RELATIONSHIP BETWEEN CO2 ASSIMILATION, PHOTOSYNTHETIC ELECTRON-TRANSPORT, AND ACTIVE O-2 METABOLISM IN LEAVES OF MAIZE IN THE FIELD DURING PERIODS OF LOW-TEMPERATURE

Measurements of the quantum efficiencies of photosynthetic electron tr ansport through photosystem II (phi(PSII)) and CO2 assimilation (phi(C O2)) were made simultaneously on leaves of maize (Zea mays) crops in t he United Kingdom during the early growing season, when chilling condi tions were experienced. The activities of a range of enzymes involved with scavenging active O-2 species and the levels of key antioxidants were also measured. When leaves were exposed to low temperatures durin g development, the ratio of phi(PSII)/phi(CO2) was elevated, indicatin g the operation of an alternative sink to CO2 for photosynthetic reduc ing equivalents. The activities of ascorbate peroxidase, monodehydroas corbate reductase, dehydroascorbate reductase, glutathione reductase, and superoxide dismutase and the levels of ascorbate and alpha-tocophe rol were also elevated during chilling periods. This supports the hypo thesis that the relative flux of photosynthetic reducing equivalents t o O-2 via the Mehler reaction is higher when leaves develop under chil ling conditions. Lipoxygenase activity and lipid peroxidation were als o increased during low temperatures, suggesting that lipoxygenase-medi ated peroxidation of membrane lipids contributes to the oxidative dama ge occurring in chill-stressed leaves.