Hormonal Activity in Detached Lettuce Leaves as Affected by Leaf Water Content

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Abstract
The interrelationship between water deficiency and hormonal make-up in plants was investigated in detached leaves of romaine lettuce (Lactuca sativa L. cv. Hazera Yellow). Water stress was imposed by desiccating the leaves for several hours in light or darkness at different air temperatures and relative humidity. In the course of desiccation, a rise in abscisic acid content and a decline in gibberellin and cytokinin activity were observed by GLC, barley endosperm bioassay and radioimmunoassay and soybean callus bioassay. Gibberellin activity began to decline in the stressed leaves before the rise in abscisic acid, the rate of this decline being positively correlated with rate of increase in leaf water saturation deficit. Recovery from water stress was affected by immersing the leaf petioles in water while exposing the blades to high relative humidity. This resulted in a decrease in leaf water saturation deficit, a reduction in abscisic acid content and an increase in gibberellin and cytokinin activity. Application of abscisic acid to the leaves caused partial stomatal closure in turgid lettuce leaves, whereas treatment with GA and kinetin of such leaves had no effect on the stomatal aperture. In desiccating leaves, GA and kinetin treatment considerably retarded stomatal closure, thus enhancing the increase in leaf water saturation deficit. The effect of desiccation in changing leaf hormonal make-up, i.e., a rapid increase in abscisic acid and a decrease in cytokinin and gibberellin activity apparently is related to a mechanism designed to curtail water loss under conditions inducing water deficiency.