Absorption and Distribution of Calcium (Ca-45) Applied to the Surface of Orange (Citrus sinensis) Fruits at Different Developmental Stages

In this study, the rate of absorption and transport of calcium (Ca) in orange fruits (Citrus sinensis cv. Fukumoto) after surface treatment with Ca-45 was evaluated by supplying treatments at different dates between fruit set and mid-phase II of growth (i.e., 30, 44, 66 and 99 days after full bloom, DAFB). Ca-45 absorption was always detected, but Ca-45 application at the fruit set was associated with the highest rates of Ca transport into the pulp (39%) compared to the other treatments (17-19%). Scanning electron microscopy SEM) images of the fruit surface showed the occurrence of stomata along with the entire rind at all the developmental stages evaluated. However, from the beginning of stage I, stomata began to collapse and develop plugs, and this became frequent as the fruit age increased. From 44 DAFB, oil gland density increased, and Ca oxalates (CaOx) were found in the fruit flavedo and albedo. Fruit Ca (not labeled) concentration increased from fruit set (30 DAFB) to 99 DAFB, although oil gland formation and rind growth and differentiation likely hindered Ca transport to internal fruit tissues (pulp). The total Ca concentration in the pedicel was always higher than that in the fruit, with no differences between the first three treatments. The information obtained in this study may be essential for improving Ca fertilizer efficacy in citrus by spray applications.