COS 263-3 - Investigating the stomatal responses to the vapor-pressure deficit (VPD) and its association with anatomical traits in grasses.

Background/Question/Methods

The maintenance of positive water status by land plants can be affected by soil and/or atmospheric water availability. The effect of water-supply drought is well-studied, but the degree and mechanisms of water-demanding drought caused by the leaf-to-air vapor-pressure deficit (VPD) is not fully-understood. While stomatal conductance (g_s) generally decreases with increasing VPD, the stomatal response is not always immediate. A stomatal “wrong-way response” (WWR)— the increase of g_s after exposure to high VPD— is often observed in angiosperms. Recent studies have begun to focus on the impact of increased VPD with warming from individuals to ecosystem levels, but grasses are often under-represented. Grasses contain a variety of unique traits, including enlarged bulliform and bundle sheath cells, and stomatal morphology, all of which may influence the relationship between g_s and changes in VPD.

In this study, the physiological responses to step-wise VPD change in two closely-related phylogenetic grass clusters were examined using LI-6400 photosynthesis system, and the VPD was altered via a dew-point generator. The response to changing VPD was determined across a range of 0.8 to 2.5 kPa with as many as 6 different steps. Anatomical traits were also measured to examine the association with VPD response.

Results/Conclusions

Stomatal sensitivity (-m) to VPD change ranged from 14.12 to 103.85 mmol ln(kPa)^-1, and the WWR, measured as a percentage change from the previous steady-state value, ranged from 0.3 to 11%. The -m value in these grasses are low, but within the range of -m values for dicot species. The WWR percentage in grasses, however, are much lower than those reported for dicot species. Interestingly, the WWR percentage remained similar across large and small step-changes in VPD. Bulliform cells covered 6.96 to 16.4% of the total cross-sectional area, and the proximity of these water reservoirs to stomata could buffer the effect of rapid VPD perturbation. These results are supported by our spatially-explicit model which examines the effect of water-storage capacitors on the short-term water status of leaves. The presence of bulliform cells often is viewed as facilitating leaf rolling during drought-stress. Our results suggest that these specialized cells may have, at the least, a secondary role in mitigating the effect of rapid VPD changes experienced by grass leaves.