Alterations in stem sugar content and metabolism in sorghum genotypes subjected to drought stress

Changes in stem sugar concentrations due to drought stress at the early reproductive stage were studied in seven sorghum (Sorghum bicolor (L.) Moench) genotypes that differ in their stem sugar storage ability. Total sap sugar concentration increased in most genotypes. ANOVA showed a significant contribution of genotype and treatment to the variation in sugar levels. Two genotypes showed little variation in total sugar levels at the fifth internode from the peduncle and five genotypes showed significant increases in total sugar levels under drought; these groups were used to compare sugar metabolism. Drought led to a decrease in catabolic sucrose synthase activity in both groups. Invertase activities increased significantly in two genotypes and correlated with the increase in reducing sugar concentrations under drought. Stem sugar hydrolysis probably had a role in osmotic adjustment under drought and correlated with retention of sap volume. However, the activities of sugar-metabolising enzymes did not correlate with their gene expression levels. After resuming irrigation, grain yields, stalk yields and juice volume at physiological maturity were lower in plants recovering from drought stress compared with the controls. In some genotypes, there were similar losses in grain yields and stem sugars due to drought, indicating photoassimilate source limitation; in other genotypes, grain yield losses were less than stem sugar losses in drought-exposed plants compared with the controls, suggesting mobilisation of sugars from the storage internodes to the developing panicle. Accumulation of stem sugars appears to be an adaptive strategy against drought stress in some sorghum genotypes.