Contribution of different genotypic roots to drought resistance in soybean by a grafting experiment

Knowledge on the existing root diversity in germplasms has been emphasized for soybean breeding to improve drought resistance. Yet, evaluation of root diversity for vast genotypes still remains challenging. Here we investigated the contribution of different genotypic roots to drought resistance by a grafting experiment. A total of 22 cultivars were used as rootstocks to graft with scions of drought-sensitive cultivar L21. Water withholding was imposed during 15 days at podding stage in pot-culture experiments. The root-related traits were measured under well-watered (WW) and water-stressed (WS) conditions. At maturity, the drought-resistance coefficient (DRC) was calculated as the yield under WS condition relative to WW yield. The DRC varied from 0.56 to 0.85 among grafts; thus, the rootstocks were categorized as drought-sensitive (DS), moderate (M) and drought-resistant (DR) types. The L21 scion grafted onto DR rootstocks resulted in higher DRC and greater drought resistance than those self-grafts. There were also significant interactions between water stress and genotype for root-related traits. With the increase of DRC, the plants showed linear increases in root length, root surface area, and root volume, but had linear decreases in root dry mass, root-shoot ratio and root tissue density under WS rather than WW conditions. We suggested that developed rooting with a low investment on dry matter identified by grafting experiments should be selected to improve drought resistance in soybean breeding.