Physiological Responses of Alfalfa to high-level salt stress: Root ion flux and stomatal characteristics

Abstract

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.