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

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Stomatal conductance of two species (a broadleaf and a conifer) increased with increasing temperature. This response was independent of carbon metabolism, plant water status, or vapour pressure difference.

Abstract

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 broadleaf angiosperm, Populus deltoides x nigra (poplar), and a needle-leaf gymnosperm, Pinus taeda (loblolly pine). Experiments were conducted in growth chambers across a leaf temperature range of 19–48°C. Manipulations of temperature were done in well-watered and drought soil conditions and under ambient (400 ppm) and elevated (800 ppm) air CO ₂ concentrations. Increases in leaf temperature caused stomatal opening at both ambient and elevated [CO ₂]. The g _s increased by 42% in poplar and by 40% in loblolly pine when leaf temperature increased from 30°C to 40°C at a vapour pressure difference of 1 kPa. Stomatal limitation to photosynthesis decreased in elevated temperature in loblolly pine but not in poplar. The ratio of net photosynthesis to g _s depended on leaf temperature, especially at high temperatures. Evaporative cooling of transpiring leaves resulted in reductions in leaf temperature up to 9°C in well-watered poplar but only 1°C in drought-stressed poplar and in loblolly pine. As global mean temperatures rise and temperature extremes become more frequent and severe, understanding the effect of temperature on g _s, and modelling that relationship, will become increasingly important.