Water deficit effects on solute contribution to osmotic adjustment as a function of leaf ageing in three durum wheat (Triticum durum Desf.) cultivars performing differently in arid conditions.

A greenhouse study was carried out using three durum wheat (Triticum durum Desf.) cultivars differing in their field performances under arid conditions (Kabir 1, poor yield stability; Omrabi 5, high yield stability and Haurani, landrace well adapted to drought). Water stress was imposed by withholding water at the seedling stage. Water potential (Psi(w)), relative water content (RWC), stomatal resistance (SR), and changes in solute concentrations were quantified: (1) as a function of leaf development during the stress period; and (2) in young expanded and growing leaves harvested at the end of the stress treatment. Psi(w), RWC and SR were almost unaffected by leaf age in controls. In contrast, solute concentrations appeared to vary in the course of leaf development. During the stress treatment, Psi(w) and RWC decreased and SR increased in all cultivars; the changes were most often largest in Omrabi 5, lowest in Haurani and intermediate in Kabir 1. Water stress also increased sugar and proline concentrations and decreased nitrate levels. Young expanded and growing leaves differed in terms of Psi(w), RWC and osmotic adjustment (OA). The capacity of OA was greater in growing than in expanded leaves, especially in the two cultivars best adapted to aridity, and allowed turgor maintenance in these genotypes. Sugars were the main solutes that contributed to OA particularly in growing leaves followed by proline and then quaternary ammonium compounds. The contributions of these organic solutes to OA tended to be higher in Omrabi 5 and in Haurani than in Kabir 1. Inorganic solutes, however, did not seem to play an important role in OA despite their high proportion in total solutes.