Solute analysis and water relations of gametophyte mutants tolerant to NaCl in the fern Ceratopteris richardii

The stl1 and stl2 mutations confer low and high levels of NaCl tolerance to gametophytes of the fern Ceratopteris richardii, respectively. As an initial characterization of these mutations, the levels of various organic solutes, tissue ion content and water relations were examined in the wild‐type and mutant strains in the absence and presence of 60 mol m‐3 NaCl stress (a level which results in a 20, 15 and 0% reduction in gametophyte growth in the wild‐type, stl1 mutant and stl2 mutant, respectively). All strains exhibited major changes in organic and inorganic solute levels and water relations in response to 60 mol m‐3 NaCl stress. Differences in organic solute levels and water relations between the wild‐type and mutant strains in the absence and in response to 60 mol m‐3 NaCl stress were minimal. Analysis of tissue ion content showed that stl1 was associated with a slight reduction in Na+ accumulation during 60 mol m‐3 NaCl stress. stl2 was associated with (1) higher constitutive levels of K+ and (2) continued selective accumulation of K+ and reduced accumulation of Na+ during 60 mol m‐3 NaCl stress. A K+/Na+ ratio close to 1 was observed in the wild‐type during 60 mol m‐3 NaCl stress, while higher ratios were detected in stl1 and stl2 (1·7 and 4·0, respectively). The findings of this study suggest that the tolerance imparted by stl1 and stl2 is associated with altered ion accumulation during NaCl stress, rather than an enhanced ability to accumulate organic solutes to be used for osmotic adjustment of the cytoplasm.