Molecular and physiological responses of Egeria densa under salt stress

Egeria densa (Planch) Casp. (Large-flowered waterweed or Brazilian waterweed; Hydrocharitaceae) is a sub-merged aquatic plant species used as a bioindicator for freshwater quality. However, there is dearth of funda-mental knowledge regarding the response of E. densa to salt stress. Expression level of antiporters EdNHX1 and EdVHA, accumulation of inorganic and organic solutes, and physiological changes in E. densa under NaCl salt shock for 7 days were evaluated. Relative expression level of EdNHX1 and EdVHA was high after 24 h of exposure to 50 and 150 mM NaCl. A positive relation between relative expression level of vacuolar EdNHX1 and EdVHA was observed (R2 = 0.76). Enrichment of Na+ in shoots was dependent on the degree of NaCl concentration and exposure period, leading to elevated Na+/K+ ratio. Free proline was increased by 100 and 150 mM NaCl over the control in each salt exposure time and peaked (19.4 mu mol g-1 DW) at 5th day under 150 mM NaCl treatment. Chlorophyll a and chlorophyll b pigments in the leaf tissues of plants under 150 mM NaCl treatment for 5, and 7 days displayed a non-significant change, therefore maximum quantum yield of PSII (Fv/Fm) and photon yield of PSII (phi PSII) were diminished. Na+ secretion into vacuolar organelles of E. densa was reported for the first time using a comparative study of expression level of vacuolar EdNHX1 and EdVHA, organic and inorganic enrich-ment, and physiological adaptation under short-term NaCl treatment. The results suggest that E. densa can tolerate very low salt concentrations (<100 mM NaCl); however, this needs to be validated under the natural estuarine environment.