SHORT-TERM EFFECTS OF ALUMINUM AT ALKALINE PH ON THE STRUCTURE AND FUNCTION OF THE PHOTOSYNTHETIC APPARATUS

A 24 h exposure of the salt-tolerant grass Thinopyrum bessarabicum (Sa vul, and Rayss) A. Love seedlings to 1 mM aluminium (Al) in nutrient s olution at pH of 9.0 resulted in a significant reduction of the biomas s. In control samples the mesophyll chloroplasts exhibited the usual l ens shape with most grana arranged in straight or slightly curving lin es, and only 6.5 % of the grana were out of order. In Al-treated plant s the mesophyll chloroplasts displayed a slightly distorted shape and distended size with most grana arranged in bow-like lines, while in th e central region of the organelle as many as 26.7 % of the grana were independent and out of order in relation to the long axis. The morphol ogical changes in the chloroplast shape and grana arrangement were pro bably due to swelling and distension of the chloroplasts in consequenc e to the altered membrane permeability. The initial in vivo chlorophyl l (Chi) fluorescence F-O, as well as the intermediate F-I and peak flu orescence F-P were increased under the Al stress: this indicated a des truction of photosystem (PS) 2 reaction centres and increased reductio n of Q(A). The (F-I-F-O)/(F-P-F-O) ratio exhibited a significant incre ase indicating higher proportion of PS2 centres unable to reduce Q(B). Changes in the chloroplast ultrastructure seemed to be the reason of photosynthetic electron transport inhibition. Yet all these changes in the photosynthetic performance and chloroplast ultrastructure were co nsidered as indirect effects of Al treatment since Al concentration in the leaves was undetectable. Disturbances in the chloroplast ultrastr ucture could be caused by a reduced uptake and/or transport of other n utrients.