Fluorescence, absorbance and photoacoustic methods were used to examine in situ various functional aspects of the photochemical apparatus of chloroplasts in potato leaves (Solanum tuberosum L.) briefly pre-exposed in the dark to a wide range of elevated temperatures (32°C–45°C). Measurements of the Emerson enhancement of photosynthetic O2 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°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 denuration of photosystem II occurred at temperatures higher than around 38°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°C had no inhibitory effect on the acceptor side of photosystem II: the rate of electron transfer from the primary (QA) to the secondary (QB) electron acceptor of photosystem II and the fraction of QB-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 P700 photooxidation by strong far-red light, was fully preserved in heat-treated potato leaves (at least, up to 45°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°C)< reduced efficiency of energy trapping by photosystem II centers < alteration of the electron flow after QA( > 42°C) <PSI( > 45°C).
Havaux, M., 1993. Characterization of thermal damage to the photosynthetic electron transport system in potato leaves. Plant Science, 94(1-2), pp.19-33.
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Category: Photonics & Optoelectronics