SEASONAL FLUCTUATIONS IN NITROGEN-FIXING ABILITY (C2H2 REDUCTION) ANDHYDROGEN UTPAKE BY ROOT-NODULES OF CORIARIA-NEPALENSIS AND DATISCA-CANNABINA

A study was made on the seasonal fluctuations in N2-fixing (C2H2 reduc tion) activity and uptake hydrogenase activity of the root nodules of Coriaria nepalensis and Datisca cannabina. Nitrogenase activity of the nodules of both plants showed biphasic curves with peaks in spring an d late summer. Acetylene reduction by the root nodules of Coriaria nep alensis was highest (16.4 mu mol. C2H4.g-1 fresh nodule wt. h-1) durin g July while the peak acetylene reduction activity (14.8 mu mol. C2H4. g-1 fresh nodule wt. h-1) of Datisca cannabina nodules was determined in April. Less than 15% of the enzyme activity was retained by the nod ules in winter. The nodules of both plant species consumed hydrogen on incubation with a gas mixture containing H2, indicating an uptake hyd rogenase activity. The H2 uptake by the excised nodules of both Coriar ia and Datisca was highest (2.16 and 1.95 mu mol. H2 consumed g-1 fres h nodule wt. h-1, respectively) in May. The presence of an uptake hydr ogenase was confirmed in nodule homogenates with phenazine metasulphat e as an artificial electron acceptor. Purified vesicle cluster suspens ions (20 mum residue) showed highest hydrogenase activity, indicating that the enzyme is associated with the endophyte.