| Increasing common bean (Phaseolus vulgaris L.) yields in drought prone environments requires a screening tool to identify resistant genotypes other than yield differences at harvest time. Both carbon isotope discrimination (CID) and osmotic adjustment (OA) have been suggested as screening criteria. However, the relationships of CID and OA to yield and its determinants (water use efficiency (WUE), harvest index (HI), and water use) under stress have not been elucidated. Two resistant and one susceptible genotypes were subjected to a vegetative water stress from 5-leaf stage (V5) to first flower (R1) and to reproductive water stress from R1 to harvest from 1991 to 1993. Seed yield varied from 28 to 352 g m-2. Vegetative stress reduced yield by 15 to 30% while reproductive stress reduced yield by 35 to 50% with one of the resistant genotypes producing a significantly greater yield than the susceptible genotype under reproductive stress. Reproductive stress significantly reduced CID by 9%. The drought susceptible genotype had 8 to 9% higher CID than the resistant genotypes under all water regimes. Similarly, the drought susceptible genotype had a higher level of osmotic adjustment under the reproductive stress. Relative reductions in biomass and water use efficiency strongly and positively correlated with relative CID reductions in both stress treatments. CID strongly and positively correlated with pod m-2 (r2 = 0.81, p < 0.01, n = 9). In addition, osmotic adjustment (r2 = 0.91, p < 0.01, n = 6) strongly and negatively correlated with yield as well as with yield determinants under the reproductive stress. It is concluded that both carbon isotope discrimination and osmotic adjustment could be useful screening criteria for selections in drought resistance studies but need further verification with more diverse genotypes. |
