Relationships between Leaf Water Status, Abscisic Acid Levels, and Stomatal Resistance in Maize and Sorghum

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Abstract
A new method for abscisic acid extraction and purification was developed to handle large numbers of small (about 125 milligrams fresh weight) samples of leaf discs. This method enabled short term changes in abscisic acid to be followed in single leaves. Water potentials, stomatal diffusion resistance, and abscisic acid levels were measured in the leaves of maize (Zea mays L. var. Wisconsin 575) and sorghum (Sorghum bicolor, hybrid NK 145) plants subjected to a drought-recovery cycle under controlled environmental conditions. The levels of abscisic acid began to rise, and the stomata closed, over a narrow range of water potential (−8 to −10 bars) in both species. Abscisic acid levels continued to rise after the stomata closed. The maximum amount of abscisic acid extracted from maize leaves was about twice that from sorghum, but this represented a similar proportional increase over the control level. In excised leaves little or no change in abscisic acid levels was found within 60 to 120 minutes after stress was applied (phase I). Subsequently abscisic acid levels rose rapidly to about 20 times the prestress level (phase II). Stomatal closure always preceded the phase II increase in abscisic acid. The aftereffects of stress differed in the two species. In sorghum control levels of both abscisic acid and stomatal resistance were regained within 24 hours of rewatering. In maize abscisic acid levels also fell rapidly, but regained control levels only after 48 hours. The resumption of normal stomatal functioning occurred after a further 24 hours. Impaired stomatal functioning after stress does not appear to be associated with high residual levels of abscisic acid in the leaf.