Research progress in the occurrence and control of bitter pit disorder in apple

Abstract：The apple (Malus pumila) is a deciduous fruit crop of the Rosaceae family and one of the four most important fruit species in the world. China ranks first in the world in per capita consumption and exports of apples. The apple industry plays a key role in the national economy, but the apple fruit quality in China is poor and the proportion of high quality fruit is low. As we all know, calcium is one of the essential mineral elements in the process of apple growth and development, and it is an important component of the cell wall, which plays an important role in maintaining cell membrane homeostasis and intracellular signal transduction. Too high or low intracellular calcium ion concentrations have a negative effect on apples. Keeping intracellular calcium homeostasis plays an important role in plant growth and development and response to stress. The lack of calcium leads to disturbances in cell metabolism, causing apple bitter pit and other physiological disorders, which seriously affect the fruit quality. Apple bitter pit is a physiological disorder caused by calcium deficiency in fruit, manifested by the rupture of flesh cells and the formation of small, dark-coloured indentations. Apple bitter pit often occurs when fruit is close to ripening and during the time of storage, and is most common in the distal part of the fruit. In apple cell, low concentrations of intracellular calcium reduce the stability of cell walls, cell membranes and membrane-bound proteins, but high concentrations are also toxic to the cell, with highconcentrations of calcium ion leading to the formation of phosphate precipitates that interfere with phosphate-based energy metabolism and compete with magnesium ions for enzyme binding sites. Ensuring intracellular calcium homeostasis is important for healthy growth of apple. Calcium homeostasis in the cytoplasm of apple involves a variety of calcium transporters and organelles, which form a complex regulatory network. Apple mainly absorbs calcium from the soil through the root and transports it to the above-ground part under the drive of transpiration pull. Since the transpiration of fruit is low, apple fruit is susceptible to physiological calcium deficiency. At the cellular level, apple mainly absorbs calcium ions through the apoplastic and symplastic pathways in the root vascular bundles, and the above-ground parts of plants can absorb calcium ion through non-vascular bundles. The root system absorbs and transports calcium ion through calcium channels, the superfamily of calcium ion/cation anti-transporters (CaCAs) and the P-type Ca2+ -ATPase, which transports calcium ion in a chelated state through the xylem to the vigorously growing canopy, including young leaves, flowers, fruits and apical meristematic tissues. In general, apple peel calcium concentration is higher than flesh, and phloem calcium content is higher than xylem. In the early stages of apple fruit growth, calcium is evenly distributed throughout the fruit, but as the season progresses there are differences in concentration, with the highest calcium content in the peel, the lowest in the flesh, and the middle in the seed and centre of the fruit. Fruit calcium content is closely related to tree factors such as cultivar, rootstock type and agronomic practices like fertilization and pruning. The calcium content in fruit trees and the distribution of calcium in the fruit are largely controlled by genes, and different stion combinations also have significant effects on apple fruit calcium content. Calcium uptake by apple is also influenced by the balance of calcium with other mineral elements and total salinity. In addition, overgrowth of fruit trees and competition for calcium between branches and fruit lead to low calcium levels in fruit. External application of various types of calcium fertilizer can increase the calcium content in fruit, improve fruit quality and increase fruit shelf life. Although calcium plays an extremely important role in preventing bitter pit in apples, a single calcium deficiency may not the key factor in triggering apple bitter pit. Compared to healthy fruit, symptomatic fruit has lower levels of calcium and boron, while the opposite is true for nitrogen, potassium, phosphorus and magnesium; the lower the calcium content in the fruit, the higher the magnesium and potassium content, and the more severe the apple bitter pit disorder. The improvement and enhancement of the fruit quality and appearance will open up more opportunities for our apple industry and will promote its healthy development. This paper reviews the relationship between apple bitter pit disorder and calcium, calcium absorption and transport in apple, the mechanism of calcium deficiency in apple and the progress in research on the control of apple bitter pit disorder, with a view to improving the calcium absorption capacity of fruit trees, increasing the calcium content in fruits to prevent apple bitter disorder, improving the fruit growth and quality during post-harvest storage, reducing the burden of fruit growers, and promoting the development of the apple industry.