Study On The Mechanism And Relative Alleviation Measures Of Ammonium Toxicity To Citrus

As one of the most important economic crops in the world,citrus cultivation area and yield ranked first among all fruits.In 2019,the cultivated area and yield of citrus in 19 provinces of China reached 2.7×106 hm² and 3.8×107 t,respectively,ranking first in the world.However,the main citrus producing areas in China are located in the tropical and subtropical regions with abundant rainfall,and the poor site conditions such as soil acidification and soil nutrient imbalance seriously restrict the sustainable development of citrus industry in China.In acidic soil,fertilization or microbial action will affect the content and proportion of NH₄⁺and NO₃^-in the soil.However,higher NH₄⁺concentration will cause the phenomenon of growth inhibition such as chlorosis of plant leaves and underdevelopment of root system,which is also related to physiological toxicity.Therefore,in view of the potential ammonium toxicity problem caused by the accumulation of NH₄⁺in acidic soils in citrus producing areas,this paper first investigated the morphological and physiological characteristics of ammonium toxicity in typical citrus rootstocks under different ammonium application levels under hydroponic conditions.Then,the conversion process of NH₄⁺-N and its effect on the toxicity of ammonium in citrus rootstocks were compared under the same ammonium application conditions under pot culture.Finally,the potential and physiological mechanism of adding nitrate nitrogen to alleviate the toxicity of ammonium in citrus rootstocks were discussed under hydroponic conditions.The main results of this study are as follows:（1）Citrus are ammonium sensitive plants.Under hydroponic conditions,when the concentration of ammonium nitrogen increased from 5m M to 20m M,the root activity decreased by59.8%,chlorophyll content decreased by 19.0%,nitrogen metabolism and antioxidant metabolism were disregulated,and membrane lipid peroxidation of leaves and roots was aggravated.When the nitrogen dosage was 10m M,the biomass of citrus was decreased,the plasma membrane rupture degree of root tip was aggravated,and the root activity was decreased.In addition,high ammonium condition can cause cation imbalance in citrus and inhibit the accumulation of Mg,K and Ca.（2）Nitrogen transformation in soil is closely related to soil pH,which significantly affects the content and proportion of NH₄⁺and NO₃^-in soil solution.The application of 400mg kg^-1 ammonium sulfate in calcareous purple soil required 10 d for the complete conversion of NH₄⁺,while the application of 400mg kg^-1 ammonium sulfate in acidic yellow soil required more than 24 d.Therefore,the acidic soil was more likely cause NH₄⁺ accumulation.Compared with no nitrogen treatment,the application of ammonium sulfate reduced the pH value of the soil solution,which in turn continued to affect soil nitrification.When 100mg kg^-1 ammonium sulfate was applied,the activities of enzymes and antioxidant enzymes in the nitrogen metabolism system of acidic and calcareous soils increased,showing obvious toxic reactions.When 400mg kg^-1 ammonium sulfate was applied,compared with the calcareous soil,the biomass,root length,root volume and root surface area of the citrus growing in the acid soil decreased significantly,which showed serious toxic phenomenon.The root length,root surface area and root volume of the citrus growing in the acid soil all reached the highest value when the ammonium application amount was 50mg kg-1,and increased by 50%,26%and 18%,respectively,compared with the treatment without nitrogen application.The lowest value was found at 400mg kg^-1 ammonium sulfate was applied,which decreased by 23%,58%and 49%compared with no nitrogen application.There was no significant effect of ammonium level on the root morphology of citrus on calcareous soil.Moreover,high ammonium content significantly reduced nitrogen concentration in roots,stems and leaves of citrus,leading to a decrease in nitrogen accumulation.（3）Adding nitrate nitrogen can alleviate the inhibition effect of ammonium stress on citrus growth.When the ratio of ammonium to nitrate was 1∶1,the biomass accumulation of Poncirus trifoliata（L.）Raf.reached the maximum,which was increased by 15.3%compared with that of pure ammonium nitrogen.When the ratio of ammonium to nitrate was 10∶1,the biomass accumulation of Citrus junos Sieb.ex Tanaka reached the maximum,which was increased by 26.9%compared with that of pure ammonium nitrogen.The mitigation mechanism was to reduce the absorption of NH₄⁺and increase the activities of GOGAT,GDH and NR enzymes to promote the assimilation of NH₄⁺.In conclusion,when the amount of ammonium nitrogen is excessive,the accumulation of ammonium nitrogen will lead to the disorder of nitrogen metabolism,inhibit the absorption of mineral elements,increase the content of reactive oxygen species and reduce the activity of antioxidant enzymes,thus inhibiting the growth of citrus.It was suggested that excessive application of ammonium nitrogen fertilizer would cause ammonium toxicity to citrus.Combination of ammonium and nitrate can alleviate the toxicity,so it is suggested that ammonium and nitrate should be applied together in citrus cultivation.