CHARACTERIZATION OF THERMAL-DAMAGE TO THE PHOTOSYNTHETIC ELECTRON-TRANSPORT SYSTEM IN POTATO LEAVES

Fluorescence, absorbance and photoacoustic methods were used to examin e in situ various functional aspects of the photochemical apparatus of chloroplasts in potato leaves (So[anum tuberosum L.) briefly pre-expo sed in the dark to a wide range of elevated temperatures (32 degrees C -45 degrees C). Measurements of the Emerson enhancement of photosynthe tic O-2 evolution and the flash-induced reduction of the oxidized reac tion center pigment of photosystem I demonstrated that selective denat uration of photosystem II in potato leaves started at a 'low' temperat ure of 32 degrees C. Analysis of the characteristics of chlorophyll fl uorescence induction in sub- and super-saturating light revealed two t argets of mild heat stress: (i) an irreversible inhibition of electron donation to PSII and (ii) a reversible reduction of excitation energy trapping by the PSII reaction centers, with the former effect being i dentified as the major determinant of the loss of photosynthesis. A se cond phase of thermal denaturation of photosystem II occurred at tempe ratures higher than around 38 degrees C, resulting in a dramatic loss of PSII-mediated electron transport. Measurements of chlorophyll fluor escence decay kinetics after short and long flashes of intense light i ndicated that high temperatures up to 42 degrees C had no inhibitory e ffect on the acceptor side of photosystem II: the rate of electron tra nsfer from the primary (Q(A)) to the secondary (Q(B)) electron accepto r of photosystem II and the fraction of Q(B)-non-reducing photosystem- II centers remained unchanged whereas the intersystem electron flow ap peared to be stimulated. It was also observed that photosystem-I photo chemistry, as probed by the photochemical energy storage in far-red li ght, the quantum yield of photosystem I for the Emerson effect and the kinetics of P-700(1 photooxidation by strong far-red light, was fully preserved in heat-treated potato leaves (at least, up to 45 degrees C ). From the presented data, one can propose the following sequence of events leading to the loss of photosynthetic electron transport in hea ted potato leaves: inhibition of water splitting (at leaf temperatures higher than 32 degrees C) < reduced efficiency of energy trapping by photosystem II centers < alteration of the electron flow after QA (> 4 2 degrees C) < PSI (> 45 degrees C).