Poster: Oxidative Stress

Abs # 124: Antioxidant levels decline in primary leaves of barley during growth at ambient and elevated carbon dioxide levels.

Presenter:	Robinson, J. Michael, robinsjm@ba.ars.usda.gov
Authors	Robinson, J. Michael (A)   Sicher, Richard C. (B)
Affiliations:	(A): USDA, ARS, Beltsville Agricultural Research Center, ANRI, Environmental Quality Laboratory (B): USDA, ARS, Beltsville Agricultural Research Center, PSI, Alternate Crops Systems Laboratory

The primary leaves of barley plants grown in high light growth chambers in elevated CO₂(100 Pa), develop symptoms of senescence, e.g., decline of chlorophyll(Chl) a and b, chloroplast protein levels, and photosynthetic rate, compared with primary leaves of barley plants grown in ambient(AMB) CO₂(36 Pa). In barley plants, grown from emergence to 17 d after seeding(DAS)in high light growth chambers, and exposed to either ambient CO₂(36 Pa) or to elevated CO₂(100 Pa), levels of antioxidants, e.g., ascorbate(ASC), dehydroascorbate(DHA), ASCtot(ASC+DHA), and total glutathione(GSHtot) were monitored in primary leaves during 9-17 DAS. Primary leaf Chl level in plants in AMB CO₂ declined 34% over 9-17 DAS, but declined 67% in high CO₂. In AMB and high CO₂, maturing primary leaf ASCtot and GSHtot levels declined over the period 9-17 DAS. However, between 9-17 DAS in AMB CO₂, leaf ASCtot declined 38-41% of the levels observed at 9 DAS but, in high CO₂, ASCtot levels declined by 45-46%. In AMB CO₂ from 9-17 DAS, GSHtot declined by 50-51%, but in high CO₂, GSHtot declined to 60-63% of the levels observed at 9 DAS. Since ASC and GSH are involved in protecting the chloroplast from damage by AOS e.g., H₂O₂, it appears that the decrease in ASCtot and GSHtot could be factors in elevated CO₂-induced photosynthetic decline in barley primary leaves. In contrast, after 13 and 17 DAS, the ASC mole fraction of the ASCtot was significantly higher in high than in AMB CO₂. This indicated that in high compared with AMB CO₂ plant primary leaves, activities of enzymes of the ASC-GSH cycle were not repressed, and the recycling of DHA to ASC in elevated CO₂ plants was functional. The greater decline of GSHtot in elevated CO₂ may reflect inhibition of photorespiration-linked GSH synthesis.