Variation in transpiration efficiency and carbon isotope discrimination in cowpea

Genotypic variation in transpiration efficiency (TE) was investigated in a set of cowpea (Vigna unguiculata (L.) Walp.) genotypes grown as isolated plants in pots and under canopy conditions in the field. In the field, plants were grown in mini-lysimeters embedded in the ground around which a crop was grown, to simulate crop canopy condition. Two moisture regimes (100 and 60% of field capacity) were imposed from 30 to 60 days after sowing in both pot and field experiments. TE was determined by measuring transpiration and dry matter (DM) produced by the genotypes during the treatment period. Genotypes differed significantly in DM although the variation in the amount of water transpired (T) was relatively small. The TE ranged from 2.2 to 3.7 g kg–1, representing a significant genotypic and environmental effect on the variation. There was a significant negative correlation (r = –0.77, P<0.01) between TE adjusted for prevailing vapour pressure deficit and carbon isotope discrimination ratio (Δ) across all the experiments and treatments. A significantly positive correlation of TE measured in pot and field experiments suggested a low G x E interaction. There was a negative correlation (r = –0.62, P < 0.01) between T and TE while the correlation of TE with net assimilation rate was non-significant, suggesting that the major cause for variation of TE in cowpea was in general associated with stomatal rather than mesophyll factors. Because of the positive relationship between T and DM, and negative relationship between TE and T, selection for high TE might therefore be associated with reduced T and hence lower DM. However, the present investigation showed a possibility of identifying specific genotypes with a combination of high TE and high net assimilation rate. The genotypes with high TE and high net assimilation rates were able to produce high DM under moisture deficit conditions.