Abstract
We used a water-centred framework (yield = transpiration x transpiration efficiency x harvest index) to investigate the effect of soil salinity on growth and yield of wheat and barley. Our working hypothesis is that salinity reduces transpiration proportionally more than transpiration efficiency. We established a glasshouse experiment with the factorial combination of four varieties (wheat: Janz, Krichauff; barley: Mundah, Keel) and three soil treatments: a control with no NaCl added, and NaCl added to achieve soil EC1:5 0.75 dS m(-1) and 1.5 dS m(-1). Pot-grown plants were watered to weight to determine transpiration and shoot dry matter was determined using a non-destructive image analysis system. Consistent with our hypothesis, salinity reduced transpiration (30-60%) proportionally more than transpiration efficiency (0-35%); transpiration accounted for 90% of the variation in shoot growth across varieties and treatments. Against this pattern, there were time- and variety-dependent responses. The rate of leaf appearance and the transpiration efficiency of Janz, Krichauff and Keel showed a two-stage response to salinity. In stage 1, salt-stressed plants maintained rate of leaf appearance and transpiration efficiency close to or slightly below those of the controls. After a clear break point where the slope changed, stage 2 was characterised by a substantial reduction in both traits. Stage 2 was not evident in salt-stressed Mundah, which maintained a relatively high rate of leaf appearance and transpiration efficiency. Across species, harvest index increased from 0.40 in controls to 0.47 at 0.75 dS m(-1). Harvest index of plants grown at 1.5 dS m(-1) was unaffected in wheat, and was reduced in barley. We propose that an understanding of the effect of salinity on crop development, growth and yield requires integration of low-level traits in a framework of resource capture, resource-use efficiency and plant allocation. Osmotic stress tolerance, Na+ exclusion, and tissue tolerance to accumulated Na+ would improve yield of salt-stressed crops to the extent that these traits contribute to the maintenance of water uptake and harvest index.