Fruit-Stalk Supplementing Calcium and Partition Regulation of Fruit Calcium for Prevention of Bitter Pit of Bagged Apple

Conventional strategies for the calcium supplementation of apples, such as via direct spray application on the fruit or administering the supplement through the root, are not suitable for apples that are cultivated using fruit bagging. Instead, calcium supplementation through the fruit stalk can avoid the inhibition created by bagging on calcium absorption of fruit and, thus, shows excellent application potential for bagged apples. Herein, a layer-by-layer self-assembly (LBLSA) method was used to prepare chitosan-sodium alginate-CaCO₃ (CSC) nanoparticles and chitosan-calcium alginate-indole acetic acid (IAA)-abscisic acid (ABA) (CCIA) nanocapsules, which were both then dispersed in chitosan thermosensitive gel. The composite gel material released Ca²⁺, IAA, and ABA in response to the signal of a citric acid/sodiumcitrate mixed solution, thereby promoting the efficient absorption of Ca²⁺. In addition, the participation of IAA and ABA effectively assisted with the development of vascular bundles in bagged apple stalks, reduced K/Ca and Mg/Ca values, and greatly promoted the reasonable partition of Ca²⁺ by activating various Ca²⁺ transporters (ACA, CAX, and CDPK). These results showed that this composite gel material can obviously reduce the incidence of bitter pit and unreasonable partitioning of calcium in cultivated apples. In summary, the bagged apple stem calcium supplement technology proposed in this study can not only effectively overcome the inefficiencies prevalent in conventional calcium supplement application methods for bagged apples, but also significantly promote the reasonable distribution of Ca²⁺ in the fruit, and is, therefore, of great research significance for the development of a new calcium supplement strategy for the prevention of bitter pit of bagged apples.