Cassava Response to Water Deficit in Deep Pots: Root and Shoot Growth, ABA, and Carbohydrate Reserves in Stems, Leaves and Storage Roots

Cassava (Manihot esculenta, Crantz) is an important staple crop for tropical climates worldwide, including drought-prone environments where it is valued for its reliable yield. The extent to which stress tolerance involves regulation of growth and carbon balance aided by remobilization of carbohydrate from various plant parts was investigated. Plants were grown in 1-meter high pots to permit observation of deep rooting while they were subjected to four soil water regimes over a 30-d period. Transpiration declined abruptly in conjunction with leaf ABA accumulation and severe leaf abscission. In water stressed plants, growth of all plant parts decreased substantially; however, a basal rate of leaf growth continued to provide some new leaves, and although growth of fibrous lateral roots was reduced, main root elongation to deeper regions was only modestly decreased by stress. In leaf blades and petioles, sugars were the predominant form of nonstructural carbohydrate and about one third was in starch; these reserves were depleted rapidly during stress. In contrast, stems and storage roots maintained relatively high starch concentrations and contents per organ until final harvest. Stems gradually lost starch and had sufficient reserves to serve as a prolonged source of remobilized carbohydrate during stress. The amount of starch stored in stems represented about 35 % of the reserve carbohydrate in the plant at the onset of water stress (T₀), and 6 % of total plant dry mass. We suggest that this pool of carbohydrate reserves is important in sustaining meristems, growing organs, and respiring organs during a prolonged stress and providing reserves for regrowth upon resumed rainfall.