Light was found to inhibit substantially (i.e. up to 88%) the production of
ethylene induced by
water stress in excised wheat leaves and from the shoots of intact plants. The relatively small amounts of
ethylene emanating fron non-stressed leaves were also inhibited by light but to a smaller degree (i.e. up to 61%). In water-stressed leaves the degree of light inhibition of
ethylene production was shown to be related to the age of the leaves; the amounts of
ethylene diffusing from young leaves (i.e. 6-days old) was inhibited 52% by light whereas in older leaves (i.e. 9-days old) it was inhibited by 85%. Previous studies [Wright (1979) Planta 144, 179-188 and (1980) Planta 148, 381-388] had shown that application of
6-benzyladenine (BA) to leaves a day before wilting, greatly increases the amount of
ethylene diffusing from the leaves following wilting (e.g. 8-fold), and to smaller degrees do applications of
indole-3-acetic acid (IAA) and
gibberellic acid (GA3). On the other hand
abscisic acid (ABA) treatment reduces the amount of
ethylene produced. In these earlier experiments the
ethylene was collected from leaves held under dark or near-dark conditions, so in the present study the activities of these growth regulators (10(-4) mol l(-1) solutions) under dark and light conditions were compared. It was found that they maintained the same relative activities on
ethylene emanation (i.e. BA>IAA>GA3>water controls>ABA) under both light and dark conditions. However, because of the inhibitory effect of light, the absolute amounts of
ethylene produced from all treatments were always much higher in the dark than in the light (usually about a 6-fold difference). An interesting effect of light treatment on
ethylene biosynthesis was found when water-stressed leaves were kept in dark chambers for 41/2 h and then transferred to light. Quite unexpectedly, instead of the rate of
ethylene production falling immediately, it continued to be produced at the dark rate (i.e. no light inhibition!) for over 2 h before the rate began to decline, and for a much longer period (i.e. in excess of 41/2 h) if the leaves had previously been sprayed with BA. Predictably, leaves placed in the light (i.e. in leaf chambers) and then transferred to darkness, immediately or very soon produced
ethylene at the dark rate. One explanation of these results, which is discussed, would be that the biosynthesis of an
ethylene precursor requires an obligatory dark stage. The possible implications of these studies to a survival role of
ethylene in plants during periods of
water stress is discussed.