Na~+ Exclusion Mechanisms Of Glycyrrhiza Species Seedlings

Glycyrrhiza uralensis and Glycyrrhiza inflata show high stress-resistance because they can grow under unfavorable conditions such as salinity soil. To understand the Na+ exclusion mechanisms of two Glycyrrhiza species, Glycyrrhiza inflata and Glycyrrhiza uralensis seedlings were cultured with a complete Hoagland nutrient solution containing 0, 50, 100, 150, 200 and 250 mmol·L-1 Na Cl. The objectives of present study were to compare the difference response of two Glycyrrhiza species to salt stress and explore its Na+ exclusion mechanisms. Therefore,this study was desingned to investigate(i) the growth characteristics of two Glycyrrhiza species and(ii) the difference response of root and shoot of two Glycyrrhiza species to Na+ and its Na+ exclusion mechanisms. The main conclusions are as follows:There was a greater inhibition on the growth of aboveground than that of the root of two Glycyrrhiza species under salt stress, because there was no significantly decrease in root dry weight. Maintain a high root biomass is important in absorbing enough nutrients and resisting salt damage of plant seedlings root under salt stress condition.The chloroplast ultrastructure of leaf show significant difference between two Glycyrrhiza species at different concentration Na Cl. At high Na Cl concentration condition, Glycyrrhiza uralensis chloroplasts became swelled significantly, outer membrane of chloroplast became swelled and blurred, starch granule became smaller and even disappeared. However, there was no influence on the ultrastructure of Glycyrrhiza inflata chloroplasts under Na Cl stress. Compared to Glycyrrhiza uralensis, Glycyrrhiza inflata have a higher salt tolerance index, implying that Glycyrrhiza inflata was more salt tolerant.The root Na+ exclusion capability of two Glycyrrhiza species showed in the below two aspects. On the one hand, the K+, Na+ selective absorption coefficient(SA) value of root of Glycyrrhiza inflata became greater than those of Glycyrrhiza uralensis and the uptake of K+ by plants from outside at salt environment significantly increased, whereas the Na+ uptake was inhibited. In higher salinity condition, the earlier development of apoplastic barriers in endodermis of roots can elongate the function zone of endodermis that resisting the apoplastic Na+ flow into the stele, therefore, the root Na+ exclusion capability of plant were improved. On the other hand, compared with other parts of plant, more Na+ accumulated in root of Glycyrrhiza inflata with increased Na Cl concentration, suggesting that root has good interception effect on Na+ that from outside salt environment. This interception effect was limit to Glycyrrhiza uralensis because it can only be observed at 0~150 mmol·L-1 Na Cl. Glycyrrhiza inflata had greater STK,Na values of root than those of Glycyrrhiza uralensis at the same Na Cl concentration, implying a stronger selective transport capability for K+ of root while Na+ was inhibited from being transported to stem. For these reasons, the significant difference of salt tolerance occurred in two Glycyrrhiza species. In addition, the ratio of vascular tissue area to total root area of Glycyrrhiza inflata and Glycyrrhiza uralensis were increased at 250 mmol·L-1 and 150~200 mmol·L-1 Na Cl, respectively. It displayed the root anatomical adaptation, which contributes to enhancement the transportation capability of root and alleviation of salt injury.Therefore, the main reasons account for a higher Na+ exclusion capability of Glycyrrhiza inflata are its high Na+ interception effect, K+ absorption and Na+ inhibition capability.Under salt stress condition, the Na+ was accumulated in different aboveground organ of two Glycyrrhiza species. Under 100 mmol·L-1 Na Cl, Na+ was mainly accumulated in stem of Glycyrrhiza uralensis. When the Na Cl concentration at 150~200 mmol·L-1 Na Cl, Na+ content is highest in lower leaf, while when the Na Cl concentration increased to 250 mmol·L-1 Na+ content in the upper leaf and upper stem was increased rapidly. Under salt stress condition, Na+ was mainly accumulated in stem of Glycyrrhiza inflata and Na+ disposed through fallen leaves, exhibiting salt efflux behavior of the seedling. There were no glands in the leaf epidermis of two Glycyrrhiza species. Na+ can be secreted by the glands and epidermal cells, and Na+ secretion content only account for 0.55% ~ 1.68% of those whole plant. Therefore, Glycyrrhiza inflata and Glycyrrhiza uralensis plants can secrete little Na+, leaf Na+ secretion is not the main Na+ exclusion way of two Glycyrrhiza species.