Plant hormones and the control of physiological processes

This review examines contemporary views of the role of plant hormones in th e control of physiological processes. Past and present difficulties with no menclature encapsulate the problems inherent in using the 'classic' hormone concept in plants, with their distinctive multicellular organization. Chem ical control may be a more relevant notion, However, control may also resid e in the responding tissue via changes in sensitivity, or as combined contr ol, where response is dictated by both sensitivity and concentration. Crite ria for demonstrating these modes of action are reviewed, as well as framew orks for deciding whether hormone transport is involved. Problems of measur ing relevant hormone concentrations are discussed. Methods for measuring an d comparing tissue sensitivity to hormones are outlined and relative contro l is introduced as a means of assessing the importance of hormonal control against a background of other influences. While animals and plants appear t o have coinherited homologueous intracellular signalling systems, at the wh ole organism level modes of hormone action may diverge. It is postulated th at the synthesis-transport-action mechanism of action may be just one of se veral possible ways that phytohormones could control physiological processe s. Twelve separate roles are discussed, and it is suggested that some of th ese could operate simultaneously to the plant's advantage.