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Metabolism and Enzymology

Diurnal Changes in Maize Leaf Photosynthesis ¹

I. Carbon Exchange Rate, Assimilate Export Rate, and Enzyme Activities

United States Department of Agriculture, Agricultural Research Service, North Carolina State University, Raleigh, North Carolina 27695-7631, Department of Botany, North Carolina State University, Raleigh, North Carolina 27695-7631, Department of Crop Science, North Carolina State University, Raleigh, North Carolina 27695-7631

Diurnal changes in photosynthetic parameters and enzyme activities were characterized in greenhouse grown maize plants (Zea mays L. cv Pioneer 3184). Rates of net photosynthesis and assimilate export were highest at midday, coincident with maximum irradiance. During the day, assimilate export accounted for about 80% of net carbon fixation, and the maximum export rate (35 milligrams CH₂O per square decimeter per hour) was substantially higher than the relatively constant rate maintained through the night (5 milligrams CH₂O per square decimeter per hour). Activities of sucrose phosphate synthase and NADP-malate dehydrogenase showed pronounced diurnal fluctuations; maximum enzyme activities were generally coincident with highest light intensity. Reciprocal light/dark transfers of plants throughout the diurnal cycle revealed that both enzymes were deactivated by 30 minutes of darkness during the day, and they could both be substantially activated by 30 minutes of illumination at night. During 24 hours of extended darkness, sucrose phosphate synthase activity declined progressively to an almost undetectable level, but was activated after 1.5 hours of illumination. Thus, the diurnal fluctuation in maize sucrose phosphate synthase can be explained by some form of light modulation of enzyme activity and is not due to an endogenous rhythm in activity. No diurnal fluctuations were observed in the activities of NADP-malic enzyme or fructose 6-phosphate-2-kinase. Phosphoenolpyruvate carboxylase was activated by light to some extent (about 50%) when activity was measured under suboptimal conditions in vitro. The results suggested that the rates of sucrose formation and assimilate export were closely aligned with the rate of carbon fixation and the activation state of sucrose phosphate synthase.

³ Permanent address: Laboratory of Chemistry, Faculty of Medicine, Teikyo University, Ohtsuka, Hachioji City, Tokyo, Japan, 192-03.

¹ Cooperative investigations of the United States Department of Agriculture, Agricultural Research Service, and the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601. Paper No. 10515 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7601. Supported in part by United States Department of Agriculture Competitive grant 5936645620 (S. C. H.), and the Japan-United State Cooperative Research Program on biochemical factors related to photosynthetic productiity of C₃ and C₄ plants (H. U.).

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