Relation between the heat-induced increase of F0 fluorescence and a shift in the electronic equilibrium at the acceptor side of photosystem 2

F-0 fluorescence and thermoluminescence (TL) were recorded simultaneously on various dark-adapted leaf samples. Above 40 degrees C, a sharp peak of TL coincided with the onset of the heat-induced F-0 rise. It results from a back-transfer of an electron from the secondary Q(B)(-) to the primary acceptor QA of photosystem 2, followed by a luminescence-emitting recombination with Tyr-D+. This demonstrates that the critical temperature at which the F-0 starts rising also corresponds to a shift towards the left of the Q(A)<->Q(B)(-) equilibrium.