Crop adaptation to edaphic constraints has focussed largely on increasing boron (B) tolerance in cereals, targeted to alkaline soils with high boron content. However, recent studies have implicated several other physicochemical constraints, such as salinity and sodicity, in reduced grain yields of cereals by restricting water extraction in the subsoil. Consequently, the value of B-tolerance may be limited on soils where multiple constraints exist. To test the contribution of B-tolerance where multiple constraints exist, near-isogenic lines of wheat and barley differing in B-tolerance were used, where growth and water extraction by crops in large intact cores, extracted from a Calcarosol profile, were measured. The effect of subsoil disturbance (deep ripping) and growing-season water supply was also investigated. Use of B-tolerant crops did not increase use of subsoil water or grain yield. Wheat and barley extracted soil water down to 0.6 m depth but not below 0.8 m. The soil B concentration of these 2 layers was equivalent (29 ν. 31 mg/kg), whereas salinity [(ECe) 7.2 ν. 8.1 dS/m] and sodicity [(ESP) 22 ν. 29%] both increased significantly with depth, implying that these 2 latter properties had a greater effect than B. Deep ripping with gypsum had no effect on grain yield. Wheat and barley grown under high water supply outyielded their counterparts grown under low water supply, although grain yield per unit of applied water for the crops under low water was 1.5 times that of the crops under high water regime. The results suggest that high salinity and sodicity, rather than B, were exerting the major effects on water extraction of wheat and barley from the deep subsoil, thus negating the effect of crop B-tolerance where multiple constraints exist. This highlights the need to breed cultivars with increased sodium tolerance, pyramided with current B-tolerance, for those crops targeted to many alkaline soils.
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