High temperature opens stomata and decouples net photosynthesis from stomatal conductance in Pinus taeda and Populus deltoides x nigra

Background/Question/Methods

The effect of temperature on stomatal conductance (g_­s) 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).

Results/Conclusions

High temperatures caused stomatal opening at both ambient and elevated CO₂. The magnitude of change in g_­s 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] g_s 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 g_s­ 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 g_s. Our results showed that temperature, independent of VPD, was an important factor affecting stomatal conductance and related photosynthesis.