THE RATE-CONSTANT OF PHOTOINHIBITION, MEASURED IN LINCOMYCIN-TREATED LEAVES, IS DIRECTLY PROPORTIONAL TO LIGHT-INTENSITY

Pumpkin leaves grown under high light (500-700 mu mol of photons m(-2) . s(-1)) were illuminated under photon flux densities ranging from 6.5 to 1500 mu mol . m(-)2 . s(-1) In the presence of lincomycin, an inhi bitor of chloroplast protein synthesis, The illumination at all light intensities caused photoinhibition, measured as a decrease in the rati o of variable to maximum fluorescence, Loss of photosystem II (PSII) e lectron transfer activity correlated with the decrease in the fluoresc ence ratio, The rate constant of photoinhibition, determined from firs t-order fits, was directly proportional to photon flux density at all light intensities studied, The fluorescence ratio did not decrease if the leaves were illuminated in low light in the absence of lincomycin or incubated in darkness In the presence of lincomycin. The constancy of the quantum yield of photoinhibition under different photon flux de nsities strongly suggests that photoinhibition in vivo occurs by one d ominant mechanism under all light intensities, This mechanism probably is not the acceptor side mechanism characterized in the anaerobic cas e in vitro. Furthermore, there was an excellent correlation between th e loss of PSII activity and the loss of the D1 protein from thylakoid membranes under low light, At low light, photoinhibition occurs so slo wly that inactive PSII centers with the D1 protein waiting to be degra ded do not accumulate, The kinetic agreement between D1 protein degrad ation and the inactivation of PSII indicates that the turnover of the D1 protein depends on photoinhibition under both low and high light.