Effect Of Potassium Chloride On Navel Orange Tree And Soil And The Difference Of Potassium Absorption Between Different Citrus Rootstocks

China is one of the main origin centers of citrus in the world,and has the largest citrus planting area and production in the world.As one of essential macronutrients for plants,potassium plays a vital role in the growth and development of citrus and formation of fruit quality.However,resource of potash in China is relatively scarce,potash fertilizer required for agricultural production is mainly rely on the importing.Potash fertilizer is mainly divided into two categories:chlorine-containing potash fertilizer and chlorine-free potash fertilizer.Citrus has been regarded as a "chlorine-sensitive crop"for a long time,and only chlorine-free potash fertilizer can be applied,which increases the cost of production.In addition,due to the lower contents of total potassium and available potassium of acid soil in main citrus producing areas,and weak ability of water and fertilizer conservation of alkaline soil,coupled with unreasonable fertilization,which led to the common problem of imbalanced potassium nutrition.This study mainly carried out two experiments:one was a field experiments which conducted in acidic and alkaline soil,respectively in 2018 and 2019.The test trees were9-year-old of‘Newhall’navel orange（Citrus sinensis Osbeck）.The annual KCl application rate for each orange tree was 250 g（T1）,500 g（T2）and 1000 g（T3）,taking K₂SO₄rate of 556 g as control（CK）.Observed the accumulation of chlorine in soil and trees,as well as the changes of nutrition of the tree,and explored whether potassium chloride can be used instead of potassium sulfate fertilization,in order to reduce the cost of potash fertilizer application for citrus production.Secondly,Trifoliate orange[Poncirus trifoliata（L.）Raf,abb.TO],Ziyangxiangcheng（Citrus junos Sieb.Ex Tanaka,abb.ZYXC）and Zhique（Citrus wilsonii Tanaka Raf,abb.ZQ）were used as experimental materials cultured in hydroponics with low level of potassium(0.1 mmol·L^-1)and normal level of potassium(3 mmol·L^-1,CK).Performance of plants,and physiological,biochemical and gene expression of different rootstocks under low potassium stress were investigated in order to lay a foundation for further understanding the cause of differences on potassium uptake by citrus.It also provides a theoretical basis for the efficient absorption of potassium by different citrus rootstocks in the future.The main results are as follows:1.Effects of Potassium Chloride on Chloride and Nutrient Content in Acid and Alkaline Soil1.1 Effects of Potassium Chloride on Chloride and Nutrient Content in Acid and Alkaline SoilThere was no significant change in the content of chlorine in pericarp and pulp after applying different concentrations of potassium chloride,The content of chlorine in soil and leaves increased significantly.T1 treatment acid soil chlorine accumulation more than alkaline soil,Compared to CK,The content of chlorine in acid and alkaline soil increased by 50.20%and 28.02%respectively;T2 treatment acid soil chlorine accumulation and alkaline soil difference;But T3 treatment of alkaline soil chlorine accumulation is greater than acid soil,Compared to CK,acid and alkaline soils increased by 79.70%and 99.36%,respectively.T3 handling,Compared with the CK,the content of chlorine in leaves of alkaline soil increased by 109.7%and 98.7% respectively,while acid soil increased by 63.8%and 58.0%,respectively.There was no significant change in relative chlorophyll,nitrogen and calcium content in leaves after potassium chloride application,However,the content of potassium and some phosphorus increased with the increase of potassium chloride application,The content of magnesium decreased slightly.1.2 Effect of potassium chloride on yield and fruit quality in acid and alkaline soilApplication of potassium chloride had no significant effect on fruit yield,single fruit weight,brightness,yellow color,soluble solids and titratable acid content.The red degree of some fruits increased significantly after application of potassium chloride instead of potassium sulfate,and the content of vitamin C increased with the increase of potassium chloride concentration.2.Effects of low potassium stress on potassium absorption and tolerance of different citrus rootstocks2.1 Comparison of potassium content and potassium absorption efficiency in different rootstocksZYXC was the least affected by low potassium stress with no yellowing symptom of potassium deficiency at the leaf tip.The degree of decreasing on potassium content in aboveground part and total potassium content of the whole plant was the lowest,and the absorption rate of net potassium was the highest among the three rootstocks under low potassium condition.ZQ was the most affected by low potassium stress,and the yellowing symptom of potassium deficiency appeared at the leaf tip after treatment.Potassium content and total potassium content in the aboveground and lower parts of plant mostly decreased among the three kinds of rootstocks,and the net absorption rate of potassium was also the lowest among the three rootstocks.2.2 Effect of low potassium stress on different chopping photosynthesis,potassium absorption-related enzyme activity,hormone levels and genesThe Fv/Fm values of the three rootstocks cultured in low potassium nutrient solution did not change compared with the control,indicating there was no photoinhibition.Compared with the control,the chlorophyll fluorescence parameters of ZYXC rootstock and ZQ rootstock slightly changed.TO rootstock showed decreased light energy absorption and electron transfer,increased heat dissipation in reaction center,and induced reversible inactivation in PSII reaction center.Compared with the control,the H⁺-ATPase enzyme activity,ethylene content,auxin content,gene expression of AHA,AUX1 and NHA1 in the root were significantly increased in ZYXC when cultured in low potassium nutrient solution,but decreased in ZQ and TO.The expression of HAK5 and TPK1 genes were up-regulated in three rootstocks.RHD2,AKT2 and TRH1 genes were up-regulated in ZYXC and TO compared to the control,while down-regulated in ZQ.AKT1 gene was up-regulated in ZYXC and TO,but slightly changed in ZQ.Compared to the control,the SKOR gene of ZYXC and ZQ had small change,but decreased in TO.3.Conclusions3.1 After short-term application of potassium chloride,the content of chlorine in soil and leaves increased significantly compared with the control,but all the levels were still under the safety threshold.Content of chlorine in the fruit did not significantly change,the yield and quality of fruit were not significantly affected,and the nutrition of leaves was less affected.It showed that it was feasible to apply appropriate amount of potassium chloride instead of potassium sulfate in navel orange orchard in acid and alkaline soil for a short period of time.3.2 ZYXC was the least affected by low potassium stress,followed by TO,and ZQ was the greatest affected.Expression of the genes related to potassium absorption and utilization,also hormone level and enzyme activity in ZYXC showed a trend of increasing,and demonstrated the strongest capacity of potassium absorption,while ZQ had a trend of decreasing,suggesting that its potassium absorption capacity was weak.