Physiological Responses Of Robinia Pseudoacaciarhizobia Symbiosis Association To Nitrogen Deficiency In Different Regions

Nitrogen（N）is an important mineral nutrient macro-element required for plant growth and development.Affected by extreme weather events and deforetaton,the availability of N in the soil gradually decreases,particularly for the vulnerable ecosystems.Many previous studies have been shown that the N content inside plants will decrease under low N stress,but plant also could promote its root development to cope with low N stress and improve its N use efficiency.Symbiosis between legumes and rhizobia in the soil could perform biological N₂ fixation to meet legume’sown N needs.However,most of the current research on how legumes respond to low N stress physiologically and N allocation solely focus on herbs such as soybean,alfalfa,and Lotus japonicus,and there are few reports on woody legume trees.Since woody legume trees play an essential role in rehabilitation of degraded forest ecosystems,improving soil nutrient availability and sustainable development of green agri-forestry.In the present study,we selected two provenances of Robinia pseudoacacia L.from Northwest China and Northeast China regions,respectively,which were cultivated in the greenhouse.Subsequently,all seedlings northwest of two Robinia provenances were treated with rhizobia inoculation and non-inoculation subjected to N stress,including normal N(9 mg·kg^-1)and low N(0.6mg·kg^-1).A total of 8 treatments were conducted.Namely,the northwestprovenance inoculated with normal N,northwest provenance inoculated with low N,northwest provenance without inoculated with normal N,northwest provenance without inoculated with low N,northeast provenance inoculated with normal N,northeast provenance inoculated with low N,northeast provenance without inoculated with normal N,northeast provenance without inoculated with low N.After two months of treated and growth in the greenhouse,plant biomass,nodulation numbers,leaf photosynthesis,leaf and root N partitioning were determined and analyzed.The main results are summarized as follows:（1）Photosynthesis and physiological characteristics of the leaves of Robinia showed that low N stress significiantly reduced the photosynthetic efficiency of Robinia from northwest provenance,but did not affect the biomass and the water use efficiency of the two provenances of Robinia.Rhizobia inoculation improved the photosynthetic efficiency of Robinia from northeast provenance.The effect of inoculation with rhizobia was more significant under low N treatment.In addition,the photosynthetic response of Robinia from northwest and northeast provenance was different between with and without rhizobia inoculation.Robinia of northeast provenance had higher photosynthetic efficiency after inoculation,but Robinia of northeast provenance had higher photosynthetic efficiency upon without rhizobia inoculation.（2）N partitioning in the leaves of Robinia showed that low N stress significiantly reduced the content of ammonium N and the activity of nitrate reductase（NR）enzyme of both Robinia provenances.Furthermore,low N could significantly increase the organic N content in the leaves of northwest Robinia plants,but had no significantly affect in northeast Robinia plants.In addition,rhizobia inoculation significantly increased the levels of different N compounds in the leaves of Robinia from northwest provenance,and improve the stress resistance of Robinia under low N stress,but rhizobia inoculation did not have a similar affect in the leaves of northeast Robinia plants.Meanwhile,the northwest Robinia plants were more sensitive to rhizobia inoculation,had higher N content,thus was more resistant to N deficiency stress compared to northeast Robinia plants.（3）N partitioning in the roots of Robinia plantsshowed that low N stress significantly reduced the levels of different N compounds.Rhizobia inoculation significantly increased the N content of the roots of northwest plants,especially upon the inoculation combined with low N supply,which was consistent with the responses of northwest Robinia leaves.Robinia roots from northwest provenance and northeast provenance had the same physiological response with their leaves under low N stress.Therefore,the roots from northwest plants had the higher N content,thus showed a stronger stress resistance to low N availbility.In conclusion,Robinia from northwest provenance in our study was more sensitive to rhizobia and showed the stronger stress resistance under low Nsupply.Our results provide a theoretical basis for establishing the physiological and ecological mechanisms of the Robinia-rhizobia symbiosis association,and further studies with responses of Robinia trees to various low N gradients are needed to elucidate the theoretical backgrounds for breeding and selection of the feasible stress-resistant Robinia provenance for reforestation programs in the degraded and vulnerable landsystems.