Effect of salt stress on growth, ion accumulation, and distribution in sainfoins seedlings

In this study, we investigated the effects of different concentrations of NaCl (0, 5, 25, 50, 100 and 200 mmol·L-1) over different time periods (0, 1, 3, 5, 7 and 9 d), after 50 mmol·L-1 NaCl treatment, on growth, ion accumulation, and distribution in 3-week-old sainfoin (Onobrychis viciifolia) seedlings. Results showed that concentrations of 5, 25, and 50 mmol·L-1 did not significantly affect plant growth, while concentrations of 100 and 200 mmol·L-1 significantly inhibited growth when compared with that in the control (0 mmol·L-1)(P<0.05). With the increase of NaCl concentrations, Na+ concentrations in both shoot and root tended to increase. Compared with the control, the addition of 50200 mmol·L-1 NaCl increased shoot Na+ concentration by 1439 times, respectively. In particular, concentrations of 200 mmol·L-1 significantly increased root Na+ net uptake rates, total Na+ amount, and relative shoot Na+ distribution by 35-fold, 93% and 45%, respectively; whereas it reduced selective transport capacity for K+ over Na+ from roots to shoots. At 50 mmol·L-1 NaCl, with increased treatment time, Na+ accumulation in both shoot and root tended to increase, but that in the roots reached a peak on the 7th day and reduced subsequently. In contrast, K+ concentrations showed a decreasing trend, to a larger degree in root than in shoot. These results indicate that under NaCl concentrations of 550 mmol·L-1, sainfoin plants could adapt to salt stress by maintaining K+ and Na+ homeostasis; while under 100 and 200 mmol·L-1, plants passively accumulated a large amount of Na+ in shoots, where ions compartmentation and selective transport capacity for K+ over Na+ were weak, thus disrupting the balance of Na+ and K+ and resulting in osmotic stress and growth inhibition.