Effects Of Salt And Water Stress On Seed Germination And Cotyledon Development Of Populus Euphratica

Populus euphratica is a desert riparian forest tree species belonging to Populus of Salicaceae,and is the only dominant tree species remaining in tertiary forests in the desert-oasis transition zone in extreme arid regions.The desert riparian forest based on Populus euphratica is an important part of the arid desert ecosystem in northwest China and an ecological link between oasis and desert.Although ecological water diversion has been carried out in tarim River basin for many years,the chance of survival of sexual reproduction of Populus euphratica in the wild is still very small,which has great potential risks for ecological community of Populus euphratica.In this paper,the seeds of Populus euphratica in the middle reaches of Tarim River and soil substrate in floodplain were used as research materials to study the changes of seed germination rate,cotyledon expansion rate and survival rate after 40 days of growth under water and salt stress.Sequential changes of germination delay,sustained germination time,cotyledon development delay and cotyledon sustained development time.By using correlation analysis and regression analysis,the critical value and limit value of tolerance in seed germination and cotyledon development of Populus euphratica were obtained by simulating regression equation.The success rate of sexual reproduction of wild Populus euphratica can be further improved by further understanding the transplanting process of seedlings of euphratica in tarim River basin,as well as the key time nodes and water and salt thresholds.Under the condition of no salt stress,the soil water content was 7.7%～33%,and the seed germination rate of Populus euphratica was more than 50%.The soil water content was 12.5%～33%,and the cotyledon expansion rate of Populus euphratica was more than 50%.The survival rate of Populus euphratica seedlings growing 40 days after seed germination was greater than 50%when the soil moisture content was17.2%～33%.When the soil salt content was less than 1.8%（intensive salinization）,the seed germination rate of Populus euphratica was more than 50%.When soil salt content was less than 1.4%（moderate salinization）,the cotyledon expansion rate of Populus euphratica was more than 50%.When soil salt content was less than 1.2%（moderate salinization）,the survival rate of Populus euphratica seedlings growing 40 days after seed germination was greater than 50%.The results showed that the moderate salinization environment seriously affected the seed germination and seedling growth of Populus euphratica from seed germination to cotyledon expansion and colonization.When soil salt content was greater than 2.4%（extreme salinization）,the seed germination rate of Populus euphratica was less than 25%.When soil salt content was more than 1.8%（severe salinization）,the cotyledon expansion rate of Populus euphratica was less than 25%.When soil salt content was more than 1.7%（severe salinization）,the survival rate of Populus euphratica seedlings growing 40 days after seed germination was less than 25%.The results showed that during the process from seed germination to cotyledon expansion to colonization,Populus euphratica seed germination and seedling normal growth had reached the lethal state under intense salinization environment,and could not grow and develop at this time.The time delay and rate of seed germination were gradually delayed and decreased under the enhanced water and salt stress,which were significantly different from the control group(S_CKW_CK).In addition to the extreme drought and salt stress(S₃W₁,S₄W₁,S₅W₁,S₃W₂,S₄W₂,S₅W₂,S₅W₃,S₅W_CK,S₅W₄,S₅W₅),the seed germination time delay was less than 3d,and the seed germination rate was fast and the seed appeared orderly.Under the same salt stress,the time delay of seed germination was complicated by different soil water content,and the boundary was slight salt stress（S₂）.Which in the light of salt stress environment(S_CK,S₁,S₂),the moisture is affecting the seed germination and cotyledon expansion and growth of the important impact factors,in the big environment,extreme drought stress and severe flooding stress（W₁,W₅）with other two moisture conditions have significant difference,the change trend of each showed a trend of opening up the u-bend.Under moderate and severe salt stress conditions（S₃and S₄）,the more abundant water,the smaller the time delay of germination,the faster the germination rate,and the trend of change at this time fluctuated greatly.In severe and extreme drought conditions（W₁ and W₂）,the time delay of germination was more than 7.5d,and the germination could not be achieved.Under(W₃,W_CK),there was no significant difference in germination time,and the germination time ranged from 2.33 to 2.5 days.In the（W₅）environment,the time lag of germination was significantly different from that of the control group,and the germination time ranged from 1.33 to 1.67 days.Under extreme salt stress（S₅）,the more abundant water,the higher the salt persecution rate,the seed germination basically reached the salt tolerance limit,but could not germinate under extreme drought（W1,W₂）.Under(W₃,W_CK,W₄),the time lag of germination had no significant difference,and the time of germination was 4.5-5.5 days.In the（W₅）environment,the time lag of germination was 2.67 days.In conclusion,under the action of water and salt interaction stress,seed germination rate,cotyledon development rate and survival rate under the action of water and salt interaction stress were basically equivalent to that under the action of water and salt interaction stress.Water and salt together restricted the growth of Populus euphratica.Under high water and high salt environment,seed germination rate,cotyledon development rate and survival rate were significantly different from that under cross salt stress,and the cross salt stress rate was higher than that under water stress at this stage.Salt promoted the growth of Populus euphratica seeds and seedlings under high water and mild salt stress,but the stress rate of water and salt interaction presented complexity at this time.In addition,the stress tolerance of Populus euphratica seedlings at seed germination stage was greater than that at cotyledon expansion stage than that at just colonization stage.