Effect Of Chlorine On Growth,Yield And Fruit Quality Of Kiwifruit And Its Mechanism

Chlorine is one of the essential nutrient elements for higher plants.The chlorine naturally present in the soil can satisfy crop growth for most crops.Some chlorine-sensitive crops are susceptible to the adverse effects of high chlorine.Therefore,chlorine is often considered harmful,and chloride-containing fertilizers are rarely or never applied on cash crops.However,kiwifruit is a crop with special needs for chlorine,many studies have reported that kiwifruit needs more than ten times the chlorine demand of ordinary crops.Kiwifruit has even been proposed as a model plant for studying chlorine,but these are mainly through hydroponics or the pot experiment confirmed.It still lack of research on the effects of chlorine on field-growth kiwifruit yield and fruit quality,the reasonable amount of chlorine application in kiwi orchard has not been clearly reported,and the mechanism of chlorine action on kiwi fruit is still unclear.Therefore,this study determined the feasibility of chlorine application in kiwifruit orchards in Shaanxi Province based on the investigation of chlorine status in kiwifruit orchards in the main kiwifruit production areas of Shaanxi Province.The field experiment and pot experiments of different levels of chlorine application were conducted.In the field experiment,the yield,quality,plant and soil chlorine content of kiwifruit for three consecutive years were determined,and the residual effects for two consecutive years were analyzed to determine the reasonable chlorine application amount for kiwifruit orchards.The pot experiment analyzed the effect of chlorine on plant growth and the distribution of chlorine in the plant,determined the critical value of chlorine tolerance of kiwifruit,and clarified the mechanism of chlorine on kiwifruit through the analysis of the transcriptome and metabolome of the leaves.The main research results are as follows:（1）Through the investigation and analysis of 154 orchards,the content of 0-40 cm soil water-soluble chlorine of kiwifruit orchards in Shaanxi Province was 3.38-203.71 mg kg^-1,with an average of 19.11 mg kg^-1.In the 0-40 cm soil,10.1%of water-soluble chlorine was above the medium level,and 48.4%was below the medium level,91.6%of the leaves were below the appropriate range of chlorine,only 8.4%of the leaves were below the appropriate range of chlorine.A large number of kiwifruit orchards in Shaanxi Province were generally deficient in chlorine,so the use of chloride-containing fertilizer should be appropriately increased.（2）The yield of kiwifruit in two treatments（S+Cl）and（Cl+S）applied with two different potassium fertilizers（K₂SO₄ and KCl）as basal fertilizer or topdressing fertilizer was significantly higher than that in treatment（S+S）only applied with K₂SO₄.The content of vitamin C in fruits under（S+Cl）treatment was significantly higher than that under（Cl+S）and（Cl+Cl）treatments.The content of trace elements in leaves and fruits under（S+Cl）treatment was also increased,and the effects on soil nutrients and p H were small.K₂SO₄ as basal fertilizer+KCl as topdressing fertilizer was the better measure to apply potassium fertilizers in kiwifruit orchard.（3）In the three-year field experiment of different chlorine application amount showed that the low chlorine application amount(Cl 170-340 kg ha^-1)increased the yield,economic benefits,and had no adverse effects on the fruit quality of kiwifruit compared with the treatment without chlorine application.Continuous application of high amount chlorine fertilizer(1480 kg ha^-1)was not beneficial to increase the yield of kiwifruit,and decreased the contents of fruit Vc,free amino acids and soluble proteins,while the content of chloride ions in plant and soil increased.In the three-year experiment,all the chlorine treatments did not cause toxicity to plants and soil,and the residual rate of Cl^-in 0-60cm soil was not more than12%.Considering the yield,fruit quality and economic benefit,the optimal application range of chlorine in kiwifruit orchard was 170-340 kg ha^-1.（4）Two years after the application of chlorine in the field was stopped,low chlorine application amount treatments still had the effect of increasing yield on kiwifruit,and the decrease of yield and Vc content caused by excessive application of chloride-containing fertilizer（Cl-910 and Cl-1480）disappeared.The application of chloride-containing fertilizer increased the content of chloride ions in plants and soil,and this effect gradually weakened with the increase of the years when the application of chloride-containing fertilizer was stopped.The leaching effect of chloride ions on deep soil（100-300 cm）decreased year by year,and the residual chlorine in the deep soil（below 100 cm）due to the application of chloride-containing fertilizer decreased rapidly after the application of chlorine was stopped.In the production process,if excessive application of chloride-containing fertilizer causes production reduction,it should be stopped in time.（5）The results of pot experiment with different chlorine application concentration showed that the dry matter weight of the plant decreased with the increase of chlorine concentration,while the chloride ion content increased with the increase of chlorine application concentration,and the Cl^-content of the same organ was different in different species.The distribution of chlorine in trees of different ages and species showed as follows:leaf>（fruit）>root>shoot>trunk,above ground>underground part,but the amount of chlorine allocation in each organ was affected by the amount of chlorine applied,the species and the age of trees,and the influence of tree age and species was more significant.The critical concentration of chlorine application was determined to be 336-545 mg kg^-1 for kiwifruit.Under non-leaching conditions,the safe concentration of chlorine in soil was<328.3 mg kg^-1,and the safe concentration of Cl^-in leaves was<23.1 g kg^-1.（6）The results of pot experiment with different chlorine application amounts showed that the SPAD and dry matter weight of leaves under low chlorine treatment（T3）were slightly higher than those under no chlorine treatment（T1）,while the dry matter weight and Vc content under high chlorine treatment（T5）were significantly lower than those under no chlorine treatment（T1）.Based on transcriptome and metabolome analysis,when the chlorine application amount was low（T3）,chlorine regulated the expression of Novel03308 and Novel03415 genes,thereby affecting the up-regulation of tryptophan metabolism,glyceride metabolism and the up-regulation of metabolites in the carbon fixation pathway in photosynthetic organisms through up-regulation of related genes.It was beneficial to the synthesis of growth hormone and carbon fixation of photosynthesis,which was ultimately beneficial to the growth of kiwifruit and the improvement of yield and fruit quality.When the amount of chlorine application was high（T5）,chlorine affected the down-regulation of metabolites in galactose metabolism,ascorbic acid metabolism,starch and sugar metabolism by regulating the expression of related genes,and reduce the direct or indirect intermediates in ascorbic acid synthesis pathway,which was not conducive to the increase of Vc content.The effect mechanism of chlorine on the growth of kiwifruit was revealed from the level of gene and metabolite.（7）Based on the comprehensive analysis of soil and leaf,this study determined the chlorine status of kiwifruit orchards in Shaanxi Province.The appropriate amount of chlorine applied in kiwifruit orchard was determined through three years’field treatments and two years’residual effect study.The critical concentration of chlorine application and the safe concentration of soil and leaf were determined by pot experiment,and the mechanism of different chlorine application on kiwifruit was analyzed from the perspective of molecular biology,which made up the gap of current research.It can provided theoretical and practical guidance for rational chlorine application in kiwifruit orchard.