SEASONAL FLUCTUATIONS IN NITROGEN-FIXING ABILITY (C2H2 REDUCTION) ANDHYDROGEN UPTAKE BY ACTINORHIZAL NODULES OF CASUARINA-GLAUCA

Casuarina glauca is an introduced actinorhizal tree grown as roadside ornamental plantation in Pakistan. Observations were made on the seaso nal fluctuations in nitrogen-fixing (C2H2 reduction) activity and upta ke hydrogenase activity of the root nodules of C. glauca. Nodules show ed significant (P less than or equal to 0.05) seasonal variation in th e nitrogenase and hydrogenase activities which were highest during Jun e (14.25 mu moles C2H4.g(-1) fresh nodule wt. hr(-1) and 2.34 mu moles H-2 consumed g(-1) fresh nodule wt. hr(-1), respectively) and lowest during December (0.46 mu moles C2H4.g(-1) fresh nodule wt. hr(-1) and 0.19 mu moles H-2 consumed g(-1) fresh nodule wt. hr(-1), respectively ). These results indicate that maximum nitrogenase activity of the nod ules coincides with the period of vigorous plant growth. The results a lso suggest that during fast growing period increased nitrogen require ments of the host plant are met by the elevated nitrogen fixation rate s by the endophyte.