Physiological Responses Of Robinia Pseudoacaciarhizobia Symbiosis To Drought Stress

In recent years,extreme climate change in arid and semi-arid regions has not only exacerbated the loss of crop yield and timber production,but also seriously affected local vegetation restoration and environmental remediation.The leguminous pioneer tree species can be used for water and soil conservation,soil fertility enhancement and soil erosion prevention due to their potential stress resistance and the ability to fix air nitrogen with rhizobia.At present,the research on the physiological response of legumes to drought stress mainly focused on herbs plants such as soybean,pea,alfalfa and licorice,but the response of woody legumes to drought stress have scarcely been reported.Meanwhile,the mechanism by which the woody legume-rhizobia symbiosis improves the water physiological state of plants under drought stress is still unclear.Black locust（Robinia pseudoacacia L.）,as a woody legume with wide distribution,strong stress resistance and nitrogen fixation ability,plays an important role in controlling soil erosion,restoring forest ecosystem functions,and improving soil nutrient availability.Therefore,we selected two black locust provenances with contrasting climatic conditions and soil backgrounds:Gansu provenance（GS）and Northeast China（Dongbei district,DB）provenance,to investigated their physiological response to drought.The one-month-old black locust seedlings were treated with inoculation and non-inoculation of rhizobia（Mesorhizobium amorphae QD9）respectively in the greenhouse,and then subjected to water treatments after 3 months of inoculation.Water treatments included normal water supply（70%of field moisture capacity）,moderate drought stress（30%of field moisture capacity）,and the rewatering process after drought stress.Both the drought and rewatering periods lasted 12 days.The experiment design includes 3 water supply level×2 rhizobia inoculation level×2provenances=12 treatments.Each treatment had 6 repetitions.During the experiment,the soil water content was kept at 70%and 30%of the field capacity for normal water group and the moderate drought stress group respectively by simple weighing methods.After water treatments,the black locust biomass,nodule number,photosynthetic characteristics of leaves,and physiological indicators of leaves and roots were measured.The main results are as follows:（1）Moderate drought stress has significantly reduced the photosynthetic rate（P_n）,transpiration rate（T_r）and stomatal conductance（G_s）of black locust seedlings compared with that under normal water supply.After rewatering,the photosynthetic characteristics of seedlings previously exposed to moderate drought could recovered to the levels of normal-watered plants.Rhizobia inoculation significantly increased the nodule number of black locust seedlings compared with the non-inoculated plants.The P_n,T_r and G_s of GS provenance were found to be significantly higher than those of DB provenance.（2）Moderate drought stress has significantly increased stable isotope natural abundance of carbon(δ¹³C),lignin,soluble sugar,malondialdehyde（MDA）and hydrogen peroxide（H₂O₂）content,but significantly reduced the water content and starch content in black locust leaves compared with that under normal water supply in the GS provenance.Rewatering treatment recovered these indicators to normal level.Rhizobia inoculation significantly increased the total nitrogen and water content,but decreased the accumulation of MDA and H₂O₂ content in the black locust seedlings leaves compared with the non-inoculated plants.The water content,total nitrogen and starch content of seedlings from GS provenance was significantly higher than those of DB provenance,while the content of soluble sugar,MDA and H₂O₂ were significantly lower than those of DB provenance.Seedlings from GS provenance showed stronger recovery capacity under rewatering treatment than DB provenance.（3）Moderate drought stress has significantly increased stable isotope natural abundance of carbon(δ¹³C),lignin,soluble sugar,MDA and H₂O₂ content,however,significantly reduced the water content,total carbon,total nitrogen,cellulose and starch content in black locust roots compared with that under normal water supply in the GS provenance.Rewatering treatment recovered some of these physiological indicators to normal level,such as natural abundance of carbon(δ¹³C),MDA and H₂O₂ content.Rhizobia inoculation significantly increased the total nitrogen content,but decreased the accumulation of MDA and H₂O₂ in the black locust seedlings roots compared with the non-inoculated plants.For provenances,the water content,total nitrogen,structural biomass and starch content of seedlings from GS provenance was significantly higher than those of DB provenance,while the content of soluble sugar,lignin,MDA and H₂O₂were significantly lower than those of DB provenance.In conclusion,moderate drought stress significantly affected the photosynthetic and physiological characteristics of the black locust leaves and roots.Rhizobia inoculation could significantly promote the growth of black locust and improve its drought resistance.The GS provenance seedlings showed stronger drought resistance under drought conditions and better physiological recovery capacity under rewatering conditions.Meanwhile,it is suggested that continuous experimental research on the physiological response of black locust seedlings with different ages to drought stress should be conducted in the future.These results provide a theoretical basis for studying the mechanism underlying the interaction between tree and soil microbe,and for the selection of high drought resistance black locust varieties.