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

摘要

A major breakthrough in our 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 four positive charges in independent “O2-evolving centers,” which undergo a series of changes in their redox state (the so-called S states). In the present paper, we have applied optical techniques (such as thermoluminescence and delayed light emission, both discovered by W. Arnold and co-workers) to monitor charge storage on the O2-evolving system in leaves from higher plants. We observed a period of four oscillations 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, is in the 20- to 30-s range; and in the dark-adapted leaves, the secondary bound plastoquinone molecule (the so-called secondary electron acceptor QB) is in equal concentration in its reduced and oxidized forms. The origin of thermoluminescence and delayed light emission, in terms of the recombination of charges on the O2-evolving and plastoquinone sides, is also discussed.