Salt tolerance of two differently drought-tolerant wheat genotypes during germination and early seedling growth

Osmotic and specific ion effect are the most frequently mentioned mechanisms by which saline substrates reduce plant growth. However, the relative importance of osmotic and specific ion effect on plant growth seems to vary depending on the drought and/or salt tolerance of the plant under study. We studied the effects of several single salts of Na+ and Ca2+−NaCl, NaNO3, Na2SO4, NaHCO3, Na2CO3, and Ca(NO3)2—on the germination and root and coleoptile growth of two wheat (Triticum aestivum L.) cultivars, TAM W-101 and Sturdy, the former being more drought tolerant than the latter. The concentrations used were: 0, 0.02, 0.04, 0.08, 0.16, and 0.32 mol L−1. Significant two- and three-way interactions were observed between cultivar, kind of salt, and salt concentration for germination, growth of coleoptile and root, and root/coleoptile ratio. Salts differed significantly (P<0.001) in their effect on seed germination, coleoptile and root growth of both cultivars. Germination of TAM W-101 seeds was consistently more tolerant than that of Sturdy to NaCl, CaCl2, Ca(NO3)2, and NaHCO3 salts at concentrations of 0.02, 0.04, 0.08, 0.16 mol L−1. The osmotic potential, at which the germination of wheat seeds was reduced to 50% of that of the control, was different depending on the kind of salt used in the germination medium. NaCl at low concentrations (0.02 and 0.04 mol L−1) stimulated the germination of both wheat cultivars. At concentrations of 0.02 to 0.16 mol L−1, Ca2+ salts (CaCl2 and Ca(NO3)2) were consistently more inhibitory than the respective Na+ salts (NaCl and NaNO3) for germination of Sturdy. This did not consistently hold true for TAM W-101. Among the Na+ salts, NaCl was the least toxic and NaHCO3 and Na2CO3 were the most toxic for seed germination. Root and coleoptile (in both wheat cultivars) differed in their response to salts. This differential response of coleoptile and root to each salt resulted in seedlings with a wide range of root/coleoptile ratios. For example, the root/coleoptile ratio of cultivar TAM W-101 changed from 2.09 (in the control) to 3.77, 3.19, 2.8, 2.44, 1.31, 0.32, and 0.0 when subjected to 0.08 mol L−1 of Na2SO4, NaCl, CaCl2, NaNO3, Ca(NO3)2, NaHCO3, and Na2CO3, respectively. Na2CO3 at 0.08 mol L−1 inhibited root growth to such an extent that germinated wheat seeds contained coleoptile but no roots. The data indicate that, apart from the clear and more toxic effects of NaHCO3 and Na2CO3 and lesser toxic effect of NaCl on germination and seedling growth, any toxicity-ranking of other salts done at a given concentration and for a given tissue growth may not hold true for other salt concentrations, other tissues and/or other cultivars. The more drought-tolerant TAM W-101, when compared to the less drought tolerant Sturdy, showed higher tolerance (at most concentrations) to NaCl, CaCl2, Ca(NO3)2 and NaHCO3 during its seed germination and to Na2SO4 and CaCl2 for its root growth. This supports other reports that some drought-tolerant wheat cultivars are more tolerant to NaCl. In contrast, the coleoptile growth of drought-sensitive Sturdy was noticeably more tolerant to NaNO3, Ca(NO3)2 and NaHCO3 than that of drought-tolerant TAM W-101. Based on the above and the different root/coleoptile ratios observed in the presence of various salts, it is concluded that in these wheat cultivars: a) coleoptile and root tissues are differently sensitive to various salts, and b) at the germination stage, tolerance to certain salts is higher in the more drought-tolerant cultivar.