Artificial calcium enrichment can significantly enhance fruit quality. Winter is the season for fruit storage, and after harvest, fruits such as apples, pears, citrus, grapes, mangoes, and longans enter the stages of transportation, storage, and consumption. While some fruits have good appearance and flavor right after harvest, they may develop issues during storage, such as apple bitter disease, pear black heart, jujube cracking, and mango depression, leading to a decline in quality and reduced storability. Scientific studies show that these physiological disorders are often caused by low calcium levels within the fruit.
The key challenge lies in improving the overall calcium nutrition of fruit trees, especially increasing the calcium content directly in the fruits. Although leaf calcium levels are commonly used for nutritional diagnosis, in practice, fruit calcium deficiency is more common than leaf calcium deficiency. In recent years, calcium deficiency in orchards has become increasingly severe, prompting researchers to focus on fruit-targeted calcium enhancement mechanisms and artificial adjustment strategies. This article introduces various calcium-increasing techniques before and after harvest to effectively reduce the occurrence of physiological diseases and extend the shelf life of fruits.
One might wonder why calcium levels in fruits tend to be low despite being an essential nutrient. The impact of calcium deficiency on fruit quality is significant—shortened storage life, bitter pox, ulcers, water core, internal breakdown, and other disorders can all result from it. Importantly, calcium is a non-mobile element in plants, making it difficult to redistribute once it's fixed in certain parts of the tree. Therefore, addressing fruit calcium deficiency requires understanding the physiological transport mechanisms of different fruit species and carefully timing calcium applications, rather than simply applying any calcium fertilizer.
Pre-harvest calcium management involves proper irrigation, drainage, and organic matter addition to improve soil calcium availability. However, excessive watering can lead to extended growth periods, larger fruit size, and lower calcium content. Moderate soil drought and air drying during the growing season can actually help increase fruit calcium levels. Applying calcium fertilizers, particularly those that increase exchangeable calcium in the soil, plays a crucial role in preventing diseases like bitter pox. Even in calcareous soils with high calcium content, fruit calcium deficiency can still occur, as seen in apples with reduced hardness and increased risk of bitter pox during storage.
Overuse of nitrogen fertilizers, especially ammonium-based ones, hinders calcium uptake and transport. Studies show that NH4+ interferes with Ca2+ absorption. Current farming practices often overemphasize nitrogen fertilization, leading to an imbalance between nitrogen and calcium, which contributes to fruit calcium deficiency. High N/Ca ratios have been linked to bitter disease in apples. Similarly, excessive potassium and magnesium can also exacerbate calcium deficiency.
During the fruiting period, foliar calcium sprays are effective. Calcium solutions should be applied to young fruits within six weeks after flowering, such as 1–2% calcium chloride spray every 4–5 weeks. A stronger solution (2.5–3%) can be used one to three times before harvest. Common calcium fertilizers include calcium chloride and calcium nitrate, while newer organic calcium products have shown promising results.
For bagged fruits, calcium sprays should be applied before bagging. For example, 0.5% calcium acetate can improve firmness and reduce cracking in litchis and cherries. However, excessive concentrations may cause leaf spots or damage.
Post-harvest calcium treatments, such as infiltration and soaking, are effective in maintaining fruit quality and reducing rot. Pressurized calcium infiltration yields the highest calcium content, up to two to three times more than spraying or temperature difference methods. However, excessive calcium treatment can affect aroma release and skin quality, potentially lowering overall fruit quality.
During storage, calcium redistributes within the fruit. In apples, calcium moves from the core to the outer layers. Injecting calcium chloride into the core can help prevent bitter pox. Combining post-harvest calcium treatments with controlled atmosphere storage and heat treatments further enhances fruit firmness and reduces spoilage.
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