Stomatal Responses to Abscisic Acid in Natural Environments

Abstract

The role of abscisic acid (ABA) as a stress hormone is now well established. Its synthesis and transport to the sites of action are stimulated under stress conditions. In leaves, ABA induces stomatal closure, thereby reducing transpiration and improving water relations. Ion- and water-uptake and root growth are also affected by ABA. These phenomena have all been studied intensively under laboratory and glasshouse conditions, but confirmation of the findings was not possible until recently because of analytical problems. With the availability of sensitive ABA assays we have investigated the role of ABA under natural conditions in almond trees and several species from the Negev desert (Israel), Ceanothus shrubs from the Chaparall in South California and Prunus and Cornus trees from a steep, dry and sun exposed habitat in the Main valley north of Würzburg. In all cases we investigated the relations between xylem sap ABA concentration, water potential and leaf conductance (g). ABA rises slightly with decreasing water potential until a threshold is reached, but thereafter a dramatic increase in xylem sap ABA can be observed. The threshold depends on both species and conditions. In some cases clear relations can only be observed between predawn water potential (a measure of soil water potential) and xylem sap ABA. When xylem sap ABA is plotted against g, in most cases there is a highly sensitive phase where tiny increases in ABA are sufficient to decrease g dramatically. ABA can be increased above this range in many cases, except in the Prunus and Cornus trees from the Main valley. Similar relations have been observed in stressed field-grown maize plants. The physiological significance of this excess ABA is not yet understood. Elevated CO₂ is an important environmental factor causing stomatal closure, and in laboratory experiments CO₂ has been shown to enhance guard cell sensitivity to ABA and vice versa. We injected ABA into the veins of cotton leaves grown in open-top chambers at high CO₂ concentration (650 μl 1^-1) under highly stressed field conditions in Arizona and measured the stomatal conductance of the treated leaves. The results confirmed the earlier observations from laboratory experiments with excised leaves and epidermal strips of interactions between CO₂ and ABA. The importance of varying guard cell sensitivity to ABA under field conditions has also been reported for Arbutus grown in a natural Portuguese macchia, nutrient deficient cotton and stressed field-grown maize plants. Our investigations indicate that ABA also has an important stress-physiological role as a root-to-shoot signal influencing stomatal responses to environmental perturbations under natural conditions.