Submitted to: International Journal of Plant Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 1, 2003
Publication Date: November 1, 2004
Citation: Robinson, J.M., Sicher Jr, R.C. 2004. Antioxidant levels decline in primary leaves of barley during growth at ambient and elevated carbon dioxide levels. International Journal of Plant Science. 165(6):965-972. Interpretive Summary: Atmospheric carbon dioxide (CO2) levels have been steadily rising, and this elevated CO2 has been shown to cause some plants to grow more rapidly. However, increased CO2 can have less desirable effects. For example, in the presence of elevated CO2, grass crops such as barley and wheat grown in high light will exhibit early senescence and/or oxidative stress. Exposure of wheat and barley to elevated CO2 in high light for prolonged periods during early stages of growth causes foliar chlorophyll degradation, decline of chloroplast carbon metabolism enzymes, chloroplast photosystems, and ultimately a decline in photosynthetic CO2 assimilation. Additional studies indicated that key antioxidants such as ascorbic acid (vitamin C) and glutathione significantly decline in level during the first 2-3 weeks of growth in barley plant primary leaves in a CO2 enriched atmosphere. This suggests that symptoms of oxidative stress and/or senescence may be due to a decline in the ascorbic acid and glutathione levels. These lower levels appear to be insufficient to handle strong oxidants or active oxygen species that are generated in high light, such as hydrogen peroxide and superoxide. This work is extremely important to grass crop breeders in developing cultivars of such crops as wheat and barley with stronger antioxidant capacities for survival in future periods when CO2 levels increase to even higher levels in the troposphere.
Technical Abstract: The primary leaves of barley plants which are grown in high light growth chambers in elevated CO2 (100Pa) often develop symptoms of senescence, e.g., chlorophyll a and b decline, chloroplast protein decline and decling net CO2 photoassimilation, compared with the primary leaves of barley plants grown in ambient CO2 (36Pa). The onset of scenescence in leaves is thought to be associated with a decline in antioxidant capacity which results in higher levels of active oxygen species (AOS) which cause oxidative stress. The purpose of the study now reported was to monitor levels of the antioxidants, ascorbate and total glutathione, in primary leaves of barley plants at 9-17 days after sowing (DAS), which were grown in high light growth chambers and were exposed from emergence to either ambient CO2 (36 Pa) or to elevated CO2 (100Pa). As the study proceeded over 9 to 17 DAS, chlorophyll level declined in primary leaves of both ambient and elevated CO2, but by 17 DAS in elevated CO2, the chlorophyll level had declined as much as 67%, while in ambient CO2 it had declined to only as much as 34%. In ambient CO2, we found that maturing primary leaf ascorbic acid (ASC) levels, dehydroascorbate (DHA) and total glutathione (GSHtot) levels declined during the period 9-17 DAS. Also, over this period, in elevated CO2, primary leaf ascorbic acid levels and total glutathione were lower at each sampling point than in plants grown in ambient CO2. Since ASC and GSHtot are factors in protecting the chloroplast from damage by AOS such as hydrogen peroxide, it appears that the decrease in ASC, DHA and GSHtot could be factors in elevated CO2-induced photosynthetic decline in barley primary leaves. After 17 DAS the ASC mol fraction of the total vitamin C was higher in leaves of CO2 enriched than ambient CO2 plants. This indicated that activities of enzymes of the ascorbate-glutathione cycle were not repressed more in the CO2 than in the ambient CO2 barley primary leaves. Thus in the CO@enriched plants, the recycling of DHA to ASC appears not to be repressed. Additionally, it is possible that the metabolism of ASC and DHA to other products is accelerated as a result of decline of CO2 enrichment, and this would explain, in part, the greater decrease of ASC and DHA. The decline of glutathione may reflect an elevated CO2 mediated inhibition of photorespiratory carbon metabolism which would inhibit the synthesis of glutathione.