Charge accumulation and photochemistry in leaves studied by thermoluminescence and delayed light emission

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Abstract
A major breakthrough in the understanding of how plants oxidize water to molecular O2 was the discovery by P. Joliot and co-workers that the O2 yield per flash, in a series of light flashes, oscillates with a periodicity of 4. This led to the concept by B. Kok and co-workers that these reactions involve accumulation of 4 positive charges in independent O2-evolving centers, which undergo a series of changes in their redox state (the so-called S states). Optical techniques (such as thermoluminescence and delayed light emission, both discovered by W. Arnold and co-workers) were applied to monitor charge storage on the O2-evolving system in leaves from higher plants. A period of 4 oscillations were observed in both thermoluminescence and delayed light emission, with maxima on flashes 2 and 6, establishing a relationship with the charge accumulation process in photosynthesis. These measurements provided additional new information: the deactivation of the O2-evolving centers, which cannot be measured by the O2 method in the leaves, in in the 20-30 s range; and in the dark-adapted leaves, the secondary bound plastoquinone molecule (the so-called secondary electron acceptor QB) is an equal concentration in its reduced and oxidized forms. The origin of thermoluminescence and delayed light emisson, in terms of the recombination of charges on the O2-evolving and platoquinone slides, is also discussed.

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