Effect of potassium application on yield, quality and photosynthetic characteristics in 'Junzao' jujube

Abstract: 【Objective】Ziziphus jujubes'Junzao'is a very important economic horticultural crop. The fruit is rich in nutrition and contains a variety of vitamins and trace elements. It tastes sweet and mellow, and is praised as a good tonic. However, in recent years, due to excessive amount of fertilizer applied to jujube trees, the fertilizer efficiency has decreased significantly. Potassium is a recognized quality element. Excessive or unreasonable application of potassium fertilizer not only reduces the benefit from the fertilizer but also causes increase in fruit cracking, decrease in quality and yield, soil crusting,and environmental pollution. In this study, we examined the effects of potassium fertilizer on yield,quality, photosynthetic capacity and tree nutrition of'Junzao'in order to screen the optimal application amount of potassium that improves fruit yield and quality while reduces fruit cracking rate.【Methods】'Junzao'trees were used as the materials for the experiment carried out for two consecutive years in Jingtai County, Gansu Province. Five concentrations of potassium fertilizer(K2O)(0, 76.5, 153, 229.5,and 306 g per plant) were used. Mature fruit, leaves and branches were collected in 2017 and 2018, and quality, yield, contents of nitrogen, phosphorus and potassium and tree nutrition were determined. Chlorophyll content and photosynthetic characteristics of'Junzao'leaves were determined during fruit expansion in 2018.【Results】1) The yield of'Junzao'was increased and the rate of fruit cracking was significantly decreased by potassium fertilization. The effect of yield increasing was strongest at 153 g per plant. Cracking rate was in the range of 3.58%-55.47%, and treatment at this dose had a significant lower fruit cracking rate than the other treatments. 2) Compared with the control(0 g per plant), potassium application significantly improved the nutritional status of'Junzao'tree, and the soluble sugars in leaves and shoots increased with the increase in potassium application. The soluble starch content in the leaves in the treatment at 306 g per plant was significantly higher than in the other treatments. 3) Potassium application significantly improved fruit quality and photosynthetic efficiency in the two seasons.The effect in the treatment at 153 g per plant was most significant. Moreover, with the increase in potassium application rate, vitamin C(Vc), soluble protein, sugar to acid ratio, contents of sucrose, fructose,nitrogen, phosphorus and potassium, net photosynthetic rate(Pn), stomatal conductance(Gs), transpiration rate(Tr), and contents of chlorophyll a, chlorophyll b, total chlorophylls, nitrogen, phosphorus and potassium increased within certain range but decreased when the potassium application exceeded this range. The intercellular CO2 concentration(Ci) and chlorophyll a/b increased at low doses and increased at higher doses, and the application rate of 153 g per plant was the lowest. Principal component analysis showed that the application of potassium at 153 g(K2O) per plant had the most significant effect in improving the yield, quality and leaf photosynthetic efficiency and in reducing fruit cracking rate of'Junzao'fruit.【Conclusion】Under normal application of nitrogen and phosphorus fertilizers, supply of potassium significantly promotes chlorophyll synthesis, improves photosynthetic efficiency, and has a dosage effect. Furrow application of potassium fertilizer(K2O) at 153 g per plant was shown to have the most significant effect. Therefore, in order to achieve high yield and quality, the recommended application amount of potassium fertilizer(K2O) is 153 g per plant in the areas along the Yellow River in Gansu Province.