Physiological Mechanism In Salt Tolerance Of Ginkgo Biloba L. Seedling

Salt stress plays an important role in plant growth and development.The study on the physiological mechanism of salt stress in Ginkgo biloba is of theoretical and practical significance in selecting improved varieties,expanding the planting area in salinized areas,and reducing the influence of snowmelting agent and other salts on the growth of Ginkgo.In thisstudy,thethree-year-old Ginkgopotted seedlings was used as plant material to explore physiological characteristics.By using conditional control method,the various concentrations of Na Cl were set as 0,0.2%,0.4%,0.6%,0.8%and 1.0%,respectively.The changes of the growth,morphological structure,photosynthetic characteristics,chlorophyll fluorescence,antioxidant system index,and osmotic adjustment system index in Ginkgo were observedunder Na Cl stress.The evaluation and monitoring indexes of Ginkgo tolerance ability were put forward,which provided theoretical basis for further study on salt tolerance mechanism of plants.The main conclusions are as follows:1.Ginkgo seedling in our study has certain salt tolerance.The survival rate is 100%in the 0.2-0.8%Na Cl treated group.With the increase of salt concentration and treatment time,the relative growth ratio of plant height and ground diameter decreased step by step.The results showed that,at 64d,the relative growth rate reached to the peak value(26.8%)under 0.2%Na Cl treatment group.It is suggested that the seedling of this species can maintain growing in good condition in salinized soil with 0.44%Na Cl.2.The tissue and structure of Ginkgo seedling have certain salt tolerance.From the result of scanning electron microscope observation,we found that a great number of pore on leave hypodermis,the difference of stomatal closure time and extent,the sink of guard cells and the upheaval of subsidiary cells.It is suggested that Ginkgo can control the pass of vapour and CO₂ through stomatal regulation,and,therefore,lower the transpiration rate and maintain the photosynthesis rate.The leave cuticle was thickened,and the leave surface was covered by tubular wax crystal.This might be a self-protection way to lower the leave temperature.A great deal of salt cluster crystal was observed in stem parenchymal cell,and little is found in root and stipe,suggesting that the Ginkgo seedling has certain ability of salt reservation in stem and avoid direct damage to leaves.The pith parenchymal cell was full of starch grain,and this might be related to salt stress and seasonal growth pattern of Ginkgo biloba.3.The photosynthesis of Ginkgo seedling has certain adaptability to salt stress.The diurnalvariation of Pn,G_s,T_r and WUE appeared as bimodal curve.This could be leading from stomatal closure under strong light exposure or inactivation of photochemical reaction by overexertion in PSII photoreaction center.With the increase of salt concentration and treatment time,value of Pn,G_sand T_rdecreased,while the value of C_i increased.The value of L_s and WUE showed rise-fall trends.In the period of stress,stomatal limitation is main reason in low concentration treatment group,while non-stomatal limitation is main reason in high concentration treatment group,putatively resulting from decrease of photosynthetic activity in high centration in mesophyll cell.Stress with 0.2%Na Cl can promote photosynthesis in Ginkgo,while carbon assimilation is inhibited in excessive salt concentration and treatment time.4.Under salt stress,synthesis of pigment photosynthesis is inhibited in Ginkgo.With the increase of salt concentration and treatment time,the content of Chlt,Chl a,Chl b and Car decrease gradually,but the content of Chlt and Chl b reached to peak at 64 d in 0.2%Na Cl treatment group,the value were1.851 mg g-1 FW?0.562 mg g-1 FW,respectively.This might be related to strengthen of chlorophyllase activity and damage of chloroplast structure caused by salt stress.At the same time,it is implicated that the ability of photosynthesis pigment capture and radiosity is not weakened.5.The chlorophyll fluorescence of Ginkgoseedling leaf is responsive to salt stress.In the early period of stress,no obvious difference was found in chlorophyll fluorescence in all treatment groups,suggesting the completeness of PSII reaction center.In the late period of stress,under the condition of low concentration of Na Cl(0.2%,0.4%),almost all the treatment groups had no significant difference compared with the control group(Fv/Fm,?PSII,q_N)and the treated group showed a decreasing trend compared with the control group at the medium and high concentrations of Na Cl(0.6%,0.8%and1.0%).The more of the concentration and earlier of the responsive time got,the more significant of the reduction showed.For example,Fv/Fm decreased significantly from 29d,36d and 50d respectively(P<0.05),while?PSII of 1.0%,0.8%and 0.6%decreased significantly from 22d,36d and 64d respectively(P<0.05).It is implicated that either the soil salinity exceed 0.87%or stress treatment is extended,which may lead to the damage of PSII or decrease of conversion efficiency of light energy.6.Ginkgoseedling can be protected from the damage of salt stress by improving the activity of antioxidant system.Under salt stress,the activity of antioxidasewas strengthened.The relative conductivity,content of leaves extractive(MDA,GSH and flavonoid),enzymatic activity(SOD,POD and CAT)and scavenging rate for free radical(DPPH and hydroxyl)all increased at first and then decreased with the increase of treatment time.The scavenging rate of CAT and hydroxyl radicals peaked at 15 d.The content of SOD,POD and GSH reached the peak at 36 d,and the content of flavonoid reached peak at 64 d.7.The salt tolerance of Ginkgo seedling can be improved by increasing osmotic adjustment substances.The soluble protein content in leavesunder 0.4%Na Cl treatment was the highest,which was1.06 mg·g^-1 in Ginkgo.The soluble sugar content increased and then decreased with the increase of salt concentration and treatment time.The proline content increased significantly at the late stage of salt stress.This indicated that soluble protein played an important role in osmotic regulation in the early stage of stress,while proline became the main osmotic regulator in the late stage of stress.8.The correlation exists between Ginkgo seedling and physiological indexes.At 64d,Data demonstrated a negative correlation between salinity and growth rate of seedling height and ground diameter.The water potential for soil and branch was significantly negatively correlated with index of leaves containing Pn,G_s,T_r,L_s,WUE,Chlt,Chla,Fv/Fm and?PSII(P<0.01).Also,the water potential was significantly negatively correlated with value of q_N and q_P(P<0.005).In addition,the water potential had significantly positive correlation with C_i,cell membrane permeability,content of SOD,POD,GSH,flavonoid and carotenoid,and scavenging rate for free radical DPPH(P<0.01).However,the water potential is not correlated with content of chlb,Car,MDA,CAT and scavenging rate for free radical hydroxyl.To sum up,the tolerance of salinity in Ginkgo is result from a comprehensive mechanism consisting of adaptability of tissue and structure,osmotic adjustment and antioxidant system.Based on main component and membership function analysis,this Ginkgo species can be planted under 0.44%Na Cl in soil.Given salt tolerance index rate of contribution,index of water potential of branch,chloroplast pigment,photosynthesis and POD activity is sensitive to salt stress,and this can be useful to monitor and estimate salinity tolerance of Ginkgo.