Acclimation of isoprene emission and photosynthesis to growth temperature in hybrid aspen: Resolving structural and physiological controls

Abstract

Acclimation of foliage to growth temperature involves both structural and physiological modifications, but the relative importance of these two mechanisms of acclimation is poorly known, especially for isoprene emission responses. We grew hybrid aspen (Populus tremula x P.tremuloides) under control (day/night temperature of 25/20°C) and high temperature conditions (35/27°C) to gain insight into the structural and physiological acclimation controls. Growth at high temperature resulted in larger and thinner leaves with smaller and more densely packed chloroplasts and with lower leaf dry mass per area (MA). High growth temperature also led to lower photosynthetic and respiration rates, isoprene emission rate and leaf pigment content and isoprene substrate dimethylallyl diphosphate pool size per unit area, but to greater stomatal conductance. However, all physiological characteristics were similar when expressed per unit dry mass, indicating that the area-based differences were primarily driven by MA. Acclimation to high temperature further increased heat stability of photosynthesis and increased activation energies for isoprene emission and isoprene synthase rate constant. This study demonstrates that temperature acclimation of photosynthetic and isoprene emission characteristics per unit leaf area were primarily driven by structural modifications, and we argue that future studies investigating acclimation to growth temperature must consider structural modifications. High growth temperature reduced area-based photosynthetic and isoprene emission characteristics in hybrid aspen, and these effects were primarily driven by structural modifications. Acclimation to high temperature also enhanced stomatal conductance, improved heat resistance of photosynthetic apparatus and increased the activation energy of isoprene emission resulting in a greater fraction of photosynthetic carbon emitted as isoprene at higher temperatures. These results emphasize the important roles of both leaf structural and physiological changes in high-temperature acclimation of photosynthesis and isoprene emission.