Alfalfa is an important salt-tolerant leguminous forage-plant in salinity areas worldwide, but its performance in high level of salt stress cannot meet the satisfactory requirement. Especially, the short-term response of alfalfa to high-level salt stress is still not clear. In the present study, thirty-day-old alfalfa Gongnong No. 1 (Medicago sativa L. cv. Gongnong No.1) seedlings were exposed to NaCl treatments at concentrations of 0 (control), 50 (moderate level), 150 (high level), and 250 mM (extremely high level). Twenty-four hours after salt stress treatment, with the increase of NaCl level plant height was slightly decreased but both shoot biomass and root length were substantially declined to a dramatic extent. Also decreased was root K + concentration. In contrast, both Na + concentration and ratio of K + /Na + showed increased trends. Root K + flux was determined using non-invasive micro-test technique (NMT) around apical root tips, wherein a clear K + influx was observed at the rate of about 0.5 nmol cm -2 s -1 under the condition without salt stress, while under salt stress at the rate of 2-3 nmol cm -2 s -1 did occur K + efflux. Accordingly, stomatal length and breadth and stomatal aperture breadth decreased with the increase of NaCl concentration, while stomatal aperture density increased with time in the first 24 h after NaCl treatment. In conclusion, as a species-specific test, alfalfa is sensitive to high-level salinity with NaCl concentrations above 150 mM in the first 24 h post salt-exposure. The key mechanism was found to be presented as the pressed stamatal conductance induced by K + -Na + unbalance which was caused by root K + efflux.

The response of plant growth to a geographical salinity gradient under the current scenario from inland to seaside implies the possible future trend of plant development in response to sea-level rise driven by global warming. In the present study, alfalfa (Medicago sativa L. cv. Gongnong No. 1) was planted in sites along a salinity gradient in Dalian City, Northeast China. Fine root growth was investigated at three depths of 0-10 cm, 10-20 cm and 20-30 cm. Meanwhile, another control experiment was conducted using potted alfalfa exposed to gradual levels of salinity at the same levels as the salinity gradients in the field to confirm the unique impact of soil salinity on alfalfa physiology. Results from both field and pot experiments indicated that the parameters of net photosynthesis rate (P N), foliar transpiration (T r) and water use efficiency (WUE) all decreased with the increase of gradient from inland to seaside. Root surface area also showed a general decline for all three soil depths, but differences among the three soil depths were not significant. Root diameter was greatest in soils at a depth of 0-10 cm, while root tip number was greatest in soils at a depth of 10-20 cm. All root morphological parameters decreased with salinity level along the geographical gradient from inland to coastal seaside, while both root surface area and root tip number positively correlated with soil ammonium and nitrate contents, respectively. Our results predicted that, driven by seawater intrusion from sea-level rise under global warming, future performance of inland alfalfa may suffer from severe soil salinity through constricting new root egress and elongation. This trend would be mainly driven by soil salinity which reduces photosynthesis by stimulating water loss in alfalfa.