Energy State and Dinitrogen Fixation in Soybean Nodules of Dark-grown Plants

Abstract

Dark treatment of 25-day-old greenhouse-grown plants of inoculated soybean (Glycine max var. Chippewa) for 1 day reduced ATP by 70%, sucrose by 60%, total adenosine phos-phates by 60%, ATP/ADP ratio by 55%, nitrogenase activity by 50%, and energy charge by 15% in nodules. The close cor-relation between nitrogenase activity and these energy param-eters indicates that they may play a major role in regulating dinitrogen fixation in the symbiotic system. The fixation of atmospheric dinitrogen by nitrogenase in nodules requires an energy supply (ATP) and reductant (NADPH) (6, 7). Based on enzymatic and physiochemical anal-yses, about four ATPs are needed for one pair of electrons transferred by nitrogenases isolated from various sources (3, 17). For 1 mole of nitrogen fixed, therefore, at least 12 moles of ATP would have to be provided. Even though it is not a tre-mendous energy drain reaction in biological processes, the pos-sible controlling role of ATP on biological nitrogen fixation is evident and known in bacterial cells (5, 9, 12. 15). Other poten-tial regulators of nitrogenase activity in the symbiotic system could be the ATP/ADP ratio and energy charge because a posi-tive correlation of the ratio and the activity has already been demonstrated by preparations of nitrogenase in vitro (1, 5, 6, 9, 12, 15). The ATP content, the ATP/ADP ratio, and energy charge, however, have never been investigated in soybean nod-ules. Also, the possible regulatory role of these parameters in N2 fixation in vivo (6) is unknown. This paper provides experimen-tal data to fill this void.