INDIRECT EFFECTS OF ALUMINUM STRESS ON THE FUNCTION OF THE PHOTOSYNTHETIC APPARATUS

Seedlings of the salt tolerant grass Thinopyrum bessarabicum were expo sed to 1 mM Al at pH 9.0 for 48 h and the photosynthetic apparatus was monitored for ultrastructural changes and perturbations in photosynth etic performance by a combination of gas-exchange measurements and in vivo chlorophyll fluorescence. Mesophyll chloroplasts displayed a dist ended shape with most grana of their central region disarranged. Alumi nium induced reductions in photosynthesis can be partly associated wit h stomatal closure, though the reduced stomatal conductance did not re duce in the same extent intercellular CO2 concentration. However, the reductions in PS II electron transport suggest an appreciable non-stom atal limitation to photosynthesis. Moreover, aluminium decreased the e fficiency of excitation energy capture by open PS II reaction centres, the photochemical quenching coefficient of PS II fluorescence and the in vivo quantum yield of PS II photochemistry, and increased the non- photochemical dissipation of excitation energy. These changes at PS II are characteristic of a saturation of photosynthetic metabolic activi ty and could be expected since Al-stress slowed down photosynthetic me tabolism and closed stomata. Since, aluminium concentration in the lea ves was undetectable, all the changes in the photosynthetic performanc e are therefore considered as indirect effects. The photosynthetic rat e was reduced at Al-treatment, but the electron transport was tightly down-regulated by carbon metabolism and there was no evidence of alter native electron sinks. Consequently, this suggests a tight linkage bet ween PS II activity and CO2 fixation under the 48 h Al-treatment. We c an suggest that Al-treatment resulted in an impaired membrane permeabi lity which affected carbon metabolism and stomatal regulation and, the reby, increased Delta pH and qE.