Relationship between CO2 assimilation, photosynthetic electron transport, and active O2 metabolism in leaves of maize in the field during periods of low temperature

摘要

Measurements of the quantum efficiencies of photosynthetic electron transport through photosystem II (phiPSII) and CO2 assimilation (phiCO2) were made simultaneously on the leaves of field-grown maize cv. LG11 and LG20.80 during the early growing season of 1995, when chilling conditions were experienced, in Essex, UK. The activities of a range of enzymes involved with scavenging active O2 species and the levels of key antioxidants were also measured. When leaves were exposed to low temperatures during development, the phiPSII:phiCO2 ratio was elevated, indicating the operation of an alternative sink to CO2 for photosynthetic reducing equivalents. The activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase and superoxide dismutase and the levels of ascorbate and alpha-tocopherol were also elevated during chilling periods. This supports the hypothesis that the relative flux of photosynthetic reducing equivalents to O2 via the Mehler reaction is higher when leaves develop under chilling conditions. Lipoxygenase activity and lipid peroxidation also increased during low temperatures, suggesting that lipoxygenase-mediated peroxidation of membrane lipids may contribute to the oxidative damage occurring in chill-stressed leaves.