The Physiological Mechanism Of Different Half-Sibling Families Of Ginkgo Biloba In Response To Salt Stress

As a well-known plant,ginkgo（Ginkgo biloba L.）is regarded as a living fossil.Ginkgo is an important economic and ecological tree,which is characterised by its edible,pharmacology,ornamental and ecological functions.Soil salinization is one of the severe problems affecting plant growth and development all over the world.More and more researchers focus on the salt tolerance mechanisms in forest trees.Therefore,it is necessary to study the physiological mechanism of ginkgo in response to salt stress and apply agronomic measures to alleviate salt stress,which will be useful for screening-excellent ginkgo germplasms with salinity tolerance that are suitable for afforestation on saline land.In this study,we selected 10 different half-sibling families of ginkgo to explore the physiological mechanisms including under four different salt concentrations（0,0.2%,0.3% and0.4%）of salt treatments.At four different periods（5,10,15 and 20 days）,the effects of salt stress on the photosynthetic system,antioxidant enzyme system and osmoregulatory system of different semi-sibling families of ginkgo seedlings were deeply analyzed.We also assessed the salt tolerance of these semi-sibling families of ginkgo and showed their different salt tolerance abilities by developing the salt tolerance evaluation method.In addition,we also evaluated the effects of different concentrations of chitosan CTS（50,100 and 200 mg/L）on the physiological and biochemical indexes of ginkgo seedlings under salt treatments.The optimal exogenous application concentration of chitosan was also defined.Our reseaches will promote salt-tolerant ginkgo germplasms breeding and lay a theoretical foundation for the mitigation of adverse effects of salinity stress on ginkgo by exogenous application of chitosan.The main findings were as follows:（1）Different concentrations of salt had significant effects on the photosynthetic characteristics,antioxidant enzyme activities,osmoregulatory substances and ion content of 10 different ginkgo half-sib families.At the early stage of salt stress（5-10 days）,the photosynthetic pigment,net photosynthetic rate,stomatal conductance,transpiration rate,Fv/Fm and other photosynthetic indexes of ginkgo showed a short-term upward trend with the increased concentration of salt in compared with the control,indicating that short-term salt stress could promote the photosynthesis to a certain extent.With the increase of time,the content of photosynthetic pigments in most of ginkgo half-sib families decreased,but the content of antioxidant enzyme activities and osmoregulation substances increased under salt stress.However,the net photosynthetic rate of the ginkgo half-sib family（Shandong（SD））still showed an upward trend and increased by 20.32% compared with the control,which indicated the half-sib family showed relatively high salinity tolerance.At the late stage of salt stress（10-20 days）,the photosynthetic indexes including photosynthetic pigment,net photosynthetic rate,stomatal conductance,Fv/Fm and activity of SOD of most ginkgo half-sib families decreased significantly,while the activity of POD,the content of osmoregulation substances and MDA increased obviously with the increase of salt concentration.The content of MDA of the ginkgo semi-sibling family（Shaanxi（XX））increased more than 1.51 times higher than that of the control,which resulted in the increase of membrane permeability as well as cell damage.All these changes indicated the ginkgo half-sib family（Shaanxi（XX））showed relatively low salinity tolerance.（2）Based on the correlation analysis and principal component analysis,we selected four typic index parameters including chlorophyll a,soluble sugars,soluble protein and maximum fluorescence（Fm）from 19 different physiological and biochemical indexes（chlorophyll a,chlorophyll b,total chlorophyll content,intercellular CO₂ concentration,Net photosynthetic rates,stomatal conductance,transpiration rate,superoxide dismutase（SOD）,peroxidase（POD）,malondialdehyde（MDA）,relative conductivity（REC）,soluble sugars,soluble protein,proline,initial fluorescence（Fo）,maximum fluorescence（Fm）,variable fluorescence（Fv）and maximum photochemical efficiency of PSII（Fv/Fm））to establish salt tolerance evaluation method for ginkgo seedlings.According to the results of the membership function based on multiple parameters from 10 different half-sibling families of ginkgo under salt treatments,the salinity tolerance of each half-sibling family from high to low was listed as follows: Shandong（SD）> Henan（HN）> Yunnan（YN）> Jiangsu（JS）> Guizhou（GZ）> Guangdong（GD）>Hubei（HB）> Shanxi（SX）> Anhui（AH）> Shaanxi（XX）.（3）According to the assessment of salt tolerance,we selected the weak salt tolerance ginkgo half-sibling family（“Shaanxi”（XX））to explore the physiological mechanism of how to mitigate adverse effects of salinity stress on the performance of ginkgo seedlings by exogenous application of different concentrations of chitosan.The appropriate concentrations of chitosan could promote the synthesis of photosynthetic pigments and improve the photosynthesis of ginkgo.Meanwhile,the exogenous application of chitosan could reduce the content of REC in ginkgo leaves.Besides,chitosan could also reduce the osmotic potential by increasing the content of osmotic regulation substance and the ratio of K⁺/Na⁺ in leaves to alleviate ion toxicity in ginkgo cells.Our results showed that it was an effective method to alleviate salt damage in ginkgo seedlings by exogenous application of chitosan with a certain concentration（50 mg/L）.