Structural characterization and deposition of stone cell lignin in Dangshan Su pear

Stone cell content is one 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. Studies of the structural features and deposition of pear lignin were rare. To understand the structural characteristics of pear fruit lignin, chemical functional groups and bond types of pear fruit lignin were studied, along with an analysis of the deposition of lignin in the stone cell. Lignin was extracted, purified and analyzed by Fourier-transform infrared (FTIR) spectroscopy and 1H nuclear magnetic resonance (1H NMR) spectroscopy. The deposition of lignin in the pear stone cell wall was detected by electron microscopy. Lignin in the pulp of Pyrus bretschneideri cv. Dangshan Su pear comprised guaiacyl-syringyl-lignin, with a relatively high amount of guaiacyl-lignin (G) compared to syringyl lignin (S). During the early development of Dangshan Su pear, lignin exhibited CO stretching in unconjugated ketoses, carbonyls and esters, but during later development, aliphatic CH stretching in CH3 (not in Ome) and phen OH occurred. The main linkages of the acetylated milled wood lignin structural units in Dangshan Su pear fruit are β-O-4, β-1, β-5 and β-β. β-O-4 was the major factor related to the solubility of lignin, with the presence of few β-5 linkages corresponding to the formation of fewer condensation-type structures, while the presence of fewer than 30% phenolic hydroxyl groups suggested that this lignin exhibited low levels of polymerization. In addition, a large number of hydrocarbon protons and side chains were present in Dangshan Su pear fruit lignin, suggesting this lignin is more easily dissolved than those with more condensation-type structures. Dangshan Su pear fruit lignin exhibited low levels of polymerization and was therefore easily hydrolyzed. Ultramicroscopy revealed that lignification extended from the corner of the primary cell wall to the remaining regions of the compound N middle lamella and the secondary wall, with lignin particles deposited along the cellulose microfibrils. Lignin particles and microfibrils were alternatively arranged until they filled up the entire cell cavity, culminating in the formation of stone cells.