Response Of Inorganic Nitrogen Utilization In Broussonetia Papyriferaand Morus Alba Seedlings To Simulated Karst Stress

Plants will respond differently and adapt accordingly under various stresses conditions,drought and high p H stress are typical abiotic stress in karst areas,which will damage the normal growth of plants and cause the loss of biomass and biodiversity of karst ecosystems.Nitrogen is an essential nutrient for plant growth and development,and is also a limiting nutrient in many terrestrial ecosystems that affects plant growth.There is significant heterogeneity in the inorganic nitrogen content of soil in karst areas,and it generally presents the characteristics of low ammonium salt and high nitrate.Nitrogen is an important component of proteins,nucleic acids,and chlorophyll in plants;The nitrogen metabolism of plants is closely related to important physiological activities such as hormone balance,ion balance,production of osmoregulation substances,and chlorophyll biosynthesis in the body.Therefore,studying the response of inorganic nitrogen utilization of woody plants to karst stress will help reveal their characteristics of inorganic nitrogen utilization under stress,thus providing guidance for inorganic nitrogen management of plants in karst areas.In this study,we selected a karst-adaptable woody plant Broussonetia papyrifera,and a non-karst-adaptable woody plant Morus alba as experimental plants,studied the physiological responses of two Moraceae species seedlings under different levels of nitrate nitrogen when nitrate nitrogen was used as the sole nitrogen source,and evaluated the relationship between nitrogen demand and supply of two Moraceae species seedlings through stable nitrogen isotope fractionation values at the entire plant scale;we studied the physiological responses of two Moraceae species seedlings under different drought treatments,p H treatments,and the interaction of high p H and drought treatments under mixed nitrogen sources,based on the high-abundance¹⁵N labeling technique,the ammonium nitrogen utilization of whole plant throughout the experimental treatment period was quantified,and then the nitrate nitrogen utilization of whole plant throughout the experimental treatment period was indirectly calculated,then we analyzed the inorganic nitrogen utilization characteristics of adapted woody plant and non-adapted woody plant under karst stress.The main results of this paper are as follows:（1）Increasing the concentration of nitrate nitrogen appropriately has a significant promoting effect on the growth of seedlings of two Moraceae species,but the application amount of nitrate nitrogen will not always be positively related to the plant biomass output.Stable nitrogen isotope fractionation occurred during assimilation of nitrate nitrogen in seedlings of both Moraceae species.The stable nitrogen isotope values of the nitrate assimilates of B papyrifera seedlings gradually increased with the increase of nitrate nitrogen concentration,therefore,the table nitrogen isotope fractionation values of the nitrate assimilates of B papyrifera seedlings gradually decreased with the increase of nitrate nitrogen concentration.This indicates that the nitrate supply of 8 mmol·L^-1did not exceed the inorganic nitrogen requirement of B papyrifera seedlings when the nitrate concentration was in the range of 0.5 to 8 mmol·L^-1.However,the stable nitrogen isotope fractionation values of the nitrate assimilates of M alba seedlings showed a trend of decreasing and then increasing with the increase of nitrate concentration,and the stable nitrogen isotope fractionation values reached a minimum at a nitrate concentration of 2 mmol·L^-1,indicating that the nitrate supply of 8 mmol·L^-1already exceeded the inorganic nitrogen demand of M alba seedlings.In general,B papyrifera seedlings have stronger nitrate nitrogen assimilation capacity than M alba seedlings.（2）Severe drought stress significantly inhibited the growth of B papyrifera and M alba seedlings,Drought stress treatment led to a decrease in stomatal conductance of seedlings of both Moraceae species,compromised their photosystem II reaction centers and electron transport chains,reduced the openness of photosystem II reaction centers,and ultimately affected the net photosynthetic rate of both B papyrifera seedlings and M alba seedlings.Due to the limitation of reducing power supply,drought treatment significantly reduced the carbon and nitrogen accumulation in seedlings of both Moraceae species.The nitrogen accumulation of the plants mainly comes from the assimilation of nitrate when the seedlings of two Moraceae species are not subjected to stress.However,with the intensification of drought stress,the amount of nitrate utilized by both Moraceae species significantly decreased,but the amount of ammonium salt utilized did not decrease linearly with the increase of drought stress.The ratio of the amount of nitrate utilized to the amount of ammonium salt utilized by two Moraceae plants gradually decreases with the increase of drought severity,the increased share of ammonium nitrogen utilization under drought stress may be an inorganic nitrogen utilization strategy adopted by two Moraceae species in response to energy deprivation.（3）By comparing the changes in biomass,photosynthesis,chlorophyll fluorescence parameters,and carbon and nitrogen accumulation of two Moraceae species under different p H environments,we found that B papyrifera seedlings has stronger p H adaptability and wider adaptation range than M alba seedlings,and can grow better under acidic or alkaline conditions,M alba seedlings are suitable for neutral or acidic environments,while alkaline environments can significantly reduce the biomass of mulberry seedlings,inhibit their photosynthesis,and affect their photosynthetic organ activity.The proportion of ammonium utilized by M alba seedlings subjected to high p H stress was significantly greater than that of the B papyrifera seedlings,which in turn led to more ammonia entering the leaf cells and resulted in damage to the photosystem II oxygen release complex,which weakened the photosynthetic capacity of M alba seedlings,which in turn led to insufficient supply of reducing power,so that the amount of nitrate utilization of M alba seedlings was significantly reduced.B papyrifera seedlings maintained high nitrate utilization when subjected to high p H stress,and B papyrifera seedlings increased leaf biomass through enhanced nitrate assimilation and reduced ammonia concentration per unit leaf,thereby avoiding damage to photosynthetic organs.One of the reasons for the adaptation of B papyrifera seedlings to high p H stress in karst areas may be that they can maintain a high nitrate utilization ability.（4）Under the alkaline environment,both mild and moderate drought stress treatments significantly reduced the biomass,stomatal conductance,transpiration rate and intercellular CO₂concentration of both Moraceae species,and affected there photosynthesis.The compound stress consisting of high p H and drought played a synergistic role in inhibiting the growth of both Moraceae species.When subjected to complex stress,the carbon and nitrogen accumulation of both Moraceae species significantly decreased compared to the control.In addition,the combined stress resulted in significantly greater inhibition of nitrate utilization in both Moraceae plants than in ammonium salt.Under mild compound stress,the nitrate utilization ability of B papyrifera seedlings is significantly stronger than that of M alba seedlings,and stronger nitrate utilization ability helps to enhance the osmotic regulation ability of B papyrifera seedlings.The relatively stronger osmotic regulation ability of B papyrifera seedlings under mild combined stress may be the reason why their photosynthetic ability is superior to that of M alba seedlings.