The Plant Hormone, Ethylene

Abstract

It is now well accepted that normal plant growth and development are controlled by compounds produced by the plant itself (referred to as endogenous plant hormones). These compounds are organic in nature, other than nutrients, that affect the physiological processes of growth and development in plants when present in extremely low concentrations. The well defined groups of these naturally occurring plant growth regulatory substances (PGRs), also termed as plant hormones or phytohormones, fall into five classes: auxins, gibberellins, cytokinins, ethylene (C2H4) and abscisic acid. Almost all plants can synthesize these PGRs, however, they also respond when they are exposed to exogenous sources of these substances most likely due to not having the capacity to synthesize sufficient endogenous phytohormones for optimal growth and development under sub-optimal climatic and environmental conditions. The general thinking is that exogenously supplied PGRs produce their effects by changing the endogenous level(s)/balance of the naturally occurring hormones, allowing a modification of growth and development depending on age and the physiological state of plant development, endogenous level of hormones, state of nutrition, and environmental conditions. Moreover, in plant tissues, excessive amounts of these exogenously supplied PGRs may be stored as conjugates and released when and where the plant needs.