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 toleran ce to gametophytes of the fern Ceratopteris richardii, respectively. A s an initial characterization of these mutations, the levels of variou s organic solutes, tissue ion content and water relations were examine d in the wild-type and mutant strains in the absence and presence of 6 0mol m-3 NaCl stress (a level which results in a 20, 15 and 0% reducti on in gametophyte growth in the wild-type, stl1 mutant and stl2 mutant , respectively). All strains exhibited major changes in organic and in organic solute levels and water relations in response to 60mol m-3 NaC l stress. Differences in organic solute levels and water relations bet ween the wild-type and mutant strains in the absence and in response t o 60mol M-3 NaCl stress were minimal. Analysis of tissue ion content s howed that stl1 was associated with a slight reduction in Na+ accumula tion during 60mol 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 60mol 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 an d 4.0, respectively). The findings of this study suggest that the tole rance imparted by stl1 and stl2 is associated with altered ion accumul ation during NaCl stress, rather than an enhanced ability to accumulat e organic solutes to be used for osmotic adjustment of the cytoplasm.