GENETIC-VARIATION OF PHOTOSYNTHETIC ELECTRON-TRANSPORT IN BARLEY - IDENTIFICATION OF PLASTOCYANIN AS A POTENTIAL LIMITING FACTOR

Cultivated (Hordeum vulgare) and wild (Hordeum spontaneum) genotypes of barley were compared for differences in photosynthetic electron transport activity and chloroplast membrane composition. Plants were grown at 21 degrees C in a controlled environment chamber (500 mu mol photons m(-2) s(-1)). Thylakoid membranes were isolated from vegetative and flag leaves and analyzed for uncoupled electron transport activity. Significant genetic variability in chloroplast electron transport was found with fourfold differences in activity observed in the genotypes tested. Quantitative measurements were made for each component of the chloroplast electron transport chain, including the development of an immunochemical assay for plastocyanin. The genetic differences in electron transport activity were related to the concentration of plastocyanin in the thylakoid membrane. The fivefold differences in plastocyanin content observed within the genotypes tested corresponded to a plastocyanin pool size of 0.8-3.8 mol of plastocyanin per mol of photosystem I reaction center. Therefore, plastocyanin was a significant limiting factor in barley genotypes expressing low photosynthetic electron transport activity.