Significance of Hydrogen Evolution in the Carbon and Nitrogen Economy of Nodulated Cowpea

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Abstract
The carbon and nitrogen economies of a single cultivar of cowpea (Vigna unguiculata (L.) Walp.cv Caloona) nodulated with either a high H2-evolving strain (176A27) or a low H2-evolving strain (CB756) of Rhizobium were compared. The two symbioses did not differ in total dry matter production, seed yield, nitrogen fixed, the spectrum of nitrogenous solutes produced by nodules for export, or the partitioning of net photosynthate within the plant throughout the growth cycle. Detailed examination of the carbon and nitrogen economy of the nodules, however, showed a significant difference between the symbioses. Nodules formed with CB756 lost less CO2 in respiration compared to the higher H2-evolving symbioses and this could have been largely responsible for a 36% better economy of carbon use in CB756 nodules during the period of maximum H2 evolution (48-76 days) and over the whole growth period (20-90 days), a 16% economy. In terms of overall net photosynthate generated by the plant, these economies were equivalent to 5% and 2% of the carbon utilized in the two periods, respectively. From the differences in H2 evolution and CO2 production by nodules of the two symbioses, the cost of H2 evolution was found to be 3.83±0.6 millimoles CO2/millimoles H2 for plants grown in sand culture and 1.69 ± 0.48 millimoles CO2/millimoles H2 for those in water culture. In both symbioses, the ratio of H2 evolution to N2 fixed varied markedly during ontogeny, indicating a significant variation in the relative efficiency and thus metabolic cost of N2 fixation at different stages during development.