Anatomy, ultrastructure and lignin distribution of stone cells in two Pyrus species

The statement “seeing is believing” expresses the importance of microscopy in basic and applied research. In this study, anatomy, ultrastructure and lignin distribution in stone cells from the fruit of two Pyrus species (Pyrus bretschneideri cv. ‘Jingaisu’ and Pyrus pyrifolia cv. ‘Kousui’) were examined by using light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) as well as autofluorescence analysis. To our knowledge, this is the first time this combined method has been used to analyze the stone cells in pear fruit. Sections stained with phloroglucinol–HCl revealed the presence of lignin in stone cells, and showed that stone cells were distributed in a mosaic pattern in the flesh, with larger stone cells concentrated around the core and smaller ones in the pericarp. There were no obvious differences in stone cell structure between the two varieties, but stone cell size and content was much greater in ‘Jingaisu’ than in ‘Kousui’. Further, lignin accounted for 29.8% of stone cell composition in ‘Jingaisu’, a significantly higher proportion than in ‘Kousui’ (24.6%); this result was confirmed by autofluorescence analysis. More detailed information on lignin distribution across the cell wall was obtained by TEM combined with the KMnO4 staining technique. In TEM images, cell walls of both pear varieties were typically divided into four layers: compound middle lamella (CML), secondary wall 1 (S1), outer secondary wall 2 (S2L) and secondary wall 2 (S2), with different staining intensities for different lignin concentrations in those regions.