Photosynthesis, symbiotic N and C accumulation in leaves of 30 nodulated cowpea genotypes grown in the field at Wa in the Guinea savanna of Ghana

Cowpea is a major food legume in Africa with relatively little information on N and C nutrition under field conditions in the continent. The aim of this study was to assess the relationship between leaf symbiotic N nutrition and photosynthetic carbon assimilation in cowpea genotypes grown under field conditions in the Guinea savanna of Ghana. The experiments were conducted in a randomized complete block design in Ghana, using 30 nodulated cowpea genotypes. Data collected from gas-exchange studies showed that genotypes with high photosynthetic rates expectedly exhibited greater stomatal conductance, high transpiration rates and increased water-use efficiency, leading to greater dry matter yield. In contrast, those with low photosynthetic rates revealed low stomatal conductance, low transpiration rates and low water-use efficiency. Quantification of symbiotic N in photosynthetic leaves indicated greater N₂ fixation in genotypes with higher photosynthetic rates, leaf transpiration and better water-use efficiency. Furthermore, measurement of C in leaves also indicated greater accumulation in genotypes with higher symbiotic N yield and total N, clearly indicating a link between C and N metabolism in photosynthetic leaves. Additionally, measures of photosynthetic N-use efficiency showed that genotypes with high levels of C-fixed per unit leaf total N also exhibited greater C-fixed per unit leaf N-fixed. Data suggest that where symbiotic legumes derive a large proportion of their N from N₂ fixation, photosynthetic C yield is much enhanced.