Evaluation on drought tolerance in sweet potato and cassava

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This study aimed to examine the root system development and plasticity in sweetpotato and cassava in relation to their drought tolerance. Among the huge genetic stock developed or collected so far, several genotypes for each species were selected for this study. Specifically, we have conducted a series of pot and root box experiments to evaluate the root system development in both species under high root-zone temperature stress and drouht stress imposed at different growth stages, for various periods and degrees. Major findings were : 1) high root-zone temperature promoted the shoot growth, while it inhibited the whole root system development in general, but development in the lateral roots of high branching orders (2nd and 3rd) was promoted ; 2) both species were very sensitive to drought stress which generally reduced shoot and root growth. However, rewatering after drought promoted especially the development of lateral roots of high branching orders ; 3) genotypic differences were also noted in the responses to drought. There was a genotype in each species that maintained or increased root growth under drought (cv. VSP-2 for sweetpotato and cv. PSR-74 for cassava). Besides, four farmers' fields in Leyte, Philippines, planted with different sweetpotato varieties were visited, in which we surveyed the root system development and soil physical and chemical conditions so that the results of the pot experiments be verified. In addition, field experiments are now being attempted to evaluate the drought tolerance including water relations and the root system development in the field. For the purpose, a rain shelter is being constructed at the moment. As such, in this study, we showed the development and responses of the root system to the soil stresses which are the least-studied part of the root crops. Efforts still continue to complete the experiments and survey, and organize the data for publication.