Abstract
Common bean (Phaseolus vulgaris L.) is the most important food legume, cultivated by small farmers and is 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 smallholder systems. Beans can derive part of their required N from the atmosphere through symbiotic nitrogen fixation (SNF). Drought stress severely limits SNF ability of plants. The main objectives of this study were to: (i) test and validate the use of 15N natural abundance in grain to quantify phenotypic differences in SNF ability for its implementation in breeding programs of common bean with bush growth habit aiming to improve SNF, and (ii) quantify phenotypic differences in SNF under drought to identify superior genotypes that could serve as parents. Field studies were conducted at CIAT-Palmira, Colombia using a set of 36 bean genotypes belonging to the Middle American gene pool for evaluation in two seasons with two levels of water supply (irrigated and drought stress). We used 15N natural abundance method to compare SNF ability estimated from shoot tissue sampled at mid-pod filling growth stage vs. grain tissue sampled at harvest. Our results showed positive and significant correlation between nitrogen derived from the atmosphere (%Ndfa) estimated using shoot tissue at mid-pod filling and %Ndfa estimated using grain tissue at harvest. Both methods showed phenotypic variability in SNF ability under both drought and irrigated conditions and a significant reduction in SNF ability was observed under drought stress. We suggest that the method of estimating Ndfa using grain tissue (Ndfa-G) could be applied in bean breeding programs 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 drought stress and these could serve as potential parents to further improve SNF ability of common bean.
Highlights
Common bean (Phaseolus vulgaris L.) is the most important food legume cultivated in the tropics of Latin America and eastern and southern Africa
If the %nitrogen derived from the atmosphere (Ndfa) in grain is closely related with %Ndfa in shoot, legume breeders would be able to select for symbiotic nitrogen fixation (SNF) based on the grain values without the need to harvest, dry and grind large volumes of shoot biomass
A significant and positive correlation values of r = 0.81*** in 2012 and r = 0.66*** in 2013 (r = 0.83*** for combined data for two seasons, Table 1) were observed between the %Ndfa values estimated with 15N natural abundance of shoot biomass at mid-pod filling growth stage and %Ndfa values estimated with 15N natural abundance in the grain under irrigated conditions; the correlation values were significant and positive under drought conditions r = 0.67*** in 2012 and r = 0.74*** in 2013 (r = 0.71*** for combined data for two seasons, Table 1)
Summary
Common bean (Phaseolus vulgaris L.) is the most important food legume cultivated in the tropics of Latin America and eastern and southern Africa. It is nutritionally rich in iron and protein, and is a source of fiber and carbohydrates that are essential in the nutrition of the population, especially in developing countries. The bean crop is cultivated by small farmers and it is often exposed to unfavorable conditions with minimum use of inputs (Beebe et al, 2013, 2008). Bean yields are affected by various biotic and abiotic stress factors. Drought affects about 60% of the bean producing regions causing yield losses between 10 and 100%.
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