Durum and Bread Wheat Differ in Their Ability to Retain Potassium in Leaf Mesophyll: Implications for Salinity Stress Tolerance

Understanding the intrinsic mechanisms involved in the differential salinity tolerance between bread wheat and durum wheat is essential for breeding salt-tolerant varieties to cope with the global salinity issue threatening future food supply. In the past, higher salinity tolerance in bread wheat compared with durum wheat has been attributed to its better ability to exclude Na+ from uptake. Here we show that another mechanism, namely more superior K+ retention ability in the leaf mesophyll, also contributes to this difference. A strong positive correlation (R-2 > 0.41, P < 0.001) was found between NaCl-induced K+ efflux in the leaf mesophyll and overall salinity tolerance in 48 wheat varieties. However, while the above correlation was strong in bread wheat, it was statistically insignificant in durum wheat. Consistent with these findings, a significantly higher relative leaf K+ content was found in bread wheat than in durum wheat. In contrast to root tissues, the role of voltage-gated K+ channels in K+ retention in the wheat mesophyll was relatively small, and non-selective cation channels played a major role in controlling intracellular K+ homeostasis. Moreover, a significant negative correlation between NaCl-induced mesophyll H+ flux and mesophyll K+ retention was found, and interpreted as a compensatory mechanism employed by sensitive varieties to regain K+ leaked into the apoplast. It is concluded that bread wheat and durum wheat show different strategies of coping with salinity, and that targeting mechanisms conferring K+ retention in the leaf mesophyll may be a promising way to improve the overall salinity tolerance in these species.