Study On Fruit Quality And Plant Nutrition Of Newhall Navel Orange In Response To Fruit Bearing Number

Since the 21st century,citrus industry has developed rapidly and has become a pillar industry for citrus farmers to shake off poverty and increase income in the hilly and mountainous areas of southern China.High-yield and high-quality fruit has always been one of the important research directions in the field of citrus research.However,the instability of citrus fruit yield and quality caused by the phenomenon of on-year or off-year is widespread,which has seriously affected the economic benefits of orchards.In this study,adult Newhall navel orange trees were used as experimental materials to research the nutrient absorption and utilization,the regulation of photosynthetic performance,and the differences of fruit yield and quality at the key phenological stages（flower bud differentiation period,physiological fruit dropping period,fruit expansion period,fruit color conversion period and maturity period）withdifferent fruit-bearing levels,and to explore the physiological mechanism of the response of fruit quality and plant nutrition of newhall navel orange to fruit bearing capacity,in order to provide a theoretical basis for flower and fruit management in citrus production and high-quality and high-yield cultivation in orchards.1.The fruit-bearing number had significant influence on the degree of on-year or off-year,fruit yield and qualityUnder the condition of this experiment,368±30 trees^-1 with the lowest fruit-bearing in 2019 had led to the occurrence of serious on-year or off-year phenomenon,of which reached 32.8%and the index of on-year or off-year increased as fruit quantity increase.。Under the local cultivation conditions,the per fruit weight decreased significantly when the fruit bearing number was more than 368±30,however,the upward trend of yield was not significant with the increase of fruit weight.In addition,with the increase of the fruit-bearing number,the fruit appearance quality index of per fruit weight,vertical diameter,horizontal diameter,fruit type index,peel thickness and brightness were significantly reduced,while the red and yellow degree of peel increased significantly;the content of titratable acid and juice yield of fruit were significantly increased,while Vc content of fruit decreased obviously,and the content of soluble solid and edible rate increased first and then decreased.2.The response mechanism of nutrient absorption and distribution to different fruit-bearing numberUnder the ecological conditions of local cultivation,the contents of N,P,K in soil and leaves,the N,P,K distribution rate from soil to leaf and the N,P nutrient of root all had positive effects on the increase of fruit-bearing capacity,and the N distribution rate between soil and leaf with higher fruit-bearing capacity was at a higher level throughout the year.The K content in leaves was at the highest level in the whole year at this stage,and the trees with high fruit-bearing number consumed more K nutrients,which led to the low K content in leaves at fruit maturity.In general,the increase of soil-leaf nutrient distribution rate and leaf N,P and K nutrient content was conducive to improve fruit juice yield and edible rate,but had a negative impact on fruit TSS and Vc content,and delayed fruit coloring.In addition,the correlation between leaf nutrition and soil-leaf nutrition allocation rate and fruit quality was the best in the fruit expansion stage,which could be used as a reference period for citrus nutrition diagnosis of fruit quality.3.The response mechanism of photosynthetic products and chlorophyll fluorescence parameters of leaves to different fruit bearing numberUnder the condition of this experiment,the improvement of PSI and PSII performance and the operation coordination between PSII and PSI in physiological fruit dropping stage were beneficial to enhancing the fruit-bearing capacity of trees;high temperature and drought in summer and less sunshine in autumn and winter inhibited the metabolic activity of PSI and the coordination of PSII and PSI operation in fruit expanding stage,while trees with higher fruit-bearing capacity could maintain higher photosynthetic physiological performance of leaves,which was conducive to the improvement of fruit quality.In addition,lower soluble sugar and starch contents in leaves during the physiological fruit-dropping stage and fruit-expanding stage were conducive to enhancing the fruit-bearing capacity and the fruit quality.4.Establishment of suitable fruit-bearing number determination model for newhall navel orangeThe results showed that the suitable-fruit bearing number of Newhall navel orange in the test area ranged from 119-173 per plant^-1,when the single fruit quality was best and reached the"L"fruit standard of navel orange,the tree yield approached the high yield standardwithout on-year or off-year phenomenon;in addition,the results of path analysis showed that the yield per plant was mainly affected by the fruit-bearing number but had less affected by the single fruit weight.The regression equation y=-18.69+0.10x₁+0.18x₂（R²=0.97）was established based on citrus fruit yield y,single fruit weight x₁ and fruit-bearing number x₂,which can be used to predict the reasonable fruit-bearing range of Newhall navel orange and achieve the goal of high yield and quality in citrus orchard.