| Calcium (Ca), as an essential element for citrus growth, constituent of cell wall and membrane system, has important effects on yield and quality of citrus fruit. Ca concentration of fruit is closely related to cell structure soundness and membrane stability with the result that impacts physiological diseases of fruit. Chongqing is one of best regions to meet the need of citrus growth where the citrus area amounts to1.26×105hm2and citrus has been a mainstay industry for the development of Three Gorges reservoir area. But the physiological diseases such as fruit cracking and oleocellosis occurred frequently, the shelf life of citrus fruit after harvest was so short that hindered citrus yield increase and quality improvement. Whether these physiological diseases related to Ca nutrition has not been certain. Many researches have been carried out on Ca nutrition of fruit trees and mainly concentrated on Ca absorbed by deciduous fruit trees, for example, apple and pear, but few on Ca nutrition of citrus especially on Ca absorption, accumulation and distribution in citrus trees. The relation of Ca and fruit quality and physiological diseases was studied by Ca supplement after fruit harvested but few reported about the effect of Ca sprayed during fruit growth on the quality and storage property of fruit.Therefore, it is necessary to research Ca nutrition in Chongqing citrus orchards and the absorption and distribution of Ca in citrus trees to make clear the physiological mechanism of Ca on fruit quality and diseases so that prevent fruit from physiological diseases, prolong the storage time and propose comprehensive management measures.The study was carried out by investigation, field test, storage test and sampling determination. It was researched that nutrition concentration of soil and leaves in Chongqing citrus orchards to make clear Ca nutrition status, the relation of Ca and other nutrient elements, the differences of Ca nutrition among citrus cultivars to determine the important factors of Ca in citrus orchards and regulate Ca nutrition reasonably in Chongqing; Ca absorption and distribution in citrus tree was studied to propose basic data for regulating Ca nutrition of citrus by digging and separating Beibei447Jincheng orange (citrus sinensis Osbeck) trees cultivated7years under field condition into different parts in a growth cycle; Beibei447Jincheng orange was investigated to research the differences of N, P, K, Ca concentrations and enzyme activities in leaves, peel and pulp between cracked fruits and normal ones. The effect of calcium on pectin, malonaldehyde (MDA) and enzymes related to hydrolysis and reactive oxygen species clearing and polyphenol oxidase (PPO) of citrus peel were studied by spraying calcium nitrate before blossom and young fruit period on Beibei447Jincheng orange to determine the physiological mechanism of fruit cracking; The effects of Ca on growth, quality and storage property of fruit and its physiological mechanism were studied by spraying Ca on citrus trees in different growth periods to propose comprehensive management measures for Ca nutrition of citrus orchards.Ca in soil and leaves of218citrus orchards in Chongqing were studied and the results showed that there was abundant Ca in Chongqing citrus orchards with the average of3316.7mg/kg, soil Ca concentration in49.5%citrus orchards exceeded3000mg/kg belonging to excessive range and that in17.9%citrus orchards was less than1000mg/kg under the range of deficiency but Ca concentration of leaves in38.3%citrus orchards (<30g/kg) under the range of deficiency and that only in0.3%citrus orchards exceeded70g/kg belonging to excessive range, which signified that soil with abundant Ca did not ensure the need of citrus trees to Ca nutrition.Soil Ca concentration was significantly positive correlated to soil pH and decreased with the decrease of soil pH value. Long-term fertilization and other anthropic factors aggravated soil acidification and then impacted Ca concentration in soil. Soil pH decreased by9.6%in84.6%citrus orchards. Soil pH was below5.5in32.7%citrus orchards. Fertilization could be main cause of soil acidification in citrus orchards. Effective measures should be taken to protect citrus orchards soil from acidification to avoid Ca deficiency in soil and citrus trees.The relation of Ca concentration in soil and citrus leaves was not certain because of the change of Ca concentrations in soil. Under the low range of available Ca concentration in soil, Ca concentration in leaves increased with the increase of soil Ca concentration and they were significantly positive correlation:y=0.0151x+16.459(r=0.608**, n=39, ro.o1=0.408); When soil Ca concentration was in low or high range, it was not correlated to Ca concentration of leaves in deficient or excessive range. As a result, deficient Ca in soil must lead to Ca deficientcy in leaves but excessive Ca in soil could result in deficient Ca in leaves, it was limited to increase Ca concentration in leaves by Ca fertilized in soil.There were large differences among Ca concentration in leaves of different citrus varieties with highest Ca in Naval orange and lowest Ca in Jincheng orange leaves, which might be related to their own hereditary character of absorption to Ca. Ca concentration of leaves in Yongchuan and Jiangjin citrus orchard were lower than optimum range but those in other areas were in the range of 30-45g/kg. Ca concentration in citrus leaves was significantly positive related to Magnesium (Mg) but negatively correlated to nitrogen (N), phosphorus (P), potassium (K), ferrum (Fe) and manganese (Mn).It was studied that Ca absorption and distribution in citrus trees and the results showed that a citrus tree absorbed119.4g Ca from soil in a growth cycle, among of it81.0%for the growth of above ground part and19.0%for roots. The crucial periods of absorption to Ca were germination period and fruit rapid-growth period which the average of Ca accumulation was66.3%and32.7%respectively. Spraying Ca on citrus trees in the two periods could be regarded as an important measure to improve Ca nutrition of citrus trees. The order of Ca concentrations in different organs of citrus trees was:leaves>branches>roots>fruits. Most of Ca absorbed by citrus tree accumulated in branches and leaves and the percentage of Ca accumulated in roots, branches, leaves and fruits were21.2%,50.7%,25.0%and3.0%, respectively. Trunk was one of major organs of citrus trees to store Ca nutrition. The concentration and accumulation of Ca in fruits were both lower than other organs, which could be one of reasons physiological diseases occurred frequently in fruits.The Ca concentrations of citrus organs were different from each other, which of leaves were higher than those of branches and those of1-year-old branches were higher than those of adult ones. The Ca concentration of young shoots increased but adult branches decreased with the growth of new organs. The Ca concentrations in cortex of branches were well above those in xylem of branches. Ca concentrations and accumulation in cortex of branches increased with the increase of branch diameter but the concentration of xylem was the opposite. There were differences in Ca requirements among citrus organs. Fruits accumulated more Ca during young fruit period and fruit rapid-growth period than other periods; Branches accumulated Ca mainly in fruit rapid growth period. Ca absorbed by roots was difficult to transport to fruit because of its low transpiration. As a result, spraying Ca in citrus trees during young fruit period and fruit rapid-growth period could be crucial measures to regulating and controlling Ca deficiency in fruits.Beibei447Jincheng orange was investigated to research the differences of N, P, K, Ca concentrations and enzyme activities in leaves, peel and pulp between cracked fruits and normal ones to study the relation of Ca and fruit cracking and its physiological mechanism. The results showed that the concentrations of Cain leaves, peel and pulp of normal fruits were higher than those of cracked ones. Fruit cracking rate was negatively correlated to Ca concentration in citrus peel significantly. The activities of polyphenol (PPO), polygalacturonase (PG) and cellulase (CX) of cracked fruits were higher than those of normal ones, but those of catalase (CAT) were lower. Fruit cracking rate was positively correlated to PPO and negatively correlated to CAT and the concentration of protopectin (PP) significantly. The concentration of MDA, the activities of PPO, PG, CX of citrus peel in fruit rapid-growth period decreased by spraying calcium while the activities of superoxide dismutase (SOD), CAT and the concentration of protopectin (PP) increased significantly. Low concentration of Ca in citrus peel resulted in increase of activities of enzymes (PPO, PG and CX) in cell-wall metabolism. The low protopectin concentration that might maintain intensity and extensibility of citrus peel was the main cause for fruit cracking. Spraying calcium to citrus trees decreased fruit cracking rate and could be regarded as an important measure to prevent fruit cracking of Jincheng orange.Ca concentration and vitamin C (Vc) concentration in citrus fruits increased significantly, the oxygenolysis of Vc was restrained, total soluble solid (TSS), the ratio of sugar and acid increased and the edible flavour maintained during storage time by spraying Ca on citrus trees in different growth periods. The activities of polygalacturonase (PG) and cellulase (CX) decreased but those of superoxide dismutase (SOD), catalase (CAT) increased because of high Ca concentration in fruits. The structure of cell was protected and lipid peroxidation was alleviated so that malonaldehyde (MDA) and soluble pectin concentration decreased, the senescence of fruit postponed and eventually kept the fruit in good during storage time. Spraying Ca on citrus trees in young fruit period was the best way to keep citrus fruit in good property during storage time.
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