Enhanced sensitivity of the photosynthetic apparatus to heat stress in digalactosyl-diacylglycerol deficient Arabidopsis

Abstract

The effect of short-term heat stress on photosynthetic electron transport (photosystems I and II) in chloroplasts of Arabidopsis thaliana deficient in digalactosyl diacylglycerol was investigated. PSII electron transport was characterized by chlorophyll fluorescence rise kinetics while the oxidation–reduction reactions of PSI complexes were studied using leaf-absorbance changes. In wild type plants exposed to temperatures above 36°C, a progressive damping of the fluorescence rise kinetics, and thus of Fv/Fm and Fv/F0, revealed the marked decline in the quantum yield of PSII. This temperature-dependent inactivation of water photolysis and light-induced plastoquinone reduction was more pronounced in Arabidopsis mutants, dgd1-2 and dgd1-3. At light intensities above 600μmolm−2s−1 under normal temperatures for growth, Arabidopsis dgd1-3 was also deficient in its capacity to quench the absorbed light energy nonphotochemically (NPQ) if compared to the NPQ efficiencies of the other two genotypes. The measurements of leaf absorbance changes at 820nm illustrated the temperature-dependent decline in PSI activity. However, the magnitudes of PSI inhibition were only half of the PSII inhibition, regardless of the plants used. These measurements also showed the suppression of cyclic electron flow around PSI which was temperature-dependent. Leaf exposure above 40°C resulted in the diversion of electron flow through Mehler reaction in which molecular oxygen acts as terminal acceptor. Notably, in the absence of fully operational PSII and cyclic electron transport around PSI, the decay of the leaf absorption at 820nm after the cessation of far-red illumination illustrated an enhanced charge recombination in PSI complexes.