Response Mechanism Of Nitrogen Utilization And Photosynthetic Physiological Characteristics Of Leymus Chinensis Under Different Nitrogen Forms To Drought Stress

Global climate change has led to frequent drought events in grassland ecosystems and a dramatic increase in atmospheric nitrogen deposition,severely affecting nitrogen availability and aboveground productivity in terrestrial plants.Nitrogen and water are important determinant of the structure and function of terrestrial ecosystems.Plant photosynthesis contributes to biochemical energy production and carbon assimilation.Improving plant photosynthetic efficiency is an important strategy to continuously increase yields and thereby to improve agricultural production and animal husbandry.Therefore,studies of nitrogen allocation in plants and the responses of photosynthetic processes to drought stress can help to predict and regulate aboveground productivity in grassland ecosystems.In this study,the dominant species Leymus chinensis in the northeast farming-pasture zone of the Songnen Plain was used to evaluate the effects of nitrogen in various forms under drought stress.The treatment groups were no nitrogen（N0）,ammonium nitrogen（NH₄）,nitrate nitrogen（NO₃）,and mixed nitrogen（NH₄NO₃）.The nitrification inhibitor dicyandiamide was added to NH₄ and NH₄NO₃treatments.Three drought stress treatments were established under each nitrogen form:low drought（LD:field water capacity 65%-70%）,medium drought（MD:45%-50%）,and high drought（HD:25%–30%）.In this study,nitrogen absorption and utilization as well as the responses of photosynthetic physiological characteristics to different nitrogen forms under drought stress and the underlying mechanisms were evaluated.The following key results were obtained:（1）NO₃ treatment increased nitrate reductase（NR）and nitrite reductase（Ni R）activities as well as NO₃^-reduction to NH₄⁺in L.chinensis,thereby promoting the absorption and utilization of NO₃^-and NH₄⁺in various organs and improving leaf and root morphogenesis.Plant growth inhibition by drought stress was alleviated by NO₃and NH₄NO₃,which promoted root and shoot growth and improved the absorption and utilization of water and nutrients,consequently facilitating the accumulation and distribution of aboveground and belowground biomass.The total biomass in the NO₃treatment group was 66.29%,23.46%,and 22.01%higher than those in the N0,NH₄,and NH₄NO₃ treatment groups,respectively.Moreover,NO₃ and NH₄NO₃alleviated the drought-induced inhibition of leaf and root growth,reduced the root-shoot ratio,improved the activity of nitrogen metabolism enzymes,promoted the uptake of nitrogen in plants,and increased plant biomass.（2）There was a significant positive correlation between nitrogen allocation to components of the photosynthetic system and photosynthetic nitrogen use efficiency（PNUE）in L.chinensis leaves.The carboxylation system and bioenergetic nitrogen allocation were the main limiting factors for PNUE when soil water was sufficient.Nitrogen allocation in the light harvesting system was the main limiting factor under severe drought stress.Under drought stress,L.chinensis maintained a higher net photosynthetic rate and PNUE by reducing nitrogen storage and structural nitrogen under NO₃ treatment;the photosynthetic rate was 13.87%and 15.39%higher and PNUE was 24.84%and 34.04%higher under NO₃ treatment than under N0 and NH₄treatments,respectively.However,under NH₄ and NH₄NO₃ treatment,L.chinensis allocated more nitrogen to non-photosynthetic organs under drought stress（57.78%and57.48%,respectively）and mainly stored nitrogen.Water use efficiency,leaf stomatal conductance,and mesophyll conductance of L.chinensis were improved,while CO₂diffusion was promoted by NO₃ under drought stress.More nitrogen was allocated to the photosystem during drought stress under NO₃ treatment,leading to an improvement in photosynthetic efficiency.（3）Drought stress significantly inhibited pigment accumulation in L.chinensis leaves under treatment with different nitrogen forms,leading to the excessive accumulation of reactive oxygen species and increased hydrogen peroxide production.It also inhibited ATP synthesis and disrupted the donor side of oxygen-evolving complex（OEC）photosystem II（PSII）,resulting in the overreduction on the acceptor side of PSI and triggering photoinhibition.The activities of ascorbate peroxidase（APX）and plant alternate oxidase（AOX）were higher after NO₃ treatment than after NH₄treatment,and these increases activated the alternate respiration（AOX）pathway and resulted in the consumption of excess electrons in the electron transport chain.NO₃treatment improved the performance index based on absorbed light energy(PI_abs)and electron transport per reaction center（ET/RC）.NO₃ reduced PSI photoinhibition and prevented PSII from drought damage by protecting against OEC and PSII damage,furtherly enhancing electron transport flux,and ensuring the stability of nitrogen in the photosynthetic apparatus of L.chinensis.The inhibition of chlorophyll accumulation in L.chinensis by drought stress was attenuated by nitrate nitrogen addition,while oxidative stress-related regulatory enzyme activity and PSII efficiency were enhanced and electron transport flux was regulated to protect PSII from oxidative damage by nitrate nitrogen addition under drought stress.（4）The photosynthetic efficiency of L.chinensis under different nitrogen forms as well as the nitrogen allocation to photosynthetic system and its components were significantly positively correlated with aboveground biomass（P<0.05）.PNUE and PI_abs were significantly positively correlated with aboveground biomass under NO₃ and NH₄NO₃ treatment（both P<0.05）.The photosynthetic efficiency of L.chinensis was significantly positively correlated with the drought response rate based on aboveground biomass（P<0.05）.The photosynthetic efficiency of L.chinensis under NO₃ and NH₄NO₃ treatment was highly sensitive to drought,which promoted photosynthetic carbon assimilation and further facilitated the accumulation of photosynthates and aboveground biomass.In conclusion,under drought stress,different nitrogen forms had different effects on nitrogen allocation and photosynthesis in leaves of L.chinensis.NO₃^--N addition promoted root morphogenesis,thereby promoting nitrogen absorption.Within-leaf nitrogen allocation should reflect trade-offs in L.chinensis,as predicted by the leaf economics spectrum,with more nitrogen allocation to metabolic processes at the expense of structure.Finally,NO₃^--N addition improved PNUE and was beneficial the stability of thylakoid membrane system and the growth of L.chinensis,therefore increasing biomass under drought stress.These results clearly demonstrate the importance of the form of nitrogen in determining nitrogen utilization and photosynthetic physiological characteristics of L.chinensis and improve our understanding of the mechanisms by which L.chinensis in meadow grasslands absorbs and utilizes NO₃^--N and NH₄⁺-N under the background of global climate change.