Role of Chemical Factors in Controlling Apical Dominance
There were indications about the existence of plant hormones in the last part of 19th century. The plant hormones had been isolated and characterized in the third decade of this century. By the middle of this century we became aware of different types of plant hormones like auxins, gibberellins, cytokinins, abscisic acid and ethylene. The first four are chemical substances that influence growth in solutions and interestingly ethylene is a gas and perhaps the only hormone of its type.
The first four plant hormones are isolated in crystal form from plant sources and their chemical structure is well characterized. Unlike the case of animal hormones, plant hormones exhibit a broad spectrum of effects on growth and development. Quite often same growth process may be regulated by more than one plant hormone. This applies to apical dominance also. Prior to the discovery of plant hormones and their role in apical dominance it was thought that apical dominance is a kind of struggle for existence and competition between apical bud and lateral buds for nutrients from the root and leaves.