Physiological and molecular analyses of seedlings of two Tunisian durum wheat (Triticum turgidum L. subsp Durum [Desf.]) varieties showing contrasting tolerance to salt stress

Salinity is one of the severest environmental stresses affecting plant productivity. In many plant species, salt sensitivity is associated with the accumulation of sodium (Na(+)) in photosynthetic tissues. Here, we provide the physiological and molecular analyses of seedlings of two Tunisian durum wheat genotypes (Triticum turgidum L. subsp. Durum [Desf.]), Mahmoudi (salt sensitive) and Om Rabia3 (salt tolerant). Na(+) and K(+) contents in leaf sheath from Om Rabia3 were significantly higher than those of Mahmoudi. However, the net uptake of Na(+) from the soil occurred at similar rates in both varieties, suggesting that Om Rabia3 has much stronger ability to limit Na(+) flux from roots to leaf blades. This mechanism could be explained by a capacity of Om Rabia3 to retain higher Na(+) concentration in leaf sheath and unload less Na(+) from the xylem to the upper shoots. When treated with 100 mM NaCl leaf sheaths of Om Rabia3 developed lower water potentials and a higher relative water contents than those of Mahmoudi. These features may arise from enhanced osmotic adjustment in Om Rabia3. Measurements of stomatal conductance, free proline and chlorophyll content also indicate that Om Rabia3 is better adapted to tolerate high salt than Mahmoudi. A correlation was obtained between the expression pattern of TaSOS1 (a plasma membrane Na(+)/H(+) antiporter) in the roots and sheaths of both wheat varieties and the Na(+) fluxes from roots to leaves. TaSOS1 transcript accumulated in Mahmoudi than in Om Rabia3, suggesting repression of TaSOS1 in the tolerant variety that reduces loading of Na(+) to the upper shoots. These results help to design new genetic screens for salt tolerance in wheat.