Limited water availability did not protect poplar saplings from water use efficiency reduction under elevated ozone

Abstract

Elevated ground-level ozone (O3) concentrations decrease photosynthetic biochemistry more than stomatal conductance (gs), leading to an overall reduction in leaf-scale water use efficiency (WUE). Global warming is expected to lead to more severe and frequent droughts resulting in stomatal closure, increased WUE, and potentially in reduced plant O3 uptake and damage. It is currently unclear how the physiological responses to O3 and water limitation interact to affect overall leaf WUE and how these WUE responses might affect ecosystem productivity. In this study, we used open top chambers to expose O3-sensitive poplar saplings to elevated O3 (E-O3) and limited water availability to explore the individual and interactive effects of these stressors on WUE. We found that leaf-scale intrinsic water-use efficiency based on gas exchange measurements (iWUEge) decreased under E-O3 due to significantly reduced photosynthetic capacity, mesophyll conductance and apparent quantum yield, while gs was not affected by the treatment. Leaf-scale iWUEge and intrinsic WUE based on isotope measurements (iWUEiso) increased in the plants receiving less water due to higher photosynthetic capacities and lower transpiration rates indicated by δ18O measurements. The overall plant growth (total number of leaves, height, stem diameter and projected area of individual leaf) was significantly reduced under low water supply. Elevated O3 resulted in significant leaf senescence, but had no other significant main effect on morphological variables. Reduced water availability prevented O3-induced decreases in leaf mass per area and increases in leaf loss. No other significant O3-water availability interactions were detected in the measured physiological or morphological variables. Our results thus suggest that drought conditions will not prevent O3 damage to photosynthetic biochemistry in poplar and that high O3 concentrations will decrease leaf-scale iWUEge regardless of future changes in plant water availability.