Stone cell distribution and lignin structure in various pear varieties

Stone cell content is a key determinant of pear fruit quality. Stone cells are formed by the deposition of lignin on primary cell walls, followed by the secondary thickening of cell walls. Few studies have focused on how the lignin structure and degree of polymerization in different varieties of pear fruit affect stone cell content. In this study, we examined the stone cell content, degree of polymerization and lignin structural characteristics of different pear varieties, which were selected based on their basic stone cell content and the size and density of their sclereids. We analyzed the lignin functional groups and bond type combinations and the organic element content in these varieties using FTIR, 1H NMR and organic elemental analysis. ‘Beijing pear’ had the highest stone cell and lignin content, while ‘Dangshan su’ exhibited the highest degree of polymerization of stone cells. The lignin and stone cell content were lower in ‘Dangshan su’ than in ‘Beijing pear’, while the stone cell density and degree of polymerization were higher in ‘Dangshan su’ than in ‘Beijing pear’, which resulted in relatively poor taste in ‘Dangshan su’. The milled wood lignin of all five varieties of pear comprises G–S lignin. The G/S ratio was highest in ‘Dangshan su’ and lowest in ‘Yali’, while the ratio between alcoholic hydroxyl and phenolic hydroxy was highest in ‘Wonhwang’ and lowest in ‘Dangshan su’. The proton content of hydrocarbons was highest in ‘Wonhwang’ and lowest in ‘Yali’. The organic element content varied among varieties, as did the C9 empirical formulas of phenylpropane structural units. Pear fruit quality is affected by the stone cell and lignin content and the degree of polymerization of stone cells, which is influenced mainly by lignin monomer composition, the G/S ratio, the phenolic hydroxyl/alcohol hydroxyl ratio and hydrocarbon proton content. ‘Wonhwang’ was lower than the other varieties examined in terms of lignin and stone cell content and the degree of stone cell polymerization, and this variety has good fruit quality.