Artificial calcium enrichment can significantly enhance fruit quality. Winter is the season for fruit storage, when large fruits like apples, pears, citrus, grapes, mangoes, and longans are harvested and then go through transportation, storage, and consumption. While some fruits may look and taste great right after harvest, they often develop issues during storage, such as apple bitter pit, pear black heart, jujube cracking, and mango depression, leading to a decline in quality and reduced storability. Scientific research has shown that these physiological disorders are primarily due to low calcium levels within the fruit. Therefore, understanding how monoammonium phosphate improves the overall calcium nutrition of fruit trees, especially by directly increasing calcium content in the fruit, is crucial.
Although leaf calcium levels have been standardized for nutritional diagnosis in fruit trees, 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 calcium improvement strategies. The following discusses artificial calcium supplementation methods 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 are so low despite being an essential nutrient. Calcium deficiency not only shortens storage life but also causes various physiological disorders, including bitter pit, ulcers, water core, internal breakdown, and porosity. Importantly, calcium is a non-mobile element in plants, making it difficult to redistribute once it's fixed in the tree. This means that simply applying calcium fertilizers isn't enough; it requires careful consideration of transport mechanisms, timing, and environmental factors like water, temperature, and nitrogen levels.
Pre-harvest calcium management includes timely irrigation and drainage, adding organic matter to improve soil calcium availability. However, excessive watering can promote vegetative growth, increase fruit size, and reduce calcium content. Controlled drought and proper air drying during cultivation can help improve fruit calcium levels. Applying calcium fertilizers, especially those that increase exchangeable calcium in the soil, plays a key role in preventing bitter pit disease. Even in calcareous soils with high calcium content, fruits can still suffer from calcium deficiency, as seen in apples that lose hardness and develop bitter pit during storage.
Excessive nitrogen fertilizer, especially ammonium-based ones, hinders calcium uptake and transport. Current fruit farming practices often overemphasize nitrogen, leading to imbalances between nitrogen and calcium, which exacerbate calcium deficiency. High N/Ca ratios have been linked to apple bitter pit. Additionally, high potassium and magnesium levels can accelerate calcium deficiency.
To meet the high calcium demand during fruiting, foliar applications should be concentrated within six weeks after flowering. Spraying 1–2% calcium chloride solution several times during this period can be effective. A few weeks before harvest, spraying 2.5–3% calcium chloride solution 1–3 times yields good results. Common calcium fertilizers include calcium chloride and calcium nitrate, while newer organic calcium products have also proven effective.
For bagged fruits, calcium sprays should be applied before bagging. For example, using 0.5% calcium acetate can improve firmness and reduce cracking in litchis and cherries. However, overly concentrated solutions may cause leaf burn or spotting.
Post-harvest calcium treatments, such as infiltration, significantly improve storage quality and prevent rot caused by pathogens. Post-harvest calcium infiltration allows calcium to penetrate the fruit through the wax layer, improving internal calcium levels and reducing surface damage. These treatments can be combined with controlled atmosphere storage or heat treatment to maintain firmness and reduce decay.
Pressurized calcium infiltration is the most effective method, resulting in 2–3 times higher calcium content compared to dip or temperature difference soaking. However, excessive calcium treatment may reduce aroma release in stored apples and cause skin damage, affecting overall 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 effectively prevent bitter pit.
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