Antioxidant Levels Decrease in Primary Leaves of Barley during Growth at Ambient and Elevated Carbon Dioxide Levels

Abstract

Previous studies revealed that the primary leaves of barley (Hordeum vulgare L. cv. Brant) that were grown in high‐light growth chambers in elevated CO2 (100 Pa) developed symptoms of senescence earlier than primary leaves of barley plants concurrently grown in ambient CO2 (36 Pa). The onset of senescence in these leaves was hypothesized to be associated with a decline in antioxidant capacity that resulted in higher levels of active oxygen species, which caused oxidative stress. In this new study, barley was grown from planting in high‐light growth chambers supplied with either ambient CO2 (36 Pa) or elevated CO2 (100 Pa). From 9 to 17 d after planting (DAP), primary leaf chlorophyll level as well as photosynthetic oxygen evolution per unit fresh mass decreased in plants in elevated CO2 to a greater extent than in plants in ambient CO2. Exposure of plants to elevated CO2 resulted in higher levels of primary leaf–free glucose, sucrose, and starch than in plants concurrently exposed to ambient CO2; this was most evident during 9–13 DAP. However, between 13 and 17 DAP, levels of foliar carbohydrates decreased to a greater extent in elevated CO2 than in ambient CO2, indicating that photosynthetic carbon metabolism was more repressed in this period in elevated CO2 treatments. In ambient CO2 over the period of 9–17 DAP, primary leaf total ascorbic acid (ASCtot) and total glutathione (GSHtot) levels decreased 38% and 51%, respectively. In elevated CO2 over this same time period, ASCtot and GSHtot decreased 45% and 63%, respectively. Since ASCtot and GSHtot are antioxidants involved in protecting the chloroplast from damage by active oxygen species such as hydrogen peroxide, we conclude that the decrease in ASCtot and GSHtot were factors in elevated CO2‐induced photosynthetic decline in barley primary leaves. Further, we interpret the greater decrease in ASCtot in elevated CO2 to be caused by a greater decrease in photosynthate partitioning and photosynthate availability, which supports ASC synthesis. We also suggest that decrease in glutathione in elevated CO2 may be related to an inhibition by elevated CO2 of photorespiratory carbon metabolism.