PHOTOSYNTHESIS AND PHOTOINHIBITION IN LEAVES OF CHLOROPHYLL-B-LESS BARLEY IN RELATION TO ABSORBED LIGHT

The response of photosynthesis to absorbed light by intact leaves of wild-type (Hordeum vulgare L. cv. Gunilla) and chlorophyll b-less barley (H. vulgare L. cv. Dornaria, chlorina-f2(2800)) was measured in a light integrating sphere. Up to the section where the light response curve bends most sharply the responses of the b-less and wild-type barley were similar but not identical. Average quantum yield and convexity for the mutant light response curves were 0.89 and 0.90, respectively times those of the wild-type barley. The maximum quantum yield for PSII photochemistry was also 10% lower as indicated by fluorescence induction kinetics (F(v)/F(m)). Just above the region where the light curve bends most sharply, photosynthesis decreased with time in thc mutant but not in the wild-type barley. This decrease was associated with a decrease in F(v)/F(m) indicating photoinhibition of PSII. This photoinhibition occurred in the same region of the light response curve where zeaxanthin formation occurs. Zeaxanthin formation occurred in both the chlorophyll b-less and wild-type leaves. However, the epoxidation state was lower in the mutant than in the wild-type barley. The results indicate that chlorophyll b-less mutants will have reduced photosynthetic production as a result of an increased sensitivity to photoinhibition and possibly a lowered quantum yield and convexity in the absence of photoinhibition.