PHOTOSYSTEM-II REGULATION AND DYNAMICS OF THE CHLOROPLAST D1 PROTEIN IN ARABIDOPSIS LEAVES DURING PHOTOSYNTHESIS AND PHOTOINHIBITION

Arabidopsis thaliana leaves were examined in short-term (1 h) and long -term (10 h) irradiance experiments involving growth, saturating and e xcess light. Changes in photosynthetic and chlorophyll fluorescence pa rameters and in populations of functional photosystem II (PSII) center s were independently measured. Xanthophyll pigments, 3-(3,4-dichloroph enyl)-1,1-dimethylurea (DCMU)-binding sites, the amounts of D1 protein , and the rates of D1 protein synthesis were determined. These compreh ensive studies revealed that under growth or light-saturating conditio ns, photosynthetic parameters remained largely unaltered. Photoprotect ion occurred at light saturation indicated by a dark-reversible increa se in nonphotochemical quenching accompanied by a 5-fold increase in a ntheraxanthin and zeaxanthin. No consistent change in the concentratio ns of functional PSII centers, DCMU-binding sites, or D1 protein pool size occurred. D1 protein synthesis was rapid. In excess irradiance, q uantum yield of O-2 evolution and the efficiency of PSII were reduced, associated with a 15- to 20-fold increase in antheraxanthin and zeaxa nthin and a sustained increase in nonphotochemical quenching. A decrea se in functional PSII center concentration occurred, followed by a dec line in the concentration of D1 protein; the latter, however, was not matched by a decrease in DCMU-binding sites. In the most extreme treat ments, DCMU-binding site concentration remained 2 times greater than t he concentration of D1 protein recognized by antibodies. D1 protein sy nthesis rates remained unaltered at excess irradiances.