Abscisic acid mediates a divergence in the drought response of two conifers

Abstract

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.

Figure 1.

Trajectories of g_s to declining Ψ_l during gradually imposed water stress (red circles) in seven individuals of C. rhomboidea (top) and P. radiata (bottom). Recovery trajectories are also plotted for five individuals of each species that were rewatered after more than 30 d without water (blue circles). Arrows indicate the progression in time through water stress and rewatering.

Figure 2.

The trajectories of foliar ABA level over increasingly negative Ψ_l during drought (red circles) and increasing Ψ_l following rehydration and recovery (blue circles) in five individuals. The inserts depict the relationship of foliar ABA level and g_s in the same individuals during drought and recovery.

Figure 3.

Pooled data (n = 5, ± sd) of the recovery of whole-plant transpiration of droughted C. rhomboidea (black circles) and P. radiata (white squares) individuals following rewatering after 33 d of drought stress. The dotted line represents the day at which all individuals from both species were fully rehydrated; best-fit regressions were sigmoidal for C. rhomboidea (r² = 0.88) and linear for P. radiata (r² = 0.93).

Figure 4.

A, Example kinetics of g_s in two droughted C. rhomboidea branches with closed stomata that were rehydrated, one with low levels of endogenous foliar ABA and the other with very high levels of endogenous foliar ABA (as noted in nanograms per gram), illustrating the dependence of stomatal reopening to maximum conductance (gray line) on the level of foliar ABA. B, The relationship between the percentage recovery of g_s and foliar ABA level in branches that were dried on the bench for varying periods of time then rehydrated in C. rhomboidea (circles) and P. radiata (squares). Each point represents a different branch dehydrated for a different time (between 10 min and 24 h), such that a large range of water potentials and ABA levels were produced prior to rehydration.

Figure 5.

The relationship between the percentage recovery of g_s in rehydrated leaves from branches taken from plants over an extended period of drought stress (indicated in days since the withholding of water) in C. rhomboidea (black circles) and P. radiata (white squares); lines represent means of data from a single branch taken each from three individuals and rehydrated at each time point (± sd).

Figure 6.

The mean percentage contribution (n = 5, ± se) to stomatal closure by foliar ABA level (black symbols) and Ψ_l (white symbols) over a prolonged period of drought stress in C. rhomboidea. The dotted line represents the day when Ψ_l contributed more toward stomatal closure than foliar ABA level in C. rhomboidea; mean Ψ_l on that day is indicated. In P. radiata (data not shown), high levels of ABA at the point of stomatal closure ensured that once stomata closed, they remained closed by the effects of ABA alone.