Effects Of Drought And Flood Stress In Growth And Defense In The Riparian Tree Species Populus

Under monsoon climate and global extreme climate events caused by human activities become more frequent,riparian vegetation is often affected by drought and floods.These water stress events will directly affect plant growth,physiology,survival and ecological relationships,and even lead to riparian ecosystems of degradation.Previous studies on riparian vegetation were mainly based on monoecious plants,while few studies on dioecious plants.Dioecious plants of Salicaceae are typical dominant tree species in the riparian zone,which provide good model to investigate how riparian plants adjust adaptive responses to riparian habitat.Previous studies have found that dioecious plants often show gender dimorphisms with environmental changes due to inconsistent reproduction costs between sexes.These studies have mainly explored the differences in growth between male and female plants.However,little is known about sexual defenses and the relationship between growth and defense under stress conditions.In this dissertation,the sexual differences in growth,photosynthesis and contents of nonstructural carbohydrate,tannins and phenols of male and female Populus cathayana and P.yunnanensis,which are endemic in China,were compared under drought or floods stress in control experiment.The research results can provide a new insight for ecological adaptation and population distribution of dioecious plants in riparian zone,but also can provide a potential technical support for the restoration of degraded riparian ecosystem.The main results are as follows:1.Differences in morphology and biomass of male and female Populus under water stressThe results showed that plant morphology（including plant height change,base diameter change,total leaf area,etc）,biomass（including leaves,stems,roots）,gas exchange,water potential,nutrient content,and non-structural carbohydrates were significantly affected by water and gender.Compared with the control（well-watered group）,water stress significantly reduced the changes in base diameter and plant height,total leaf area,biomass,net photosynthetic rate,stomatal conductance,transpiration rate,instantaneous water use efficiency,leaf nitrogen,leaf phosphorus,leaf potassium,stem nitrogen,stem phosphorus,root carbon,root phosphorus,root starch,root soluble sugar,root reducing sugar,root sucrose,root fructose of male and female Populus;Under drought treatment,the root dry mass,total weight,net photosynthetic rate,stomatal conductance,transpiration rate,and root soluble sugar,root sucrose,root fructose content were significantly higher than those in the flooded treatment group.Compared with female plants,the base diameter change,plant height change,total leaf area,aerial root number,biomass（including leaves,stems,roots）,net photosynthetic rate,stomatal conductance,instantaneous water use efficiency,root phosphorus content,and stem sucrose content of male plants under water stress were significantly higher,while the root-shoot ratio and intercellular CO₂ concentration were significantly lower.Under drought treatment conditions,the stem carbon,stem phosphorus,root potassium,leaf fructose,stem reducing sugar,and stem sucrose of the male plant were significantly higher than that of the female plant,while the stem potassium and soluble sugar of the female plant were significantly higher than that of the male plant.Under flooding treatment,the contents of root phosphorus and root potassium of female plants were significantly higher than that of male plants,and the number of aerial roots of male plants was significantly higher than that of female plants.2.Differences in defense capabilities of male and female plant under water stressThe results showed that the contents of condensed tannins and total phenols in plant tissues were significantly affected by water stress and gender,and the responses of these substances was different between the two Populus.Although the total phenol content of female P.cathayana under the control was lower than the male plant,there was no gender difference under water stress;the tannin content of P.cathayana was always higher in the female plant than that in the male plant,and water stress increases the tannin content differences between genders.In P.yunnanensis,the contents of condensed tannins and total phenols of female plants under water stress were lower than that of male plants,especially in stem.3.The relationship between male and female Populus growth and defenseThe biomass of P.cathayana and P.yunnanensis showed a negative correlation with the total phenol and condensed tannin content of the plants（except for the correlation between the tannins of the male P.cathayana and the biomass）.Under the same biomass,the total phenol content of female P.cathayana and P.yunnanensis were lower than that of male plants.Moreover,the condensed tannins content was lower of female P.yunnanensis and higher of female P.cathayana than that of the male plant at a given biomass.These results show that there had sexual differences in plant growth,photosynthesis,nutrient content,non-structural carbohydrates,and plant defense performance under flooding and drought treatments.Male plants suffered less negative effect in growth than that females under water stress,while plant defense was increased under flood stress.In totally,when under water stress,P.cathayana females showed a better defense ability than male,while P.yunnanensis males showed a better defense ability than females.These results indicated that water stress amplified the sexual differences in growth and defense,and females with higher reproduction cost had to face a stronger dilemma in growth and defense tradeoff than males under water stress.These results provide some new insights for population structure and dynamic of the dioecious riparian Populus and vegetation restoration in riparian zone.