Variation in transpiration efficiency and carbon isotope discrimination incowpea

Genotypic variation in transpiration efficiency (TE) was investigated in a set of cowpea (Vigna unguiculata (L.) Walp.) genotypes grown as isolated pl ants 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 w as 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 an d dry matter (DM) produced by the genotypes during the treatment period. Ge notypes differed significantly in DM although the variation in the amount o f 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 (Delta) across all the experiments and treatm ents. A significantly positive correlation of TE measured in pot and field experiments suggested a low G x E interaction. There was a negative correla tion (r = -0.62, P < 0.01) between T and TE while the correlation of TE wit h net assimilation rate was non-significant, suggesting that the major caus e for variation of TE in cowpea was in general associated with stomatal rat her than mesophyll factors. Because of the positive relationship between T and DM, and negative relationship between TE and T, selection for high TE m ight therefore be associated with reduced T and hence lower DM. However, th e present investigation showed a possibility of identifying specific genoty pes with a combination of high TE and high net assimilation rate. The genot ypes with high TE and high net assimilation rates were able to produce high DM under moisture deficit conditions.