High temperature opens stomata and decouples net photosynthesis from stomatal conductance in Pinus taeda and Populus deltoides x nigra
The effect of temperature on stomatal conductance (gs) and corresponding gas exchange parameters was studied in two tree species with contrasting leaf anatomy and ecophysiology – a narrow leaved gymnosperm, Pinus taeda, and a broad leaved angiosperm, Populus deltoides x nigra. Experiments were conducted in growth chambers across a temperature range of 10 to 49 °C. Manipulations of temperature were done in well-watered and droughted conditions, and under ambient and elevated air CO2 concentrations ([CO2], 400 and 800 ppm, respectively).
High temperatures caused stomatal opening at both ambient and elevated CO2. The magnitude of change in gs with temperature was larger in Pinus taeda than in Populus deltoides x nigra. Stomatal conductance in Pinus taeda increased by 106 and 160 % and in P. deltoides x nigra by 20 and 81 % when air temperature increased from 30 °C to 40 °C and from 30 °C to 49 °C, respectively, at a vapor pressure deficit (VPD) of 3.5 kPa and ambient [CO2]. At the elevated [CO2] gs increased with the temperature too but the conductances were in average by 12 and 21 % lower in Pinus taeda and Populus deltoides x nigra, respectively. Evaporative cooling resulted in reduction in leaf temperature up to 9 °C in Populus deltoides x nigra and up to 2 °C in Pinus taeda. The lower leaf temperature had a positive effect on net photosynthesis at extreme temperatures. Stomatal limitation to photosynthesis decreased in elevated temperature in Pinus taeda but not in the Populus deltoides X nigra. Consequently, intercellular CO2 concentration varied greatly with temperature in P. taeda, but not P. deltoides x nigra. Net photosynthesis was impaired from gs in temperatures 40 °C and higher. The ratio of photosynthesis to stomatal conductance declined at higher temperatures in both species, highlighting the inability of existing photosynthetic models to correctly predict gs. Our results showed that temperature, independent of VPD, was an important factor affecting stomatal conductance and related photosynthesis.