The citrus cultivation area in Mazandaran province, north of Iran, is about 120000 ha with about 3 million tons of annual production. Mandarin is one of the most important citrus varieties in this region and its cultivation area in Mazandaran province is about 25000 ha, with annual production about 500000 tons per year. One of the most important physiological abnormalities in the Satsuma mandarin cultivar is puffiness, which is a serious problem in the citrus industry, especially in the old orchards of the north region of Iran. These puffed fruits are easily damaged during the process of harvesting, sorting, packing, transportation, and storage. Therefore, the puffy fruits are not desirable in the market. Application of calcium is known to reduce peel puffiness to some extent. Calcium penetrates into the epidermis of the peel cells and is then quickly absorbed by lower epidermis. This reduces or prevents the activity of enzymes that cause peel puffiness. Therefore, the peel Ca content can have a great impact on reducing physiological disorders, especially fruit puffiness, and increasing the net profit of producers.
Materials and methods
In order to investigate the fruit puffiness status and the effect of calcium nitrate spray on quality and puffiness of Satsuma mandarin fruit, two separate experiments were conducted. In the first experiment, the rate of Satsuma mandarin (Citrus unshiu 'Sugiyama') fruit puffiness in different orchards, in the eastern region of Mazandaran was compared. In the second experiment, the effect of calcium nitrate application as foliar spray on reduction of fruit puffines was examined. The experiment was performed in a commercial orchard, with sour orange rootstock, based on a randomized complete block design, with six treatments and four replications. Treatments included: T1. Control (no calcium nitrate spray); T2. Calcium nitrate spray in late May; T3. Calcium nitrate Spray in mid- June; T4. Calcium nitrate spray in early July; T5. Calcium nitrate spray in late May and mid- June; T6. Calcium nitrate spray in late May, mid- June and early July.
Results and discussion
The results of the first experiment showed that the percentage of puffed fruits in the orchards of different regions varied from 27 to 46%, and the number of puffed fruits increased significantly from the early of November. The results obtained from the second experiment indicated that calcium nitratespray in T5 (late May and mid- June) and T6 (late May, mid- June and early July) treatments had a significant effect on puffiness, so that the number of puffy fruits reduced by about 20% compared to the control, but this anomaly did not completely prevented . Also, calcium nitrate spray had a significant effect on fruit Ca concentration. The highest concentration of Ca in whole fruit, fruit peel and inner skin (albedo) occurred in T5 and T6 treatments. Therefore, calcium nitrate spray is recommended at least twice (late May and mid- June) up to three times (late May, mid- June and early July) in Satsuma mandarin orchards. The peel puffiness is not only associated with changes in albedo cells, but also by significant changes in the cells of lower epidermis. Using calcium nitrate foliar application allows calcium to penetrate directly into the fruit through the cuticle. This suggests that the mechanism that controls the calcium is more physical than physiological. In fact, it keeps the stomata open and facilitates the transpiration. Fruit puffiness is not completely controlled by calcium application, but it is significantly reduced to an economic level. Developing tissues have an urgent and continuous need for calcium. This calcium is supplied by xylem vessels and depends strongly on transpiration rate from the surface of that organ. Calcium deficiency and the resulting disorders occur when low transpiration and the flow of raw sap cannot provide enough calcium for the development of those tissues. In different regions of Mazandaran province, the number of sunny hours is low, especially in the first stage of fruit growth, and the relative humidity is high. Therefore, there is a possibility of calcium deficiency in most years, especially in tissues that have low transpiration. Calcium deficiency in the fruitlets development stage reduces the cellular communication between peel cells and increases the possibility of puffiness. The maximum absorption of calcium in citrus fruits is approximately from after fruit set to 45-50 days after flowering. After June drop, calcium absorption decreases rapidly. In citrus fruits, most of the calcium enters the albedo tissue in the first phase of fruit growth until about 100 days after flowering; then the calcium is evenly distributed between the albedo and the pulp. In the case of citrus fruits, after the physiological drop, the wax of fruit peel surface develops rapidly. The increase of wax will reduce the efficiency of calcium absorption.
In general, the percentage of puffed fruits in the orchards of different regions varied from 27 to 46%, and the number of puffed fruits increased significantly from early November. Also, spraying calcium nitrate in the first stage of fruit growth (after fruit set to about 20 to 30 days after the physiological drop of the fruits) caused an increase in calcium content of the fruit peel, especially in the inner skin (Albedo), and reduced the rate of puffing fruits. Therefore, based on the results of this research, it is recommended to spray calcium nitrate 2 -3 times, with intervals of approximately 15 days, from late May to early July.