Photosystem-II Activity Is Decreased by Yellowing of Barley Primary Leaves during Growth in Elevated Carbon Dioxide.

Leaf yellowing was studied in 10-18-d-old barley seedlings (Hordeum vulgare L. cv. Brant) grown at ambient (38 Pa) and at elevated (68, 100, and 140 Pa) 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 observed 10-12 d after sowing (DAS). Total Chl levels in primary leaves were 35%, 64%, and 78% below maximal levels in the 38, 68, and 100 Pa CO2 growth treatments, respectively, when measured 18 DAS. Losses of Chl in 18-d-old primary leaves were similar in the 100 and 140 Pa CO2 treatments. Decreases of Chl a and Chl b in response to CO2 enrichment were comparable in isolated chloroplast preparations and in intact 18-d-old barley primary leaves of plants grown at 38 and 68 Pa CO2. Total thylakoid membrane proteins, the Chl a/b binding protein (LHC-II), and D1 protein levels were also lower in chloroplast preparations from plants grown in the elevated compared to the ambient CO2 treatment. Both ferricyanide reduction and whole-chain electron-transport rates (H2O --> methylviologen) were significantly lower for chloroplasts from plants grown at 68 Pa CO2 compared with those grown at 38 Pa CO2. However, photosystem-I-dependent chloroplast photoreductions did not differ between CO2 treatments. The results indicated that the CO2-dependent yellowing of barley primary leaves adversely affected photosystem-II activity. Growth in elevated CO2 may have increased the susceptibility of photosystem-II to light damage.