Salt Tolerant Mechanisms Of Three Glycyrrhiza Species

Glycyrrhiza inflata，Glycyrrhiza uralensis and Glycyrrhiza glabra belongs to the genusLeguminosa Glycyrrhiza and are used as herbal medicine. They show high stress-resistancebecause they can grow under unfavorable conditions such as salinity soil. But salt toleranceparameters like threahod and salt tolerance mechanisms of three Glycyrrhiza species have notbeen determined. The objectives of present study were to compare the difference response ofthree Glycyrrhiza species to salt stress and explore its salt tolerance mechanisms. Therefore，this study was desingned to in vestigate (i) the growth characteristics such as seedgermination, external morphology of seedlings，anatomy of adaptability of plants under saltstress，and (ii) physiological regulatory mechanisms such as photosynthetic physiologicalregulation，ion balance and osmotic adjustment，antioxidant enzyme system，and (iii)secondary metabolism such as licorice acid，total flavonoids accumulation，distribution ofplants under salt stress in three Glycyrrhiza species. Based on the above research，the effectsof saline soil improvement and the yield and quality of Glycyrrhiza were evaluated bysalinization land planted trials. These results could provide theoretical basis for cultivationand the high-efficiency utilization of resources of three Glycyrrhiza species. The mainconclusions are as follows:The sequence of salt injury level of seed was NaHCO3>Na2SO4>NaCl. There was asignificant difference of salt tolerance threshold among three species. G.inflata exhibitedgreater germination ability under salt stress than G. uralensis and G.glabra. So it is suitablefor being planted in the soil mainly with chloride-sulfate containing0~1.3%salt. TheNaHCO3tolerance of G.glabra was the highest and can be planted in the abandoned salineland containing NaHCO3. The slat tolerance of G.uralensis was the weakest. Therefore，theoptimum salt concentration for G.uralensis was0~0.7%. The seeds of three Glycyrrhizaspecies all show characteristics of delay germination under high salt stress and rapidgermination after recovery water，which is saline environment adaptation strategy to ensurethat the seed can avoid salt damage.Low NaCl concentration had no significant effect on the fresh weight，dry weight andglycyrrhizic acid content of three Glycyrrhiza species seedling，but higher than200mmol·L-1NaCl inhibited their growth. The threahod of salt for the seedling growth of Glycyrrhizainflata，Glycyrrhiza uralensis and Glycyrrhiza glabra was278.17，151.52and118.18mmol·L-1NaCl，which was calculated by the fitting equation for the relationship between salttolerance index and salt concentration. In contrast to leaf and shoot growth，root growth isusually less affected by salt stess. The downregulates of biomass in the aerial parts maymaintain root biomass accumulation，to ensure absorption of water and minerals of plantseedlings root and resist salt damage. Compared with the seed germination stage，theseedlings of three medicinal licorices have greater salt tolerance.Salinity induced anatomical changes in roota，stems and leaves. These changessuggested that three Glycyrrhiza species showed anatomy of adaptability to saltstress.Vegetative organs at different developmental stages had different anatomyadaptabilities to salt stress. Salinity increased palisade tissue thickness of young leaves andarea of vascular system of young stems and roots，which may have greater photosynthetic capacity and transport of water from root to leaves. Moreover，salinity increased corticalthickness and enhanced development of root casparian band in root endodermis，whicheffectively filtered salt ions and improved barrier of roots to prevent the passage of anexcessof ions to the xylem. In contrast to young leaf，stems and roots，salinity (increased)stimulates sizes and vacuolization in parenchyma cells of old leaf，stems and roots. It couldbe interpreted as an adaptive mechanism to diluted salts ions and regulate the internalconcentration of solutes. The changes of Glycyrrhiza inflate anatomical structure showed thehighest tolerance to salinity.With increasing NaCl concentration，the uptake of K+，Ca2+and Mg2+of threeGlycyrrhiza species plants from outside salt environment significantly increased whereas theNa+uptake was inhibited. For the concentration of NaCl was0～100mmol·L-1，thepreferential accumulation of Na+in roots over leaves can be interpreted as a mechanism oftolerance. Under the treatments of200mmol·L-1and above，most of Na+were transported tothe lower leaves and then disposed through old leaves fall，exhibiting salt efflux behaviour ofthe seedling. Under salt stress，the transport systems selective for K+were at work. Thus theupper leaves maintained a high K+/Na+ratio，which was beneficial to the growth of seedling.Additional，the salt-tolerant root accumulated Ca2+and Mg2+as well as synthesis ofglycyrrhiza acid and proline to increase osmoregulation ability，maintaining cell membranestability and easing the Na+toxicity. Depending on these ways，the Glycyrrhiza speciesseedlings can adapt to saline environment. In contrast to Glycyrrhiza inflata，The lowerleaves of Glycyrrhiza uralensis and Glycyrrhiza glabra accumulated more Na+as tomembrane permeability increased and induced damage in memberanes,which suggested thation balance and osmotic adjustment of Glycyrrhiza uralensis and Glycyrrhiza glabra wereweaker than Glycyrrhiza inflate.Protective enzyme activities of stress-induced three Glycyrrhiza species changed withrepact to different severity and duration of the salt treatment and plant organs. Enzymeactivities in leaves markedly increased after1d，otherwise it was similar to those in root ofcontrol plant and increased after15d. SOD、APX、CAT、GR activities of Glycyrrhizauralensis increased in the early phase of salt stress but decreased with prolonged salttreatment. On the contrary，these enzyme activities of Glycyrrhiza inflate markedly increasedafter15d salt stress while they were unchanged at high salt，suggesting a build-up of aprotection against oxidative damage. Additional，the enzyme activities increased in root anddecreasd in leaves. This phenomenon indicated a possible blance of between enzymeactivities and energy consume of plant.The energy heat dissipation of PSⅡ system was improved through stomata closure andtranspiration reducation induced by the low salinity.Thus，photosynthetic apparatus can beprotected from damage. At high salinity，non-stomatal factors were an important regulatemechanism for medicinal licorice to adapt to salt environments. There was no sign that thephotosynthetic apparatus of Glycyrrhiza inflata was stressed，which owing the pathway tosacrifice a part of the cost of carbon assimilation. This pathway could be Mailer reaction andphoto respiration，but it is absent in Glycyrrhiza uralensis and Glycyrrhiza glabra. Underhigh salinity stress，the degree of damage to the photosynthetic apparatus in Glycyrrhizaglabra was higher than Glycyrrhiza uralensis. However，the photosynthetic apparatus of Glycyrrhiza uralensis can restored through a sharp increased energy heat dissipation of PSⅡsystem in the late phase of salt stress.Total flavonoid and glycyrrhizic acid content of tissues and organs of three Glycyrrhizaspecies significantly increased under salt stress. Histochemical observations indicated thatdistribution of flavonoids in epidermis，periderm，glandular hairs，capsule cells and cytoplasmof parenchyma cells of the root，stem and leaf would allow them to perform the antioxidantand osmotic function to decrease salt stress injuries. There were a large distribution andaccumulation of glycyrrhizin from the root parenchyma cells to the peripheral skin tissue，which has important function to improve water retention capacity of plant root cells in aridand saline water environment.With the increase of planting year，the salt accumulated in plant was transfered to thestem from the root. So Glycyrrhiza inflate can significantly reduce the salt content of salinesoil. About2.36g/m2salt can be absorbed from soil after Glycyrrhiza inflata have beenplanted for three years. Total flavonoids of1～3years old Glycyrrhiza inflate had reachedstandards of pharmacopoeia. However，glycyrrhizic acid content in root had no significantincrease with the increasing of planting year. Considering an allowance for banlence of yieldand quality，2～3years old Glycyrrhiza inflata in reclamation of saline and alkaline land isopportunely harvested.