Cell wall modifications in the pericarp of litchi (Litchi chinensis Sonn.) cultivars that differ in their resistance to cracking

Abstract

SummaryChanges in structural calcium and galacturonan concentrations, the degree of methylesterification of pectins, the activities of pectin methylesterase (PME) and soluble and wall-bound peroxidases (POD), and amino acid compositions of structural proteins in the fruit pericarp (skin) were studied in cracking-resistant ‘Huaizhi’ and cracking-susceptible ‘Nuomici’ litchi. Structural calcium concentrations were higher in ‘Huaizhi’ than in ‘Nuomici’ and decreased from 22 d to 52 d after anthesis (DAA), then increased. Galacturonan concentrations increased over time and were higher in ‘Huaizhi’. Methylesterification of pectins increased from 50% at 22 DAA, to 55% at 78 DAA in ‘Huaizhi’, but fluctuated around 48% in ‘Nuomici’. PME activity was significantly higher in ‘Nuomici’ than in ‘Huaizhi’ in the later stages of fruit development (> 50 DAA). The activities of soluble (SPOD) and ionically wall-bound POD (IWBPOD) increased with fruit development. Maximum SPOD activity was similar in both cultivars, whereas the peak activity of IWBPOD was higher in ‘Nuomici’ than in ‘Huaizhi’. Total concentrations of SDS-soluble amino acids were similar in the two cultivars. Total structural proteins (SDS-insoluble amino acids) decreased with fruit development. Among the SDS-insoluble amino acids, hydroxyproline was the only one that increased over time. The results suggest that higher levels of structural calcium and galacturonans may contribute to cracking resistance in ‘Huaizhi’, while a higher activity of IWBPOD, which catalyses the formation of phenolic cross-linkages and an irreversible increase in the rigidity of cell walls, may be associated with cracking susceptibility in ‘Nuomici’. Structural proteins may not be involved in resistance to cracking in litchi cultivars.