Effect of the host legume on acetylene reduction and hydrogen evolution by Rhizobium nitrogenase.

The relative efficiency (RE) of N(2) fixation (RE = 1 - [H(2) evolved in air]/[acetylene reduced]) was investigated in a Rhizobium strain lacking uptake hydrogenase activity (Hup(-)). Variation in RE of such strains presumably reflects changes in the electron allocation coefficient of nitrogenase. By artificially extending the normal dark period of 24-day-old Pisum sativum L. cv ;Alaska' inoculated with the Hup(-)R. leguminosarum strain 3740, reproducible changes in RE were obtained. The RE showed no change during a normal 8-hour night, but a significant increase in RE was measured after 20 hours in the dark. Upon returning to the normal 550 microEinsteins per square meter per second light treatment, RE declined to previous levels within 2 hours. If, after the 20-hour dark treatment, plants were returned to 90 or 190 microEinsteins per square meter per second or maintained in the dark, RE did not decline significantly. The RE varied inversely with changes in soluble sugar content of root nodules. A similar pattern of changes in RE during an extended dark period and subsequent light treatment was measured in 28-day-old Alaska peas and in the Hup(-)R. trifolii strain 162X99 in symbiosis with Trifolium subterraneum L. cv ;Woogenellup.' These results suggest that Rhizobium cells may produce short-term alterations in the electron allocation coefficient of nitrogenase in response to physiological changes. The observed changes in the bacterial RE favored N(2) reduction over proton reduction when soluble sugars provided by the host plant declined.