Differences And Related Mechanisms In Effects Of Ammonium Nitrogen On The Invasive Plant Xanthium Sibiricum And Its Native Congener X. Sibiricum

Ammonium（NH₄⁺）and nitrate（NO₃^-）nitrogen are two inorganic nitrogen forms that can be absorbed and utilized by plants.NH₄⁺,as an important inorganic nitrogen source,plays an important role in plant growth and development.From the perspective of energy consumption,it is more advantageous for plants to use NH₄⁺as their main nitrogen source.However,most plants prefer to absorb and utilize NO₃^-,while being sensitive to NH₄⁺.When NH₄⁺is the main nitrogen source in the environment,its high concentration will cause high ammonium stress to plants,or even toxic effects often occur.Previous studies conducted by our laboratory showed that the invasive plant Xanthium sibiricum prefer to use nitrate nitrogen,while its native congener X.sibiricum prefer to use ammonium nitrogen,under which biomass of the former is lower than that of the latter.In addition,X.sibiricum showed toxic symptoms under hydroponic culture with high ammonium nitrogen.In order to explore the mechanisms of ammonium toxicity of X.sibiricum and the detoxification mechanisms of X.sibiricum,the two Xanthium species were first treated with eight concentrations of NH₄⁺(0.25,0.5,1.0,2.5,5.0,7.5,10.0 and 12.5 mmol L^-1)for 7 d in this study.The differences in the effects of different ammonium nitrogen levels on the two Xanthium species were evaluated,and the critical ammonium nitrogen level,under which toxic effects of ammonium occurred,was determined.Then,the two Xanthium species were further treated with three ammonium nitrogen concentrations(0.5,2.0 and 8.0 mmol L^-1)for different times to explore the differences and related mechanisms in the effects of different ammonium nitrogen concentrations and treatment time on the two Xanthium species.This study has not only important theoretical significance,but also potential value for preventing and controlling X.sibiricum by regulating soil nitrogen forms.Through observing and measuring leaf shape,size,color,chlorophyll content,total biomass and root-shoot ratio of the two Xanthium species under different ammonium nitrogen concentrations,we found that the increase of ammonium nitrogen concentration significantly inhibited growth and development of Xanthium sibiricum.The ciritical ammonium nitrogen concentration,under which Xanthium sibiricum showed ammonium toxicity symptoms,was about 2.5 mmol L^-1.For Xanthium sibiricum under high ammonium treatment,leaf chlorosis occurred,leaf edge was slightly curled,leaf top was dry;and chlorophyll content,total biomass and root/shoot ratio were all significantly reduced.However,X.sibiricum was not poisoned under each ammonium nitrogen concentration.The increase of ammonium nitrogen concentration was beneficial to the growth of X.sibiricum within a certain range,its leaf chlorophyll content and total biomass all significantly increased.With the increase of ammonium nitrogen concentration and the prolongation of treatment time,the content of free ammonium nitrogen in Xanthium sibiricum increased,while the activities of ammonium assimilating enzymes such as glutamine synthetase and glutamate synthetase changed little.At the same time,the contents of hydrogen peroxide and malondialdehyde in Xanthium sibiricum increased,while superoxide dismutase activity and the contents of antioxidants such as reduced glutathione content were low,so the plants would be seriously poisoned by ammonium and damaged by oxidation.In contrast,under high ammonium stress,the activities of ammonium assimilating enzymes such as glutamine synthetase and glutamate synthetase in X.sibiricum are higher,which reduces the concentration of ammonium nitrogen in plants and alleviates the toxic effect of ammonium.At the same time,the content of hydrogen peroxide in X.sibiricum is less affected by ammonium nitrogen concentration and treatment time,and the activity of superoxide dismutase and the content of reduced glutathione in its plants are high,which can effectively remove active oxygen and reduce oxidative damage to leaves.The results of this study showed that under high ammonium stress,ammonium assimilation capacity and antioxidant capacity of Xanthium sibiricum were weak,and it was vulnerable to ammonium toxicity,while X.sibiricum could alleviate the toxic effects of ammonium by increasing the assimilation of ammonium nitrogen and the quenching of reactive oxygen species.Increasing soil ammonium nitrogen content might affect the invasion of X.strmarium.