Hydrogen uptake — measurement of photosynthate limitation in nodules of cowpea miscellany hosts

Nodule nitrogenase and H 2 uptake activities were determined in normal (undecapitated) and decapitated plants (removal of shoot 24 h before measurements) of green gram ( Vigna radiata (L.) Wilczek) and black gram ( Vigna mungo (L.) Hepper) inoculated with two Hup + Rhizobium strains which had H 2 recycling ability in excess to the rates of H 2 produced by nitrogenase in nodules. A significant decline in nodule nitrogenase activity was observed in decapitated plants as compared to uncut control plants at 40 d and 50 d of plant growth. However, nodules of both, control and decapitated plants, of both host species showed hydrogen supported enhanced acetylene reduction activity (ARA). The H 2 uptake rates of the two strains varied depending on host as well as on stage of plant growth. However, based on the relative ratio of H 2 consumed per mole of C 2H 2 reduced in case of normal photosynthate supply as well as from the interruption of photosynthate supply (decapitated plants), it appears that photosynthate supply remains a limiting factor in nitrogen fixation under normal conditions of plant growth during symbiosis. The nitrogenase enzyme had the ability to utilize energy equivalents approximately 1 mole H 2 oxidized by uptake hydrogenase per mole of C 2H 2 reduced at 40 d of plant growth under normal photosynthate supply conditions.