Influence of Drought on Mesophyll Resistance to CO2 Diffusion and its Impact on Water-Use Efficiency in Trees

Abstract:

The resistance to CO₂ diffusion inside leaves (mesophyll resistance; r_m) during photosynthesis is often comparable in magnitude to stomatal diffusion resistance, and varies among species and across environmental conditions. Consequently, photosynthesis is strongly limited by r_m at low internal CO₂ partial pressures, such that its variation may determine patterns of leaf water-use efficiency (WUE). Reduction in stomatal conductance with drought typically increases WUE, but also decreases photosynthesis. In theory, the decrease in photosynthesis could be countered by reduction in r_m while maintaining high WUE. It is still uncertain how drought-related changes in r_m affect short- and long-term WUE strategies of different tree species.

We conducted field observations of instantaneous WUE and ¹³C discrimination in two dominant conifer species (Pinus contorta and Picea engelmannii) in SE Wyoming over the seasonal dry-down period in the summer of 2015. r_m was examined by on-line ¹³C discrimination using isotope laser spectroscopy. Controlled environment studies on three conifer species (P. contorta, P. engelmannii, and Abies lasiocarpa) and one angiosperm (Populus tremuloides) are in progress. We hypothesize that the plasticity of r_m in response to drought accounts for significant adjustments in photosynthetic capacity and WUE. Needle leaf conifers are known to have relatively high r_m, and we expect them to show greater improvements in photosynthesis and WUE when r_m is decreased compared to angiosperm tree species.