Effects of lime concentration and spray coverage on the severity of citrus sunburn and fruit quality

Abstract:【Objective】Sunburn is a serious physiological disease in citrus production, which leads to significant loss of fruit quality and yield. With the frequent exposure to high temperature and strong light in summer, the occurrence of sunburn is increasing and causes serious economic losses in citrus in- dustry. To date, there are many ways to reduce the damage of sunburn, such as spraying sunscreen, spraying water, bagging, stamping shade net, carrying out sod-culture, and building shelterbelt. Among these methods, spraying lime suspension on the canopy of citrus trees is regarded as the most economi- cal and applicable way. However, there are different opinions on suitable lime concentration for reduc- ing sunburn, and there has been no systematic study on the control effect with lime suspension. In this study, we compared the fruit and leaf surface temperatures, the incidence of sunburn, and the quality of fruit at the different treatments with spraying of lime suspension at different concentrations and coverag- es, to establish suitable concentration and coverage for sunburn control.【Methods】Two cultivars,‘Newhall’navel orange and‘Orah’mandarin, were used as the materials for the experiment. Five concen- trations of lime [0 g·L-1 (CK), 5 g·L-1, 10 g·L-1, 20g·L-1, and 40 g·L-1] and two spraying coverages (30% and 50%) were used. The surface of leaves and fruit in the outer canopies were sprayed before the  commence of summer high temperatures. The surface temperatures of the leaf and the fruit near sun- burn spot were measured with an infrared thermometer. After the end of high temperature season, the degree and incidence of sunburn were statistically analyzed. The quality indexes, including peel chromi- nance, pericarp thickness, peel hardness, flesh recovery, and contents of vitamin C, titratable acids and soluble solids were measured at the commercially mature stage. Principal component analysis of these quality parameters was carried out to establish the optimal treatment.【Results】Compared with the con- trol, the treatment of spraying lime suspensions significantly improved the leaf light reflectance in both varieties. With the increase in concentrations or spraying coverage, the reflectance of leaves increased significantly. At the same time, spraying lime suspension effectively reduced the surface temperature of the leaves and the fruit, but there was a ceiling value in the cooling effect with the lime suspension. Above the ceiling value, the surface temperatures of the fruit and the leaf showed no significant change with the further increase in the concentration or spraying coverage. The ceiling value was shown to be 10 g·L-1 in‘Orah’mandarin. In contrast, the incidence and degree of sunburn varied with the increase in lime concentration or spray coverage. The lowest incidence of sunburn was found in the treatment at the concentration of 40 g·L-1 and the coverage of 50%. The application of lime suspension affected the quality of fruit, and the most significant effect was found in the hardness of fruit peel, which was signifi- cantly increased compared with the control. In addition, the treatment also affected the brightness of the peel, with significant reduction in L value but no significant effect on a and b values. Spraying lime sus- pension increased vitamin C content in the fruit. In‘Newhall’navel orange, vitamin C content was sig- nificantly increased in all treatments compared with the control. Titratable acid content and soluble sol- id content in the treatments were also higher than the control, but only a few of them were significantly different from the control. Beyond the above indicators, there was no significant difference among the treatments. Principal component analysis of the quality indexes showed that the score of control treatment was 0.109 7 (ranking the fourth) in‘Orah’mandarin, and 0.125 6 (ranking the fourth) in‘Newhall’navel orange. When the concentration of lime suspension was higher than 20 g·L-1, the compre- hensive score was higher than the control. Among these treatments, the one with the concentration of 40 g·L-1 and coverage of 50% had the highest comprehensive score. The results showed that the best lime treatment to reduce fruit sunburn and improve quality was to spray the canopy with lime at the concen- tration of 40 g·L-1 with 50% coverage【. Conclusion】The application of lime suspension can significant- ly reduce the incidence of fruit and leaf sunburn, and has no significant adverse effect on fruit quality. For sunburn control, it is recommended to spray 40 g·L-1 lime suspension with a coverage of 50%.