Both nitrogen and calcium fertilization management are vital for crops, where an imbalance of these elements can cause both physiological and yield problems. It has been proposed that nitrogen absorption, particularly ammonium, is in part dependent on calcium supply. Moreover, the balance between these two nutrients could be a key indicator of plant growth in some species. Tomato, one of the most cultivated crops worldwide, can also be widely affected by nutritional imbalance. Using large amounts of N fertilizers could lead to an imbalance with other nutrients and, thus, detrimental effects in terms of plant development and yield. Here we show that ammonium excess has a negative impact on plant development and results in calcium deficiency. Moreover, a deficit in calcium nutrition not only affects calcium concentration but also leads to a restriction in N uptake and reduced N concentration in the plant. These effects were evident at the seedling stage and also during flowering/fruit set. Using PCA analysis, we integrated both phenotypic and nutritional imbalances in seedlings and grown plants. Interestingly, the Ca/N ratio appears to be a key indicator to monitor appropriate N and calcium nutrition and more importantly the balance between both. Maintaining this balance could be an essential element for tomato crop production.

In this study, the rate of absorption and transport of calcium (Ca) in orange fruits (Citrus sinensis cv. Fukumoto) after surface treatment with ⁴⁵Ca 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). ⁴⁵Ca absorption was always detected, but ⁴⁵Ca 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.