Cassava Drought Tolerance Mechanisms Re-Visited: Evaluation Of Drought Tolerance In Contrasting Cassava Genotypes Under Water Stressed Environments

Abstract

Cassava (Manihot esculenta Crantz) is a perennial root crop from the neo-tropics, which is considered a food security crop against famine in drought prone areas. The objectives of this work were to identify traits in cassava that contribute to drought tolerance and evaluate the potential to use these traits in cassava breeding programs. Two sets of cassava genotypes containing 45 and 15 lines were used to assess drought response under field and screen house conditions. Morpho-physiological traits evaluated included sugar and starch contents in leaves and stems, abscisic acid (ABA) accumulation, leaf retention (LR), relative water content (RWC), leaf canopy temperature (T), leaf-air temperature difference (Td), leaf chlorophyll greenness (CG) and root growth, among others. In field-grown potted plants at the early phase of water deficit, leaf ABA in the afternoon was positively correlated with soil water content and negatively correlated with T and Td in the morning, suggesting that genotypes with stomatal closure in the afternoon conserved water, permitting morning stomatal opening at DAY 30 of water deficit. Storage root weight was not correlated with aboveground fresh biomass (AGB), whereas it was strongly correlated with partitioning index. Broad sense heritability and phenotypic standard deviation were sufficiently high to predict that response to selection would be successful for several traits including PH, T, Td, CG, and PI. Genotypes CM 3306-9 and MBRA 165 ranked high for yield and partitioning index in both environments. Sampling a set of 15 genotypes during the progression of stress onset showed that fibrous roots and stems accumulated ABA at an early stage, while leaf and stem ABA were negatively correlated with carbohydrate accumulations. Leaves in both control and water stress treatments maintained high RWC. Root and stem carbohydrate reserves were gradually depleted during stress. Genotypes previously characterized as stress tolerant had a smaller reduction in biomass when compared to their susceptible counterparts. I conclude that several of the traits examined could be useful in phenotyping genotypes for drought tolerance.