The relationship between leaf water potential ψ leaf and the levels of abscisic acid and ethylene in excised wheat leaves.

The amount of diffusible ethylene from excised wheat leaves (Triticum aestivum L. cv. Eclipse) increased when they were subjected to water stress. The quantity of ethylene produced was related to the severity of the stress, reaching a maximum at a leaf water potential ψ leaf of approximately-12 bars. Irrespective of the severity of the stress, the maximum rate of ethylene production usually occurred between 135-270 min after applying the stress and then the rate declined. Part of the decline may have been due to an oxygen deficiency in the leaf chambers. In excised water-stressed leaves there was a sigmoid relationship between increasing ethylene and abscisic acid (ABA) levels and decreasing leaf water potential values. The two curves were displaced from each other by approximately 1 bar, with ethylene evolution leading that of ABA accumulation. The maximum rate of increase in ethylene occurred between-8 and-9 bars and for ABA between-9 and-10 bars. A significant increase in the levels of these two plant growth regulators was found when the ψ leaf decreased outside the normal diurnal ψ leaf range by 1 bar for ethylene and 2 bars for ABA. Because of the sigmoid nature of the curves there was no distinct threshold ψ leaf value triggering-off an increase in ethylene or ABA, but with ABA the curve became very steep at a ψ leaf value of-9.3 bars and this could be looked upon as a kind of "threshold" value.It seems unlikely that the stress-induced ethylene evolution in excised wheat leaves stimulated the accumulation of ABA, because when the leaves were subjected to a substantial water stress (e.g. ψ leaf bars) ABA increased immediately and at a faster rate than ethylene.