PHOTOINHIBITION AND D1 PROTEIN-DEGRADATION IN PEAS ACCLIMATED TO DIFFERENT GROWTH IRRADIANCES

The relationship between the susceptibility of photosystem II (PSII) t o photoinhibition in vivo and the rate of degradation of the D1 protei n of the PSII reaction center heterodimer was investigated in leaves f rom pea plants (Pisum sativum L. cv Greenfeast) grown under widely con trasting irradiances. There was an inverse linear relationship between the extent of photoinhibition and chlorophyll (Chl) a/b ratios, with low-light leaves being more susceptible to high light. In the presence of the chloroplast-encoded protein synthesis inhibitor lincomycin, th e differential sensitivity of the various light-acclimated pea leaves to photoinhibition was largely removed, demonstrating the importance o f D1 protein turnover as the most crucial mechanism to protect against photoinhibition. In the differently light-acclimated pea leaves, the rate of D1 protein degradation (measured from [S-35]methionine pulse-c hase experiments) increased with increasing incident light intensities only if the light was not high enough to cause photoinhibition in viv o. Under moderate illumination, the rate constant for D1 protein degra dation corresponded to the rate constant for photoinhibition in the pr esence of lincomycin, demonstrating a balance between photodamage to D 1 protein and subsequent recovery, via D1 protein degradation, de novo synthesis of precursor D1 protein, and reassembly of functional PSII. In marked contrast, in light sufficiently high to cause photoinhibiti on in vivo, the rate of D1 protein degradation no longer increased con comitantly with increasing photoinhibition, suggesting that the rate o f D1 protein degradation is playing a regulatory role. The extent of t hylakoid stacking, indicated by the Chl a/b ratios of the differently light-acclimated pea leaves, was linearly related to the half-life of the D1 protein in strong light. We conclude that photoinhibition in vi vo occurs under conditions in which the rate of D1 protein degradation can no longer be enhanced to rapidly remove irreversibly damaged D1 p rotein. We suggest that low-light pea leaves, with more stacked membra nes and less stroma-exposed thylakoids, are more susceptible to photoi nhibition in vivo mainly due to their slower rate of D1 protein degrad ation under sustained high light and their slower repair cycle of the photodamaged PSII centers.