The rate coefficient of repair of photosystem II after photoinactivation

During photosynthesis, photoinactivation and repair of photosystem II (PSII) occur simultaneously, resulting in a net loss of functional PSII under a given irradiance. This study determines the rate coefficients for the partial processes, allowing the calculation of the partial rates at any concentration of functional/non-functional PSII. The rate coefficient of photoinactivation was obtained from the onset of photoinactivation of PSII in leaf segments of Capsicum annuum L. in the absence of repair, and was in turn used to obtain the rate coefficient (k_r) of repair of PSII when repair was occurring. The value of k_r was found to be near maximum at an irradiance as low as 29 μmol photons m^-2s^-1 and peaked at or somewhat above the growth irradiance; however, it declined on further increasing the irradiance, possibly due to oxidative stress. The value of k_r was considerably decreased by elevating the CO₂ to about 1%, particularly at low irradiance, probably due to acidification of the stroma to a pH outside the range that is optimal for protein synthesis. The method of determining k_r is convenient to apply, not relying on radio-labelling and pulse-chase experiments.