Negative correlation between the ratio of K+ to Na+ and proline accumulation in tobacco suspension cells

The concentrations of K+, Na+, and proline and the ratio of K+ to Na+ (K+/Na+) were analyzed in NaCl-unadapted and NaCl-adapted tobacco (Nicotiana tabacum) cells in suspension culture. At 3 to 5 d after inoculation, the NaCl-unadapted cells cultured in 100 mmol L-1 NaCl saline culture medium (Na100 medium) accumulated 28.7 mmol L-1 proline with a low ratio of K+ to Na+ (= 2.8) and the NaCl-adapted cells cultured in the Na100 medium contained 6.28 mmol L-1 proline with a high K+/Na+ ratio (greater than or equal to 7.5). The contents of amino acids for the NaCl-adapted cells in the Na100 medium were similar to those for the NaCl-unadapted cells in a modified LS medium (standard medium). At 14 d after inoculation, the NaCl-unadapted cells in the Na100 medium contained 4.77 mmol L-1 proline and restored the K+/Na+ ratio from 2.8 to 6.2. These results indicate the presence of a negative correlation between the K+/Na+ ratio and proline accumulation and suggest that a balance between the K+/Na+ ratio and proline accumulation may be the factor involved in determining the salt tolerance of plant cells.