NITROGEN FIXATION AND CARBON METABOLISM IN VICIA FABA ROOT NODULES OVER ONTOGENY

Abstract

The evolution of growth, nodulation, nitrogen fixation, and activities of root-nodule enzymes related to sucrose breakdown (sucrose synthase, alkaline invertase), pentose phosphate pathway (glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase), malate dehydrogenase, phosphoenolpyruvate carboxylase, and NADP-dependent isocitrate dehydrogenase, were followed over the complete life-cycle of faba bean in a growth chamber. The aim was to study the ontogenic changes of these parameters to obtain information on the carbon metabolism in nodules ofVicia faba, an amide-exporting legume. The maximum values of the relative growth rate and the specific acetylene-reducing activity were registered during the vegetative period. At pod-filling, the specific and total acetylene-reducing activity per plant declined in parallel with the enzyme activities of carbon metabolism. Contrary to reports for other legume species, inV. fabaroot nodule activity of sucrose synthase exceeded alkaline invertase values by 2-fold or more during the vegetative period. The activity of the two enzymes was similar at flowering and pod formation. The enzymes of carbon metabolism registered two maxima, one before and one after a trough (day 32), which marked the change from the vegetative to the reproductive period.