Effects of different concentrations of citric acid and oxalic acid on yield, quality and nutrient absorption of grape

Abstract: 【Objective】Shine Muscat grape is characterized by large berry size, yellow-green peel, crisp and juicy flesh, and rose fragrance, with soluble solids content higher than 20%. In addition, this grape variety has the advantages of disease resistance, high yield, high quality, and good storage and transportation performance. It has been favored by consumers, with high economic value and great market potential. Due to excessive expansion of fruit production and unscientific fertilization, soil hardening and the soil ecosystems deterioration is becoming more and more serious, leading to tree malnutrition and fruit quality decline. All these ultimately seriously restrict the high-quality and efficient development of the grape industry. Citric acid and oxalic acid, as rhizosphere exudates, can activate mineral elements in soil by affecting the rhizosphere microenvironment and microbial community, and participate in the nitrogen cycle, thereby enhancing plant nutrient absorption. Therefore, citric acid and oxalic acid have the potential as fertilizer synergists, but there are few reports on their effects in improving the quality and aroma of grapes and on utilization rate of fertilizers. The effects of citric acid and oxalic acid combinedwith NPK fertilizer on berry quality, nutrients, and aroma substances were examined and the optimum organic acid application concentrations were screened to provide a reference for the research and development of special fertilizer for improving the quality and aroma of Shine Muscat grape.【Methods】In this paper, application of NPK fertilizer alone was used as the control, and the combination with application of 5% or 10% citric acid (LC or HC, respectively) or 5% or 10% oxalic acid (LO or HO, respectively) was set. Berry quality, yield, nutritional components, and aroma content were determined.【Results】 Compared with the control, fruit firmness was significantly increased by HO treatment but significantly decreased by citric acid treatment. Compared with the control, the grape yield of HO treatment was significantly increased by 36.18%. The soluble solids and soluble sugars in the berry under LO treatment were the highest, significantly increased by 18.75% and 69.21% compared with the control, respectively. Compared with the control, the ratio of sugar to acid in fruits treated with low concentration of organic acids (LC and LO) increased significantly, and vitamin C content in treatments with high concentration of organic acids (HC and HO) was significantly increased. There was no significant difference in grape appearance quality under citric acid treatment, and HO significantly increased grape appearance quality, while LO significantly decreased. There was no significant difference in grape flesh flavor sensory score between the citric acid treatments and the control, but oxalic acid treatments significantly improved grape flavor sensory score. In addition, the oxalic acid treatments also significantly increased the comprehensive score of grape sensory evaluation compared with the control, by 21.40% and 11.53% in LO and HO, respectively. Compared with the control, LO treatment significantly reduced nitrogen, phosphorus, and potassium contents in the leaves. There was no significant difference in the nitrogen content in fruits between citric acid and oxalic acid treatments. Fruit phosphorus content in the treatments with citric acid or oxalic acid was significantly higher than the control. The contents of nitrogen, phosphorus, and potassium in fruit in HC treatment were the highest, and phosphorus and potassium in fruit were significantly increased by 60.42% and 24.02%, respectively, compared with the control. The differences in aroma compounds in different treatments showed that the composition and content of aroma substances in Shine Muscat fruit were significantly changed by citric acid or oxalic acid application. Citric acid and oxalic acid increased aroma compounds in the berries. As for aroma compounds in the flesh of Shine Muscat grape, the relative content of aldehydes in each treatment was in a range of 61%-94%. The LO treatment had the highest aldehyde content and HC had the lowest. The highest relative content of alcohols was found in HC treatment (36%), while that in the other treatments was only 1%- 4% . In the treatment of HC, the highest number of aroma substances were detected, which was 38, followed by 37 in HO. The result indicated that high concentrations of citric acid or oxalic acid increased the variety of aroma substances in grapefruits.【Conclusion】Oxalic acid treatment improved the yield and quality of Shine Muscat grape, and the best effect was found in 10% oxalic acid.