Possible role of soil alkalinity in plant breeding for salt-tolerance

Bennett et al. [1] present the phylogenetic distribution of salt-tolerant species on a phylogeny of 2684 grasses in the context of plant breeding for salt-tolerance. Salt-tolerance, they conclude, is an evolutionary labile trait that has evolved a number of times across many lineages; this is at odds with the record of difficulty in breeding salt-tolerant crops.

One potential explanation for this disconnect is the association between soil salinity and alkalinity; many saline soils are also alkaline due to the presence of sodium (Na)-carbonates (see [2] for a review of salt-affected soils). Combined alkaline and salt-stresses are more deleterious to plant growth than salinity alone [3–5]. Thus, the failure of laboratory-bred NaCl-tolerant cultivars to give good yields under field conditions may reflect combined soil sodicity and alkalinity. Indeed, Yang et al. [4] have shown that for even for Chloris virgata, a natural alkali-resistant halophyte, the inhibitory effects of alkali stress (from NaHCO₃ and Na₂CO₃) on relative growth rate and stored energy were significantly larger than those of salt-stress (from NaCl and Na₂SO₄). Other Chloris species are tolerant of alkaline pH (e.g. Chloris gayana [6] and Chloris barbata [7]), and it would be interesting to see whether there is any evolutionary relationship between alkali- and salt-tolerance in Chloris and more widely in the Chloridoideae. Has there been coevolution of alkali- and salt-tolerance in grasses? If yes, this may provide new avenues for plant breeding for salt-tolerance.

Both alkali- and salt-stress impact photosynthetic productivity and metabolism, but they may involve different physiological and molecular processes ([8–11] and references therein). However, plant breeding efforts continue to focus on NaCl-tolerance [12]. In Australia, where decades of research on plant breeding for salt-tolerance have been invested, 50% of soils are calcareous [13] and therefore contain significant concentrations of carbonate and bicarbonate ions (alkalinity). Sodic soils occupy about 27% of Australia [14], including large tracts of agricultural land; most have calcareous subsoils. This implies that a wider range of salts and their potential interactions need to be considered if we are to successfully breed salt-tolerant crops that are high-yielding under field conditions. It also suggests that new breakthroughs in food security are likely to arise at the intersection of disciplines (plant and soil sciences).