Salt tolerance of Glycyrrhiza inflata seedlings in Xinjiang and its ion response to salt stress

Aims The objectives were to examine the response of Glycyrrhiza inflata to NaCl stress and understand its salinity tolerance and salt tolerance mechanisms.
Methods By the method of hydroponics, seedlings were cultured in plastic pots filled with a complete Hoagland nutrient solution containing 0, 50, 100, 200 and 300 mmol·L^–1 NaCl. After four weeks, we measured the fresh weight, dry weight, water content, shoot height, root diameter, membrane permeability, glycyrrhizic acid, proline, malonaldehyde content and ion content of different parts of the plants.
Important findings Low NaCl concentration had no significant effect on the fresh weight, dry weight and glycyrrhizic acid content of G. inflata seedlings, but ≥200 mmol·L^–1 NaCl inhibited growth. The optimal salt range for seedling growth was 0–278.17 mmol·L^–1, which was calculated by the fitting equation for the relationship between salt tolerance index and salt concentration. With increasing NaCl concentration, the uptake of K⁺, Ca²⁺ and Mg²⁺ by plants from outside a salt environment significantly increased, whereas the Na⁺ uptake was inhibited. For the 0–100 mmol·L^–1 concentration of NaCl, the preferential accumulation of Na⁺ in roots over leaves can be interpreted as a mechanism of tolerance. For concentrations of 200 mmol·L^–1 and above, most Na⁺ was transported to the lower leaves and then disposed through leaf fall, exhibiting salt efflux behavior of the seedling. Under salt stress, the transport systems selective for K⁺ were functioning. Thus the upper leaves maintained a high K⁺/Na⁺ ratio that was beneficial to the growth of seedlings. In addition, the salt-tolerant root accumulated Ca²⁺ and Mg²⁺ as well as synthesized glycyrrhizic acid and proline to increase osmoregulation ability, maintain cell membranestability and reduce Na⁺ toxicity. These are the ways that G. inflata seedlings are adaptive to a saline environment.