COMPETITION BETWEEN THE PHOTOSYNTHETIC AND THE (CHLORO)RESPIRATORY ELECTRON-TRANSPORT CHAINS IN CYANOBACTERIA, GREEN-ALGAE AND HIGHER-PLANTS - EFFECT OF HEAT-STRESS

By measuring the effect of cyanide on the flash-induced redox reaction s of the cytochrome (cyt) b(6)/f complex we carried out a comparative study in order to characterize the interaction between the photosynthe tic and the respiratory electron transport systems in cyanobacterial ( Synechococcus sp. PCC 6301) and green algal (Chlamydomonas reinhardtii ) cells, and in tobacco (Nicotiana tabacum L. cv. Petit Havana SR1) pr otoplasts. We found that the addition of 1 mM KCN resulted in a signif icant acceleration of the rereduction-rate of cyt f(+). This enhanceme nt of the activity of the cyt b(6)/f complex apparently occurred with the same mechanism in prokaryotes and eukaryotes, and its dependence o n the concentration of KCN in eukaryotes ruled out an origin in mitore spiration, superoxide dismutase and plastocyanin, strongly suggesting that a cyanide-sensitive terminal oxidase, a putative component of chl ororespiration, competes with photosystem 1 (PS1) for electrons from t he plastoquionone (PQ) pool. Concerning the physiological role of the competition between the (chloro)respiratory and the photosynthetic ele ctron transport systems, our data obtained with cyanobacterial and alg al cells incubated at elevated temperatures (30-50 degrees C) showed t hat the respiratory control over photosynthesis became significant in cells exposed to heat-stress.