Cellular Distribution of Ions in Salt-stressed Cells of Populus euphratica and P. tomentosa

The authors used suspension cells of a salt-resistant poplar genotype Populus euphratica and a salt -sensitive genotype P. tomentosa to examine genotypic differences in intracellular compartmentation of ions under NaCl stress. Cells of P. euphratica were treated with 52, 137, 223, 309, 395 and 480 mmol·L^-1 NaCl respectively and P. tomentosa was salinized with 137 mmol·L^-1 NaCl. Spot energy dispersive X-ray microanalyses show ed that salt treatment significantly increased Na and Cl levels in cell compartments:cell wall, cytoplasm and vacuole. Preferential accumulation of Cl relative to Na in cytoplasm and vacuole indicates that Na and Cl may translocate independently from apoplast to symplast. Compared with P. tomentosa, P. euphratica accumulated much less salt ions in cytoplasm at 137 mmol ·L^-1 NaCl. Furthermore, with increasing salinity, Na and Cl levels in cytoplasm of P. euphratica were remained low er than that in walls when NaCl below 223 mmol·L^-1. Collectively, these results demonstrate that P. euphratica has a higher capacity to restrict the entry of salt into cytoplasm. Sharp rise of vacuolar fraction of Na and Cl was observed in P. euphratica when external salinity exceeded 223 mmol ·L^-1, indicating that P. euphratica is effective to sequester salt ions into vacuole. In contrast, P. tomentosa had almost no ability on salt compartmentation. K and Ca concentrations in P. euphratica were not reduced by increasing salinity, whereas the decline of these nutrients was observed in both cell wall and cytoplasm of P. tomentosa. In conclusion, P. euphratica has a greater ability on selective ion uptake, salt exclusion and ion compartmentation under saline conditions, which contribute to salt tolerance.