Root properties and proline as possible indicators for drought tolerance in soybean
Use of an efficient selection tool in breeding for complex traits such as soil water-limitedinduced- stress (WLIS) tolerance is of paramount importance in order to reduce the genotype by environment interaction effects that affect the heritability of the trait and reduce the associated cost implications of testing in multiple environments. Hence this study was carried out in order to determine the response of drought tolerant and -sensitive soybean genotypes to soil WLIS conditions at different growth stages. Furthermore, the study evaluated seedling root properties and proline accumulation as possible selection criteria for soil WLIS in soybean. The association between proline accumulation and yield-based drought tolerance indices was investigated. Results showed that different soil WLIS levels had varied effects on the morphological traits measured. The severe (30% deficit irrigation) and moderate (50% deficit irrigation) soil WLIS treatments drastically affected the morphological traits. Soil WLIS stress imposed during poddevelopment growth stage affected the seed yield of soybeans much more than drought stress imposed during the flowering and vegetative growth stages. Soil WLIS reduced soybean seed yield, 100-seed mass, seed mass, seed number per plant, number and mass of pods per plant, plant height, node number and plant biomass among the morphological traits measured. Significant differences were observed among genotypes for seed yield and seed mass per plant in response to soil WLIS. Seed mass mean difference is the relative performance of a genotype in a soil WLIS in contrast to a non-stress environment. Seed mass mean difference proved to be a useful index for soil WLIS tolerance, as tolerant genotypes were discriminated from the sensitive ones. Tolerant genotypes showed lower reduction in seed mass per plant after severe WLIS, compared with that of sensitive genotypes. Soybean genotypes showed varied seedling shoot- and root morphology in response to soil WLIS. Seedling root traits with potential use as soil WLIS tolerance indicators include tap root length to shoot ratio and deep rooting ability. Drought tolerant genotypes showed deep rooting ability and larger root-to-shoot ratios compared to the drought sensitive genotype. This is a coping mechanism of drought tolerant genotypes to soil WLIS. Furthermore, the adopted ‘deep-pot’ system that was used to phenotype the roots of the soybeans was effective in studying the root system and is recommended for similar studies. Similarly, soil WLIS had a significant effect on proline accumulation amongst the soybean genotypes. Proline concentration tended to increase with an increase in soil WLIS. It was observed that proline concentration tended to increase much more in the tolerant genotypes than the sensitive genotypes. This was true for both the controlled glasshouse (pot and lysimeter) and field experiments. Results also showed that proline accumulation under soil WLIS conditions correlated positively with yield potential (Yp and Ys) as well as the tolerance indices geometric mean productivity (GMP), mean productivity (MP) and tolerance index (TOL). This suggested that proline is a useful index in screening for drought tolerance in soybean. The proline determination essay proved to be easy and cost-effective as multiple samples from different sites can be evaluated at the same time since it only requires few leaf samples to be carried out.