Factors determining stomatal and non-stomatal (residual) transpiration and their contribution towards salinity tolerance in contrasting barley genotypes

Eighty barley (Hordeum vulgare L.) genotypes of different geographical origin and contrasting in salinity stress tolerance were grown under glasshouse conditions and exposed to high salinity stress (300 mM NaCl) for four weeks to investigate the relationship between leaf gas exchange, tissue ionic relations, and plant salinity tolerance. Four weeks after the treatment commenced, stomatal conductance, stomatal density, residual transpiration, chlorophyll content, leaf sap Na, K, Cl and leaf sap osmolality were measured. Responses to salinity stress differed greatly among the genotypes. The overall salinity tolerance significantly correlated with leaf Na⁺ content, osmolality and the residual transpiration. At the same time, no significant correlation between salinity tolerance and stomatal conductance was found. The residual transpiration in stressed plants correlated negatively with the leaf sap osmolality of control plants. A significant correlation was found between changes in the residual transpiration and changes in leaf Cl⁻ content but no such correlation was found for leaf Na⁺. Higher stomatal density was correlated with higher osmolality under both salinity stress and control conditions. The stomatal density correlated negatively with the residual transpiration under salinity stress conditions but positively with K⁺ accumulation in the shoot under both control and salinity stress conditions. Higher relative stomatal conductance correlated with higher residual transpiration and lower stomatal density under salinity stress. Interestingly, stomatal conductance correlated very strongly with Cl⁻ accumulation in the shoot under stress conditions but negatively under control conditions. Taking together, these results suggest that increasing stomatal density as well as minimization of the residual transpiration may be a promising way of improving water use efficiency and increase salinity tolerance in barley. Our data also show that residual transpiration is strongly affected by the number of stomatal pores on the leaf surface.