13.6.1—
Introduction

Abscisic acid (ABA) is a plant hormone which now ranks in importanance with the auxins, gibberellins, cytokinins, and ethylene. Interest in the physiology and chemistry of ABA has grown greatly since the structure was established in 1965. During the 1950's and early 1960's a number of laboratories were engaged in research on growth-inhibiting substances. ABA was first isolated from cotton plants and was named abscisin II by a team from Addicott's laboratory (Ohkuma et al., 1963). During the same year Wareing's research team (Eagles & Wareing, 1963) isolated an active substance from Acer leaves which they named dormin. Abscisin II and dormin are the same substance which is now called abscisic

Figure 13.8
Structure of abscisic acid.

acid (Fig. 13.8). Plant tissues of all ages appear to synthesize and inactivate ABA. The number of various plant responses affected by ABA is very large. Generally, the physiological processes are related to senescence or abscission and growth retardation or inhibition. ABA appears to act as an abscission accelerating hormone in many fruits and leaves. Furthermore, it also tends to induce dormancy in some woody plants. ABA has been shown to move from the leaves to the apical bud to bring about a dormant condition. In potato, the

levels of inhibitors, including ABA, decrease during the quiescent period prior to renewed growth. ABA in extremely low concentration, moreover, prolongs the dormancy of excised potato buds.

Recently, ABA has been shown to be involved in the responses of many plants to stress conditions. As the ABA concentration increases in the leaves the stomata close. In this way, it appears that ABA is directly involved in the opening and closing of the stomata, thus regulating the rate of transpiration. Through this mechanism, ABA appears to protect the plant through conditions of water stress or drought.

At present the mechanism of action of ABA is not clearly understood. However, the available evidence indicates that ABA affects transcription as shown by reduced activity of chromatin-associated RNA polymerases. In other cases the mechanism of action of ABA appears to involve regulation of the translation of long-lived messenger-RNA, whilst its effects on stomata probably involve the regulation of membrane permeability.