Three cassava (Manihot esculenta) genotypes were grown in greenhouse conditions and subjected to water deficit treatments to: (a) assess the extent to which abscisic acid (ABA) and osmotically active solutes accumulate in cassava leaves during water deficit and solute contributions to osmotic adjustment (OA), and (b) determine whether cassava leaves substantially increase their content of dehydrin-like polypeptides during water deficit. The concentration of ABA, sugars (sucrose, glucose, and fructose), proline, and potassium salts (K-salts) were determined in mature and expanding leaves. ABA content increased under water deficit, consistent with a putative role in regulating solute accumulation, and thus OA. The extent of osmotic adjustment, although modest, was higher in young folded leaves than in mature leaves. K-salts were the major contributors to total osmolyte concentration in both mature and expanding leaves, accounting for approximately 60% of osmotic potential. The concentration of K-salts increased in response to water stress and was positively correlated with the extent of OA. In contrast, total sugars (sucrose + glucose + fructose) decreased during water deficit, showing a negative correlation with OA. Although the concentration of proline in mature leaves increased in response to water stress its contribution to the total change in osmotic potential was insignificant. Thus, in the current study, cassava used K-salts as its primary osmolyte, and did not substantially adjust its osmotic solute concentration during the 6-day water deficit. Expression of dehydrins was not found in any of the genotypes or leaf stages. (C) 2003 Elsevier B.V. All rights reserved.
