Heterogeneity of photosystem II reaction centers as influenced by heat treatment of barley leaves

Three functionally distinct populations of PSII reaction centers differing in the ability to keep the primary accepters in a reduced state and to tran sfer electrons to PSI were estimated using chlorophyll fluorescence measure ments in primary barley leaves exposed to elevated temperatures in the rang e of 37-51 degrees C, The capacity of the PSII reaction centers to perform at least one light-induced charge separation was not affected by a 5-min he at treatment at temperatures up to 51 degrees C, The first population conta ining Q(B)-non-reducing centers corresponded to 15-20% of the total PSII ac tivity up to 45 degrees C. In a second population, PSII reaction centers ma intained Q(A) reduction under light in the presence of oxygen, but not in t he presence of a strong artificial PSI electron acceptor, methyl viologen, In a third population that gradually increases from zero at 37 degrees C to about 60% at 45 degrees C, the PSII centers were not able to keep Q(A) in the reduced state even in the presence of oxygen as the sole electron accep tor, Three electron transport pathways, the pseudocyclic one involving both PSII and PSI, the NAD(P)H-dependent pathway mediated by PSI alone after th e loss of activity in some PSII centers, and the PSI-driven ferredoxin-depe ndent route enhanced by weakly efficient PSII centers that are able to prov ide only catalytic amounts of electrons, are suggested to create a proton g radient in chloroplasts of heat-stressed leaves thus protecting PSII reacti on centers from photodamage.