Contrasting responses of two C4 desert shrubs to drought but consistent decoupling of photosynthesis and stomatal conductance at high temperature

How desert shrubs respond to drought and heat remains poorly understood. We investigated how drought affected the morphology and physiology of two C4 species with different root architecture, Calligonum arborescens and Haloxylon ammodendron, in the Taklamakan Desert. We measured leaf traits, chlorophyll fluorescence and temperature responses of photosynthesis after-3 years of experimental drought and asked: (1) Do both species adjust similarly to drought? (2) Does transpirational cooling increase at high temperature, and if so, how is this affected by drought? Drought plants of H. ammodendron had shorter and thinner leaves than controls, and lower chlorophyll content, photochemical efficiency, and electron transport rate. In contrast, C. arborescens leaves became shorter and thicker. Drought reduced maximum photosynthesis by 63 % and 21 % in H. ammodendron and C. arborescens, respectively. The optimum temperature did not change significantly in H. ammodendron and decreased by-2 degrees C in C. arborescens. Surprisingly, above 40-45 degrees C, stomatal conductance (gsw) and transpiration increased-even under drought stomata partially reopened. This consistent uncoupling of photosynthesis and gsw suggests that widely-implemented stomatal optimization models may poorly reflect high-temperature behaviors in dryland ecosystems. Our study established the basis for predicting the eco-physiological responses of C4 species in hyper-arid ecosystems to climate change.