Traits That Differentiate Drought Tolerant and Susceptible Genotypes Of Cassava: Growth Partitioning, Reserve Carbohydrates, Abscisic Acid and Leaf Water Loss.

Cassava (Manihot esculenta Crantz.) is a storage root crop of importance in tropical regions where periodic dry season and drought affect performance.  Cassava genotypes that differ in performance in ecosystems with various water regimes were subjected to water stress (WS) during storage-root initiation and early development.  Two experiments were conducted: the first with 15 lines in screen house conditions; the second with 45 lines growing in pots in the field.  Relative water potential remained high during WS while growth was drastically decreased.  Leaf temperature data indicated that tolerant lines closed stomata to a greater extent than susceptible lines, due to stomatal sensitivity, as the leaves of tolerant lines accumulated a higher concentration of ABA during stress.  Tolerant lines had higher leaf retention after 60 d of WS.  Tolerant lines had higher starch levels in leaves and stems, while fibrous roots had higher sucrose levels. Tolerant genotypes had a higher partitioning index than susceptible genotypes during the WS, associated with a larger number of storage roots initiated and larger storage root biomass, while they were shorter and had less fibrous root biomass.  Tolerant lines were indistinguishable from susceptible lines in above ground biomass. These findings suggest that early evaluation of storage root number, partitioning index, and associated traits at an early stage of cassava storage-root development could be an effective approach by which cassava genotypes are screened for favorable drought tolerance response.