Effect of salinity on water relations of wild barley plants differing in salt tolerance

Background and aims Certain lines of wild barley (Hordeum spontaneum) are more tolerant of salinity than others. The physiological basis of this difference is examined in a comparative study of a saline-tolerant and saline-intolerant line that emphasizes plant water relations. Methodology Effects of salt-treatment (75 mM maximum) extending from a few hours to 3 weeks were quantified in 8-day-old seedlings of a saline-sensitive wild barley line ('T-1') and a less saline-sensitive line ('20-45'). Plants were grown in nutrient culture. Levels of mRNA of the HtPIP2; 4 aquaporin (AQP) gene were determined together with a range of physiological responses including root hydraulic conductivity, osmotic potential of root xylem sap, transpiration, leaf relative water content, root water content, leaf water potential, leaf sap osmolality, leaf length, leaf area and chlorophyll content. Principal results Salt treatment inhibited transpiration and hydraulic conductivity more in salt-tolerant '20-45' plants than in salt-sensitive 'T-1'. In '20-45', the effect was paralleled by a fast (within a few hours) and persistent (3 days) down-regulation of aquaporin. In salt-sensitive 'T-1' plants, aquaporin down-regulation was delayed for up to 24 h. Greater tolerance in '20-45' plants was characterized by less inhibition of leaf area, root fresh weight, leaf water content and chlorophyll concentration. Leaf water potentials were similar in both lines. Conclusions (i) Decline in hydraulic conductivity in salt-treated barley plants is important for stomatal closure, (ii) lowered transpiration rate is beneficial for salt tolerance, at least at the seedling stage and (iii) changes in AQP expression are implicated in the control of whole plant hydraulic conductivity and the regulation of shoot water relations.