Effect Of Chlorine On Citrus Growth And Its Migration In Soil-citrus System

Chlorine is an essential nutrient element for plants.Citrus is considered a chlorine-sensitive crop,which has been discouraged from using chlorine-containing potassium fertilizers for a long term,and potassium sulfate has been employed as the main potassium fertilizer.The accumulation of sulfate would easily lead to soil acidification and heavy metals accumulation.While the long-term non-application of chlorine-containing fertilizers and the leaching of chloride（Cl^-）in soil may lead to chlorine deficiency and affect the yield and quality of citrus.Appropriate chlorine-containing fertilizers application to chlorine-sensitive crops such as tobacco and potatoes can promote yield and quality,but few reports focused on the application of chlorine-containing fertilizers to citrus.This field trial was conducted in a 5-year continuous chlorine-containing fertilizers application in citrus orchards,investigating the impact of chlorine levels on the growth,yield and quality of citrus,and the composition and structure of soil microbial communities.The soil-tree transport properties of chlorine were explored with the soil core lysimeter experiments.A nutrient solution incubation experiment was set up to further investigate the growth response mechanisms to different chlorine levels of citrus.The main findings of this trial are as follows.（1）Various chlorine levels improved the yield and quality of Newhall and Satsuma to varying degrees.Yields of Newhall and Satsuma increased by 13.33-37.89%and 20.76-38.71%,respectively,and the maximum yield reached at the chlorine application range of450-485 kg ha^-1 and 435-568 kg ha^-1.Meanwhile,application of chlorine-containing fertilizers also increased the fruit weight,peel thickness,soluble solids,and Vc content,of which increased by 7.73%-14.90%（for Newhall）and 5.53%-14.17%（for Satsuma）,4.23%-11.29%and 1.11%-5.39%,1.12%-4.52%and 0.71%-2.29%,1.24%-11.75%and 2.28%-11.59%,respectively,and titratable acid content decreased by 4.92%-9.64%and 4.28%-9.74%,respectively.（2）The content of Cl^-in citrus orchards soil,leachate,and trees significantly increased by application of chlorine-containing fertilizers,but the content of Cl^-in soil and trees did not increase significantly with the years of chlorine-containing fertilizer application.The reason was that Cl^-leached out in a relatively short period.Within 77 days of the chlorine-containing fertilizer application,68-82%of Cl^-leached into the leach solution and deeper soil layers beyond 60 cm.The Cl^-content of soil and citrus were also lower in years with higher rainfall.Cl^-leaching led to increased leaching of Calcium ion(Ca²⁺),and potassium ion（K⁺）,while potassium sulfate application led to increased leaching of NO₃^-and Mg²⁺.The uptake of chlorine by the citrus improved nitrogen and potassium content,and thus positively affected the fruit quality.The content of Cl^-was in order of leaf>peel>pulp.In addition,the long-term application of sulfur-containing fertilizers leads to a yearly increasing sulfur in soil and citrus leaves,and the excessive sulfur in leaf negatively affected the fruit quality.（3）Compared with no chlorine applications,5-years continuous application of chlorine-containing fertilizers increased the contents of K,Ca,Mg,and EC value and decreased the content of N in the soil;significantly enhanced the activity of soilβ=1,4-glucosidase（βG）,cellulose disaccharide hydrolase（CBH）,β-1,4-xylosidase（βX）andβ-1,4-N-acetylaminoglucosidase（NAG）;Microorganisms were more limited by phosphorus nutrients when chlorine application was below 75 kg ha^-1and more limited by carbon nutrients when chlorine application was between 150-450 kg ha^-1.Chlorine-containing fertilizers increased the abundance of beneficial bacteria such as Gliocladiopsis,Lophiostoma Opinomyces,and salt-tolerant bacteria such as Subdoliganulum,decreased the abundance of harmful bacteria such as Didymella,wonexibacter,Helicobacter and nitrifying bacteria such as Granulicella and Badyrhizobium.But,the abundance of the microbial community was increased generally.Soil nutrients（P,K,and Ca）and soil extracellular enzyme（βX,βG,and CBH）were positively correlated with soil microbial community abundance.Above all,it was speculated that chlorine-containing fertilizers could improve the contents of soil nutrients and thus increased the activities of extracellular enzymes,which further accelerated the abundance of microbial community,and promoted nutrient uptake by citrus.（4）Under nutrient solution culture conditions,the low and high chlorine levels inhibited the growth of citrus.This was manifested by decreased number and development of chloroplasts in the leaves,inhibited the activity of Rubisco enzyme,and the reduced chlorophyll content and starch quantity,which reduced photosynthetic capacity under low chlorine levels,induced yellowed leaves,short and thick root length systems,and increased lateral roots but decreased new roots.High chlorine levels induced reactive oxygen species（ROS）damaging organelle structure and severely caused organelle lysis and apoptosis,and further caused leaf thickening recoil,obstructed root system development,root tip expansion,and even resulted in serious leaf yellowing wilt,until withered.Appropriate chlorine levels increased the number of chloroplasts and starch,promoted cell expansion,and enhanced photosynthesis and plant growth.Plants subjected to low and high chlorine stresses increased cell wall thickness to resist chlorine stress by increasing antioxidant enzyme activity and antioxidant substance contents.Low chlorine levels promoted the catabolism of starch and sucrose and the synthesis of malic acid to maintain stomatal movement.The synthesis of galactinol and cottonseed was enhanced to eliminate cellular ROS and maintain cellular homeostasis.The synthesis of lignin primes such as methyl eugenol and coniferin was also boosted to improve lignin content.High chlorine levels promoted the synthesis of soluble sugars such as myo-inositol,sucrose,and maltose to maintain osmotic pressure homeostasis.The expression of genes associated with the phenylpropanoid biosynthesis pathway and the synthesis of lignin was promoted regulating cell wall extension and remodeling;the activity of CLC and some NPF transport proteins was increased to promote the efflux of Cl^-and to reduce the toxic effects of chlorine overload on cells.Appropriate chlorine levels acidify the cytoplasm and increase the expression of Expansin protein-coding gene and the synthesis of growth hormone,resulting in cell expansion and growth promotion.In conclusion,in the citrus growing areas with more rainfall in the south,after the application of chlorine-containing fertilizers,most of the Cl^-was leached out of the deep soil and only a little Cl^-remained in the soil tillage layer.The composition and structure of the soil microbial community was improved,enhanced the nutrient uptake of citrus was enhanced,and the growth,yield and quality of citrus were increased.Farmers can choose to apply containing-potassium chloride fertilizers in appropriate amounts,thus reducing fertilizer costs and increasing income.