Estimation of phenotypic variability in symbiotic nitrogen fixation ability of common bean under drought stress using 15N natural abundance in grain
tCommon bean (Phaseolus vulgaris L.) is the most important food legume, cultivated by small farmers andis usually exposed to unfavorable conditions with minimum use of inputs. Drought and low soil fertility,especially phosphorus and nitrogen (N) deficiencies, are major limitations to bean yield in smallholdersystems. Beans can derive part of their required N from the atmosphere through symbiotic nitrogenfixation (SNF). Drought stress severely limits SNF ability of plants. The main objectives of this study wereto: (i) test and validate the use of15N natural abundance in grain to quantify phenotypic differences in SNFability for its implementation in breeding programs of common bean with bush growth habit aiming toimprove SNF, and (ii) quantify phenotypic differences in SNF under drought to identify superior genotypesthat could serve as parents. Field studies were conducted at CIAT-Palmira, Colombia using a set of 36 beangenotypes belonging to the Middle American gene pool for evaluation in two seasons with two levelsof water supply (irrigated and drought stress). We used15N natural abundance method to compare SNFability estimated from shoot tissue sampled at mid-pod filling growth stage vs. grain tissue sampledat harvest. Our results showed positive and significant correlation between nitrogen derived from theatmosphere (%Ndfa) estimated using shoot tissue at mid-pod filling and %Ndfa estimated using graintissue at harvest. Both methods showed phenotypic variability in SNF ability under both drought andirrigated conditions and a significant reduction in SNF ability was observed under drought stress. Wesuggest that the method of estimating Ndfa using grain tissue (Ndfa-G) could be applied in bean breedingprograms to improve SNF ability. Using this method of Ndfa-G, we identified four bean lines (RCB 593,SEA 15, NCB 226 and BFS 29) that combine greater SNF ability with greater grain yield under droughtstress and these could serve as potential parents to further improve SNF ability of common bean.