Leaf Photosynthetic Metabolism and N2 Fixation at the Flowering Stage in Three Genotypes of Cowpea [Vigna unguiculata (L.) Walp.]

Biological nitrogen fixation (BNF) in cultivars of cowpea is not yet understood. The hypothesis proposed by this study is that lack of basic technology, including sufficient mineral nutrition, combined with periods of water shortage during the crop cycle leads to insufficient photosynthetic metabolism in the flowering stage in nodulated plants. Two experiments were conducted in northeastern Brazil, one under field conditions and another in a greenhouse at near optimal conditions. Two of the principal cultivars selected for northeastern (Mulato and Gurguéia) and one for northern Brazil (Milênio) were evaluated regarding physiological, biochemical and biometric variables between the late vegetative and early reproductive stages under mineral nutrition and BNF treatments. Gas exchanges, chlorophyll fluorescence and leaf contents of soluble sugars, amino acids, proteins and chlorophyll of inoculated plants were similar to plants fertilized with mineral nitrogen, in the three cultivars in both experiments, with emphasis on Gurguéia. Leaf nitrogen concentrations and the number and dry weight of nodules were higher in Mulato and Gurguéia compared to Milênio. Milênio and Gurguéia under BNF showed higher total dry weight compared to controls; however, the number of pods per plant was higher in inoculated plants compared to controls. In conclusion, based on these results under non-stressful conditions, cultivars selected for the northeastern region inoculated with an effective rhizobia strain and receiving a mild dose of mineral nitrogen can ensure development similar to that verified in plants fertilized only with correct nitrogen fertilization.