Abscisic Acid Mediates a Divergence in the Drought Response of Two Conifers

During water stress, stomatal closure occurs as water tension and levels of abscisic acid (ABA) increase in the leaf, but the interaction between these two drivers of stomatal aperture is poorly understood. We investigate the dynamics of water potential, ABA, and stomatal conductance during the imposition of water stress on two drought-tolerant conifer species with contrasting stomatal behavior. Rapid rehydration of excised shoots was used as a means of differentiating the direct influences of ABA and water potential on stomatal closure. Pinus radiata (Pinaceae) was found to exhibit ABA-driven stomatal closure during water stress, resulting in strongly isohydric regulation of water loss. By contrast, stomatal closure in Callitris rhomboidea (Cupressaceae) was initiated by elevated foliar ABA, but sustained water stress saw a marked decline in ABA levels and a shift to water potential-driven stomatal closure. The transition from ABA to water potential as the primary driver of stomatal aperture allowed C. rhomboidea to rapidly recover gas exchange after water-stressed plants were rewatered, and was associated with a strongly anisohydric regulation of water loss. These two contrasting mechanisms of stomatal regulation function in combination with xylem vulnerability to produce highly divergent strategies of water management. Species-specific ABA dynamics are proposed as a central component of drought survival and ecology.