Physiological and developmental changes in cassava (Manihot esculenta Crantz) under water deficit

Cassava (Manihot esculenta Crantz) is an important food crop in the tropics. It is widely used in drought-prone climates, but little is known about the mechanisms by which cassava responds to water deficit environments.;Cassava genotypes from five climatic zones of Brazil were grown in greenhouse conditions and subjected to water deficit and recovery treatments during the vegetative growth stage. In response to water deficit, cassava leaves rapidly accumulated the plant hormone abscisic acid (ABA) and correspondingly halted leaf expansion growth. Young leaves accumulated more ABA than mature leaves. Recovery was rapid: one day after rewatering, ABA concentrations returned to control levels. Other studies indicated that stomatal closure also occurred early during water deficit and recovered rapidly after rewatering.;To further evaluate the water deficit response with respect to leaf area growth, leaves at 15 nodal positions, representing several developmental stages from expanding to meristematic, were sampled at 2-day intervals during stress and recovery. Water deficit decreased leaf area growth due to inhibition of both cell expansion and cell division, with about equal sensitivity. In addition, the rate of new leaf production decreased during stress, accounting for about 40% of the leaf area reduction due to the stress. The extent to which the three component processes--cell expansion, cell division, and leaf initiation--were inhibited in a particular leaf nodal position corresponded with that leaf's concurrent developmental stage during the 8-day stress. All three processes recovered, partially, after relief of stress.;The role of osmotic adjustment (OA) in cassava drought response was evaluated by measuring the osmotic component of leaf water potential in mature, expanding, and folded leaves. Mature and expanding leaves expressed lower osmotic adjustment than folded leaves. The solutes primarily responsible for OA were potassium salts, whereas sugars (sucrose + glucose + fructose) and proline did not contribute to OA. Expression of dehydrins was not found in any genotypes or leaf stages.;I conclude that cassava responds to water deficit by rapidly closing stomata, thus maintaining high water potential with minimal osmotic adjustment. It halts leaf growth during the stress, but rapidly recovers after rainfall resumes.