  We propose a dynamic mechanism for the reversible regulation of
photochemistry in plants under varying light environments. We employ a
three-level quantum model to take into account the correlations between charge
donors and charge acceptors immediately before photoexcitation, and show that
under steady and coherent driving of light, the efficiency of charge transfer
is inversely proportional to the intensity of incident light, which can be
suppressed so severely that it becomes a limiting factor on photosynthetic
electron transport. These results are analyzed to gain insight in the light
responses of photosynthetic parameters. We discuss the implications of thermal
fluctuation in the light source used in photochemical experiments, and argue
that in high light conditions, the quantum yields measured with an incandescent
lamp may be higher than those measured with a laser, a manifestation of thermal
fluctuation in lamp illumination. Our new picture renders a consistent
interpretation of a wide range of experiments, including plastocyanin-dependent
electron transport in photosystem I, biphasic redox kinetics of P700 and
wavelength-dependent quantum yields, and provides a donor-side scheme for the
onset of irreversible damage to photosystem II.
