Plant hormones and water stress

In recent years, the involvement of plant hormones has become a subject of interest in plant water relations. The interest was initially stimulated by research into leaf ageing and plant senescence and the role of cytokinins in these processes. Plant water stress and some other stresses enhance senescence. They also bring about reduction in the levels of endogenous cytokinins. Exogenous cytokinins retard leaf senescence and may stimulate stomatal opening. Later, interest in the subject gained momentum from the various observations of the role of abscisic acid in stomatal opening. Abscisic acid brings about rapid stomatal closure, and its endogenous levels in leaves increase rapidly when plants are subjected to water stress or several other stresses. Hypotheses and data relevant to the possible role of hormones in plants subjected to environmental stresses are presented and discussed.