Improving Quality and Antioxidant Characteristics of Pulp and Peel of Citrus During Storage by Pre-Harvest Spray of Calcium Chloride on Fruit

 Calcium is a key plant nutrient with significant functions including reducing fruit senescence; calcium deficiency has been involved in several physiological disorders in fruits. Pre-harvest foliar application of calcium chloride (CaCl2) is an efficient and safe strategy to maintain or improve the fruit quality during storage. Also, calcium significantly increases the membrane and cell wall density and reduces the physiological disorders of the fruit. Thus, the aim of this study was to evaluate the effect of pre-harvest CaCl2 sprays of citrus trees at three fruit development stages on improving fruit quality and antioxidant capacity during storage. To conduct this research, citrus trees of three different varieties (‘Thomson’, ‘Moro’ and ‘Page’) were sprayed with different concentrations of CaCl2 (0, 1, 2 and 4%) at three fruit development stages (120, 140 and 160 days after full bloom). After harvesting, citrus fruits were stored at 5°C and 85% relative humidity for 60 days. Then, weight loss and juice percentage, total carotenoids, superoxide dismutase (SOD) and ascorbate peroxidase (APX) activity, and total antioxidant capacity (IC50 content) of the fruits peel and pulp were determined on 0 (harvesting time), 30 and 60 days. Data were analyzed using SAS (Ver.9.1) and significance of the differences between the means was computed using Tukey test.    Results showed that weight loss percentage in fruit treated with CaCl2 (especially in 2 and 4% treatments) of all three cultivars was significantly less than control during storage. Total carotenoid of pulp of Thomson and Moro cultivars increased with increasing of CaCl2 concentrations compared to control during storage. In Page mandarin, the carotenoid content of pulp was not affected by calcium chloride treatment. Results also showed that SOD activity in peel and pulp of treated fruits was higher than control at harvest time. At the time of harvesting, the peel APX activity of Thomson and Moro cultivars (control) was higher than those treated by calcium. Antioxidant capacity (based on IC50 content) of peel and pulp varied depending on the cultivar and the treatment. That is, the highest antioxidant capacity was determined in ‘Thomson’ at harvest time and in treated fruits with high level of CaCl2. In the present study, the CaCl2 pre-treatment was sprayed on the citrus trees and the results indicated that pre-harvest calcium chloride significantly decreases weight loss percentage and increases antioxidant capacity, SOD activity and total carotenoid contents in citrus fruits during storage. It seems that the accessibility of sufficient CaCl2 during plant growth has a strong influence on concentrations of bioactive compounds and overall antioxidant properties of citrus fruits. Overall, it is suggested that an appropriate CaCl2 pre-treatment of citrus tree can help improve or maintain pulp and peel quality during storage. It is further suggested, using chlorine-free calcium sources in southern Iran with alkaline soils.