Genetic Studies of Physiological and Morphological Traits Associated with Drought Tolerance in Cassava Genotypes

Abstract

This study was undertaken to identify key traits that are closely related to cassava yield under drought stress and also identify stable high yielding cassava genotypes under varying environments. A survey was conducted using in semi-structured questionnaire in three districts in the Northern Region involving 120 farmers to identify farmers’ perception on drought in cassava cultivation, production constraints, mitigation strategies and preferences for improved cassava genotypes. To identify ideal genotypes that meet farmers’ preferences, 150 cassava genotypes from local and exotic sources were assembled and assessed for diversity using morphological traits and simple sequence repeat markers. Subsequently 20 genotypes were selected and evaluated under irrigation and no irrigation to assess genetic variability in abscisic acid content, carbon isotope ratio, stomatal conductance, leaf temperature and root yield. Stability of genotypes for physiological and yield traits were also assessed using the additive main effect and multiplicative interaction (AMMI) and GGE biplot analyses. The survey indicated lack of credit as the most important constraint facing cassava cultivation in the region. Drought was the second most important constraint and the intensity was observed to be increasing. Majority of the farmers also preferred early maturing cassava varieties that are high yielding with good plant type and marketability. Factorial analysis of the morphological traits grouped the genotypes based on their origin with few exceptions. Principal component analysis further identified plant height, branching habit, distance between leaf scars, colour of end branches, root yield, harvest index and number of roots per plant as the traits contributing most of the variability in the groups. High level of heterozygosity was revealed by the simple sequence repeat markers which grouped the genotypes into seven distinct clusters irrespective of sources. Genetic variability was established for abscisic acid content which was higher under stress than irrigation. ABA content was negatively correlated with root yield, harvest index and above ground biomass yield meaning it can be used as indirect selection criteria against unproductive genotypes. Narrow genetic variation was observed for carbon isotope ratio which was higher under irrigation than no irrigation. Carbon isotope ratio was positively correlated with above ground biomass yield but not root yield. Stomatal conductance and leaf temperature were significantly different among genotypes and environments but genotype x environment interaction was not significant. Broad sense heritability estimates were high for most of the traits except stomatal conductance, above ground biomass yield, root number and stem diameter. AMMI analysis of plant height, severity of cassava mosaic disease, root yield, root length/girth ratio, above ground biomass yield and harvest index indicated stronger effect of environment than genotypes for all traits except CMD. The study established for the first time relationship between ABA content and cassava root yield on the field. Extension of roots to lower soil depts (L/G ratio) was also found to be detrimental to storage root yield. It was also found from this study that carbon isotope ratio influence above ground biomass and not storage root yield under stress conditions. Based on AMMI selections and the GGE biplot analysis, three genotypes, MM96/1751, UCC2001/449 and 00/0203 were identified as high yielding and stable across environments. These can be used as donor parents in improving local farmer-preferred varieties. Six genotypes (UCC2001/449, 96/1708, MM96/1751, 00/0203, 96/409 and I91934) had significantly higher root yield than the best local farmer preferred variety. These can be tested on-farm for official release to farmers for cultivation.