Photosynthesis in a desert tree is driven by the highest light and temperature

Among living tree species, Acacia raddiana (Savi) and Acacia tortilis (Forssk), species of the legume family, populate some of the hottest and driest places on earth. Our research investigates the physiological processes underlying the unique survival of these trees in their extreme environmental conditions. We measured Acacia trees in their natural habitat once a month for two years to unravel the photosynthesis dynamics and water relations. Leaf gas exchange and leaf water potential were measured, as well as atmospheric and soil parameters. Daily and annual gas-exchange curves showed higher carbon assimilation during noon and in summer, when temperature and radiation were maximal (44°C, 2000 µmol m^-2 s^-1), and the air was dry (21% RH). Additionally, we found that the maximum rate of carbon assimilation was at PAR (photosynthetic active radiation) of 3000 µmol m^-2 s^-1. Our results suggest that water did not drive net carbon assimilation but rather light and temperature, which are already close to their maximum in our hyper-arid ecosystem.