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Probing Electron Transport through and around Photosystem II in vivo by the Combined Use of Photoacoustic Spectroscopy and Chlorophyll Fluorometry
Author(s) -
Havaux Michel
Publication year - 1998
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.199800027
Subject(s) - chemistry , plastoquinone , photosystem ii , electron transport chain , photochemistry , spectroscopy , photosynthesis , biophysics , thylakoid , chloroplast , biochemistry , physics , quantum mechanics , biology , gene
Photoacoustic spectroscopy and chlorophyll fluorometry were used in combination to monitor in vivo several aspects of the photosystem II (PSII) function (O 2 evolution, electron transport, and energy storage) in leaves of various plant genotypes. Exposure of barley leaves to 45°C in low light induced a rapid and complete disruption of the O 2 ‐evolving complex of PSII, as shown by the suppression of the photobaric component of the photoacoustic signal. The photoacoustically measured loss of O 2 evolution during heat stress (i) was confirmed by polarographic measurements of O 2 exchange by the leaf samples, (ii) was shown to be independent of the activity of the carbonic anhydrase, and (iii) was not correlated with a concomitant suppression of the photochemical energy storage and the electron transport activity of PSII. The maintenance of a significant PSII photochemical activity in the absence of O 2 evolution is indicative of cyclic electron transport around PSII which bypasses the O 2 ‐evolving complex. The modulation‐frequency dependence of the energy stored during cyclic PSII activity, its inhibition by the herbicide diuron, and its insensitivity to the plastoquinol antagonist DBMIB (2,5‐dibromo‐3‐methyl‐6‐isopropyl‐ p ‐benzoquinone) indicated that Q B , the secondary electron acceptor of PSII, is involved in the electron cycle and that the rate‐limiting step of the cycle is a reaction with a half‐time of ca. 1.5 ms. In the absence of heat stress, overreduction of the plastoquinone pool by strong illumination or by leaf infiltration with DBMIB was not sufficient to induce appreciable cyclic electron transport around PSII. The electron cycle was insensitive to preferential excitation of carotenoids with a blue‐green light and was not significantly affected by the operation of the xanthophyll cycle. Interruption of the electron cycle by diuron during prolonged heat treatment in moderate light was accompanied by a marked peroxidation of the chloroplast thylakoid membranes, as revealed by the appearance of chlorophyll thermoluminescence bands in the temperature range 70–120°C, demonstrating the photoprotective function of electron cycling around PSII.