Two-Step Salt Stress Acclimatization Confers Marked Salt Tolerance Improvement in Four Rice Genotypes Differing in Salt Tolerance

In this study, four rice genotypes were evaluated for their salt tolerance characteristics in terms of ion concentrations and expression levels of OsHAK1, a gene that is involved in the regulation of K ion uptake. Salt tolerance profiles were assessed at 0, 0.5, 1.0, 1.5 and 2.0% NaCl. It was observed that the growth of salt-sensitive (RD6 and Sakon Nakhon) and moderately salt-tolerant (Niaw Ubon 2) cultivars were completely inhibited at 1.5% NaCl, with satisfactory growth observed for salt-tolerant (Pokkali) cultivar. One- and two-step treatments were compared for their efficacy in inducing salt tolerance in rice plants. One-step treatment was ineffective for producing rice plants with improved salt tolerance, while two-step treatment was a promising approach, producing salt-tolerant rice plants growing vigorously at 1.5% NaCl. Inductively coupled plasma–optical emission spectrometry indicated that salt-sensitive cultivars produced from two-step treatment showed improvements in their abilities to maintain Na+/K+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {Na}^{+}/\hbox {K}^{+}$$\end{document} ratios. Real-time qRT-PCR revealed that a remarkable increase in the OsHAK1 transcript level was not observed in plants grown at 1.5% NaCl. Besides, the OsHAK1 transcript level was not related to Na and/or K concentrations, and Na+/K+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {Na}^{+}/\hbox {K}^{+}$$\end{document} ratios, suggesting that other genes involved in K+\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {K}^{+}$$\end{document} uptake regulation may express at higher levels and therefore, OsHAK1 expressed at low levels.