Calcium and Boron Foliar Fertilizer to Relieve Cracking of 'Liuyuezao' Pummelos

'Liuyuezao' pummelo is highly prone to cracking, which seriously affects its quality. The aim of this study was to illustrate the effect of foliar sprays of calcium (Ca) and boron (B) and their combined treatments on the fruit cracking and quality of 'Liuyuezao' pummelos during the fruit expansion period (40-55 days after flowering). Analysis of 12 mineral elements of the pericarp by ICP-MS revealed that the three treatments significantly increased the content of calcium and boron in the corresponding pericarp. These treatments effectively reduced the enzyme activities of pectin methylesterase (PME), polygalacturonase (PG), pectin lyase (PL), β-galactosidase (β-Gal), and cellulase (Cx) in the peel and down-regulated the expression of corresponding cell wall-degrading enzyme genes. Calcium, boron, and their combination treatments reduced water-soluble pectin (WSP) in the peel. Simultaneously, they inhibited the degradation of CDTA-soluble pectin (CSP) and Na₂CO₃-soluble pectin (NSP), thereby stabilizing the cell wall structure. Additionally, these treatments enhanced fruit skin break force (Bf) and elasticity (Ela), ultimately decreasing the fruit cracking rate. Diversification analysis showed that Ca and B elements significantly increased the sugar and vitamin C (Vc) content of 'Liuyuezao' pummelo fruits and reduced their organic acid content, thus improving fruit quality. The study provides new ideas on the use of fertilizer interactions to control fruit cracking and improve the quality of the pummelo fruit.