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

Fluorescence, absorbance and photoacoustic methods were used to examine in situ various functional aspects of the photochemical apparatus of chloroplasts in potato leaves (So[anum tuberosum L.) briefly pre-exposed in the dark to a wide range of elevated temperatures (32 degrees C-45 degrees C). Measurements of the Emerson enhancement of photosynthetic O-2 evolution and the flash-induced reduction of the oxidized reaction center pigment of photosystem I demonstrated that selective denaturation of photosystem II in potato leaves started at a 'low' temperature of 32 degrees C. Analysis of the characteristics of chlorophyll fluorescence induction in sub- and super-saturating light revealed two targets 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 identified as the major determinant of the loss of photosynthesis. A second phase of thermal denaturation of photosystem II occurred at temperatures higher than around 38 degrees C, resulting in a dramatic loss of PSII-mediated electron transport. Measurements of chlorophyll fluorescence decay kinetics after short and long flashes of intense light indicated that high temperatures up to 42 degrees C had no inhibitory effect on the acceptor side of photosystem II: the rate of electron transfer from the primary (Q(A)) to the secondary (Q(B)) electron acceptor of photosystem II and the fraction of Q(B)-non-reducing photosystem-II centers remained unchanged whereas the intersystem electron flow appeared to be stimulated. It was also observed that photosystem-I photochemistry, as probed by the photochemical energy storage in far-red light, 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 heated 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 (> 42 degrees C) < PSI (> 45 degrees C).