Effect of potassium-magnesium ratio on ‘Zaosu’ pear fruit quality

Abstract:【Objective】Potassium (K) and magnesium (Mg) are essential macro nutrients for plants. Excessive K in plants has an antagonistic effect on Mg uptake, particularly when Mg is low in soils. De- spite the well-known role of Mg in various critical functions, there is surprisingly little research about the role of Mg nutrition in fruit crop production and quality. Hence, Mg is often considered a“forgotten element”. However, Mg deficiency is increasingly becoming an important limiting factor in intensive fruit crop production systems, especially in soils fertilized with high K. Research has shown that reason- able combination of potassium and magnesium had a positive effect on the yield and quality of vegeta- ble and fruit crops. The effects of different potassium-magnesium ratio on sugars, acids and mineral ele- ments in‘Zaosu’pear fruit were studied in order to provide reference for applying potassium and mag- nesium fertilizers to produce high quality fruit.【Methods】Field experiment was performed in a pear or- chard located in Xingcheng city, Liaoning province, China, during 2018 to 2019. Seven-year-old‘Zao- su’pear trees at a distance of 4m between rows and under standard horticultural practices and disease and pest control were selected. Each treatment consisted of 3 biological replicates each consisting of two-tree plot. Six treatments were set with different potassium-magnesium ratios including 0, 2.69,  5.38, 10.76, 16.14 and 21.52. The potassium and magnesium fertilizers were applied to the soil at 20 cm under the base of each tree at 15 days and 60 days after full blooming. During the harvest season (Au- gust 20, 2019), twenty fruit in the outer canopy were randomly collected. All the samples were washed and air dried. Some of fruit were used to determine the quality traits including contents of soluble sol- ids, soluble sugars and total organic acids, sugar components and acid components. Solid/acid ratio and sugar/acid ratio were calculated. The remaining fruit samples were dried at 75-80 °C for determinations of potassium, calcium and magnesium using an inductive coupled plasma emission spectrometer (iCAP 6200).【Results】The potassium-magnesium (K/Mg) ratio had significant effects on the contents of po- tassium, calcium and magnesium in fruit. With the increase in K/Mg ratio, the content of potassium in fruit first increased and then decreased, and the contents of calcium and magnesium first decreased and then increased. When the K/Mg ratio was 10.76, the potassium content in fruit was the highest while the contents of calcium and magnesium were the lowest, and therefore the ratios of K/Mg and K/Ca in the fruit were the highest. There was an extremely significant difference between the treatment with a K/ Mg ratio of 10.76 and that with a K/Mg ratio of 0. K/Mg ratio had a significant effect on soluble solids in pear fruit and was significantly positively correlated with total acid content; the nitrogen content in the fruit was significantly negatively correlated with the sugar/acid ratio; the potassium content in the fruit was strongly positively correlated with the soluble sugar content in the fruit (R=0.970); the magne- sium content in the fruit was significantly negatively related to soluble solids. The K/Mg ratio in the fruit was significantly positively correlated with the content of soluble solids. The soluble solid content, the solid/acid ratio and the sugar/acid ratio were the highest under the treatment with the K/Mg ratio of 10.76. The sweetness index and flavor index increased as the K/Mg ratio in fertilization increased from 5.38 to 10.76, but not significantly. The content of fructose in‘Zaosu’pear fruit was the highest, ac- counting for 52.52% of the total sugar content. Sorbitol and sucrose increased first and then decreased with the increase in K/Mg ratio. Citric acid and malic acid were the main organic acids in‘Zaosu’pear fruit. Citric acid was negatively correlated with K/Mg ratio in fruit, and quinic acid had an extremely significant positive correlation with shikimic acid【. Conclusion】The quality related to sugars and acids in‘Zaosu’pear fruit was improved by combined application of potassium and magnesium fertilizers. Lower K/Mg ratio increased the fruit flavor indexes, solids/acid ratio and sugar/acid ratio. When K/Mg ratio was 5.38-10.76, fruit quality was the best. Therefore, this K/Mg ratio range is recommended for fertilization in‘Zaosu’pear production.