which at a very low concentration initiate a particular physiological process like enhancement of production of a-amylase in barley seed endosperm or acceleration of a specific m-RNA or initiation of a major physiological process-involving number of specific steps like germination, flowering, senescence, etc. There are just five plant growth regulators in plants.
They are auxins, gibberellins, abscisic acid, cytokinins and ethylene. They are also called as plant hormones. But, the term hormone is in fact borrowed from animal, as hormones were first discovered and ' well characterized in animal systems. Unlike in the case of animals, each one of these five groups of 'plant hormones' can influence/ initiate a number of physiological processes. The same hormone can promote growth at one concentration and retard growth at some other concentration in the case of same organ like a shoot, bud, root etc. Same physiological process like germination can be influenced positively by two different types of hormones (gibberellins and cytokinins). Plant physiologists have recommended the use of the phrase "plant growth regulators" (PGRs) to these compounds.These are naturally occurring as well as synthetic compounds which can be included under PGRs. Some scientists call the former as hormones. In recent times there is much discussion whether the term hormone should be used for plants as plant and animal hormones are markedly different in their origin, structure and mode of action.