Short-term Thermal Acclimation Increases Ribulose 1, 5 Bisphosphate Carboxylase/Oxygenase Activity and Content and Enhances Heat Stress Tolerance of Photosynthesis in Cucumber

Abstract

We investigated whether short-term thermal acclimation (growth temperature of 38 ℃ for 4 days) enhanced heat stress (leaf temperature of 45 ℃ for 10 minutes) tolerance of the CO₂ fixation system of photosynthesis in cucumber leaves. We found that thermal acclimation did not influence the gross photosynthetic rate (PG) or the maximum quantum yield of photosystem II (Fv/Fm), but significantly increased the transpiration rate (TR) of the leaves. An increase in TR due to thermal acclimation increased the nitrogen and magnesium content of leaves, resulting in an increase in the content of RuBisCO. However, the activation state of RuBisCO was significantly decreased by thermal acclimation. Although the PG, Fv/Fm, and initial RuBisCO activity in non-acclimated leaves decreased significantly on heat stress, those of acclimated leaves recovered to 69, 88, and 89 % of those before heat stress, respectively. Thermal acclimation maintained a high level of content and total activity of RuBisCO after heat stress, but low levels in the ratio of initial to total activity of RuBisCO. These results suggest that the quantitative intensification of RuBisCO by thermal acclimation makes up for its decreased activation state and leads to the recovery of photosynthetic potential after inhibition from heat stress.