Probing electron transport through and around photosystem II in vivo by the combined use of photoacoustic spectroscopy and chlorophyll fluorometry

Photoacoustic spectroscopy and chlorophyll fluorometry were used in combina tion to monitor in vivo several aspects of the photosystem II (PSII) functi on (O-2 evolution, electron transport, and energy storage) in leaves of var ious plant genotypes. Exposure of barley leaves to 45 degrees C in low ligh t induced a rapid and complete disruption of the O-2-evolving complex of PS II, as shown by the suppression of the photobaric component of the photoaco ustic signal. The photoacoustically measured loss of O-2 evolution during h eat 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 th e carbonic anhydrase, and (iii) was not correlated with a concomitant suppr ession of the photochemical energy storage and the electron transport activ ity of PSII. The maintenance of a significant PSII photochemical activity i n the absence of O-2 evolution is indicative of cyclic electron transport a round PSII which bypasses the O-2-evolving complex. The modulation-frequenc y dependence of the energy stored during cyclic PSII activity, its inhibiti on by the herbicide diuron, and its insensitivity to the plastoquinol antag onist DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone) indicated tha t Q(B), the secondary electron acceptor of PSII, is involved in the electro n cycle and that the rate-limiting step of the cycle is a reaction with a h alf-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 DBM IB was not sufficient to induce appreciable cyclic electron transport aroun d PSII. The electron cycle was insensitive to preferential excitation of ca rotenoids 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 temperatur e range 70-120 degrees C, demonstrating the photoprotective function of ele ctron cycling around PSII.