Calcium application regulates fruit cracking by cross-linking of fruit peel pectin during young fruit growth stage of citrus

• Distribution and content of calcium in the intercellular tissues of the fruit pericarp affect the development process of the fruit pericarp. • Calcium participates in the structural formation of pectin substances in the fruit pericarp during the development of citrus fruits. • The young fruit stage is a critical period for calcium supplementation to prevent fruit cracking. We conducted a meticulous analysis to elucidate the physiological responses to calcium supplementation at the young fruit growth and fruit expansion stages, focusing on its impact on fruit pericarp development. Our study demonstrated that calcium application significantly reduced fruit cracking by 71.09 %, increased fruit load by 16.73 %, and enhanced yield by 17.39 %. Notably, calcium application during the young fruit growth stage had a more pronounced effect on yield improvement. TEM-EDS spectral analysis revealed that calcium application increased calcium distribution within pericarp intercellular spaces, facilitating cross-linking of demethylated pectin with calcium ions and altering pectin composition. This led to a predominance of stable ionic and covalently bound pectins during fruit pericarp thickening, reducing labile water-soluble pectins. This change significantly inhibited the degradation and softening processes of the fruit pericarp by enzymes such as cellulase, xylanase, and polygalacturonase, allowing the fruit pericarp to maintain a higher degree of firmness. The denser cellular arrangement and increased hardness effectively combated fruit cracking. Our findings underscore the pivotal role of calcium in pericarp development and provide a physiological basis for reducing fruit cracking.