Effects of PEG, NaCl and Na₂CO₃ stresses on Suaeda glauca and Suaeda salsa seed germination

Suaeda glauca and S. salsa are native halophytes in China with a key role in the improvement and restoration of salt marsh ecologies. There is a lot of works on the response of Suaeda plants to salinity, but works are limited about the effects of drought and alkali stress on S. glauca and S. salsa, especially at germination stage. The aim of this study was to evaluate the impacts of PEG, NaCl and Na₂CO₃ stress on the germination of S. glauca and S. salsa seeds. The in-vitro seed germination test was carried out with various solutions of PEG-6000 (29 mmol·L^-1, 38 mmol·L^-1, 45 mmol·L^-1and 50 mmol·L^-1), NaCl (100 mmol·L^-1, 200 mmol·L^-1, 300 mmol·L^-1 and 400 mmol·L^-1) and Na₂CO₃ (70 mmol·L^-1, 140 mmol·L^-1, 210 mmol·L^-1and 280 mmol·L^-1) in iso-osmotic concentrations (-0.46 MPa, -0.92 MPa, -1.38 MPa and -1.84 MPa). We measured the germination rate, germination index, mean germination time, early seedling growth after 7-day treatment and final germination rate after another 7-day recovery. The results showed that with increasing osmotic stress intensity, the germination rates of two species were depressed under PEG, NaCl and Na₂CO₃ treatments, while -0.46 MPa treatment did not differ significantly from that of the control (fresh water). The negative effect of NaCl on S. glauca seed germination was less than iso-osmotic PEG and Na₂CO₃ treatments. By contrast, the effects of iso-osmotic PEG, NaCl or Na₂CO₃treatments on S. salsa seed germination were not significantly different. The germination rate of S. salsa was positively related to osmotic potential. Recovery study indicated that PEG, NaCl and Na₂CO₃ stress had no negative effects on the final germinate rate of S. glauca and S. salsa, meaning that the inhibition of PEG, NaCl and Na₂CO₃ stress on seed germination was due to osmotic pressure rather than ion toxicity. In addition, mild NaCl treatments (-0.46 MPa for S. glauca, and -0.46 and -0.92 MPa for S. salsa) promoted S. glauca and S. salsa seedling elongation, while PEG and Na₂CO₃ treatments inhibited it. In iso-osmotic conditions, radicle and hypocotyl lengths of S. glauca and S. salsa seedlings under NaCl treatment were greater than those under PEG and Na₂CO₃ treatments. Compared with S. glauca, S. salsa seedlings grew better under -0.46 MPa and -0.92 MPa NaCl treatments, but worse under -0.46 MPa Na₂CO₃ treatment. The findings suggested that, 1) S. salsa seeds exhibited strong resistance to PEG, NaCl and Na₂CO₃ stress, and its resistance to PEG and Na₂CO₃ stress was greater than that of S. glauca. 2) S. salsa had stronger capacity to establish seedlings under NaCl stress than S. glauca, but its tolerance to mild Na₂CO₃ stress was weaker than that of S. glauca.