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Carbon Metabolism in Alfalfa Root Nodules: Developmental Patterns of Host Plant Enzymes Before and After Shoot Removal¹

Dinitrogen fixation in the nodules of perennial forage legumes usually decreases rapidly after the harvest of shoots and then increases as new shoots are produced. In this study the activities of enzymes of carbon metabolism which may indirectly support dinitrogen fixation and subsequent assimilation were measured in the plant fractions of alfalfa (Medicago sativa L.) nodules before and after shoot removal. Nodule enzymes of sucrose and glucose metabolism (invertase, glucose-6-phosphate dehydrogenase) and organic acid metabolism (malate dehydrogenase and NADP-isocitrate dehydrogenase) decreased rapidly in activity (gram fresh weight basis) after shoot removal. Except for NADP-isocitrate dehydrogenase, the decreases in specific activity of these enzymes after shoot removal were similar to that for soluble protein suggesting general proteolytic degradation. Amylase activity showed no clear response to shoot removal. These data suggest that when alfalfa shoots are removed, root nodule capacity to metabolize some hexoses and organic acids is reduced while the capacity to degrade nodule starch is not greatly influenced.

Key Words: Medicago sativa • Carbon-nitrogen interactions • Harvest management

² Research associate (formerly graduate research assistant) and associate professor of agronomy, Dep. of Horticulture and Agronomy, respectively, Univ. of Wisconsin-Madison, WI 53706.

Received for publication August 5, 1983.