Photosystem-II activity is decreased by yellowing of barley primary leavesduring growth in elevated carbon dioxide

Leaf yellowing was studied in 10-18-d-old barley seedlings (Houdeum vulgare L. cv. Brant) grown at ambient (38 Pa) and at elevated (68, 100, and 140 P a) CO2 partial pressures in controlled-environment chambers. Maximal total chlorophyll (Chl) concentrations of primary leaves from all four CO2 growth treatments were 0.36 +/- 0.01 g m(-2), and these concentrations were obser ved 10-12 d after sowing (DAS). Total Chi levels in primary leaves were 35% , 64%, and 78% below maximal levels in the 38, 68, and 100 Pa CO2 growth tr eatments, respectively, when measured 18 DAS. Losses of Chi in 18-d-old pri mary leaves were similar in the 100 and 140 Pa CO2 treatments. Decreases of Chi d and Chi b in response to CO2 enrichment were comparable in isolated chloroplast preparations and in intact 18-d-old barley primary leaves of pl ants grown at 38 and 68 Pa CO2. Total thylakoid membrane proteins, the Chi alb binding protein (LHC-II), and D1 protein levels were also lower in chlo roplast preparations from plants grown in the elevated compared to the ambi ent CO2 treatment. Both ferricyanide reduction and whole-chain electron-tra nsport rates (H2O --> methylviologen) were significantly lower for chloropl asts from plants grown at 68 Pa CO2 compared with those grown at 38 Pa CO2. However, photosystem-I-dependent chloroplast photoreductions did not diffe r between CO2 treatments. The results indicated that the CO2-dependent yell owing of barley primary leaves adversely affected photosystem-II activity. Growth in elevated CO2 may have increased the susceptibility of photosystem -II to light damage.