Anatomy and lignin deposition of stone cell in Camellia oleifera shell during the young stage

As the by-products of edible oil production with rich lignin, the reserves of Camellia oleifera shell were abundant and had a great economic value. Lignin was the most important limiting factor during the conversion of plant biomass to pulp or biofuels, which mainly deposited in the stone cells of C. oleifera shells. Thus, its lignin deposition made the function of stone cells in the ripening process of the shell clearer, and provided a theoretical basis for the potential utilization of the biomass of C. oleifera shells. In this study, the paraffin embedding method was used to investigate the development and difference of stone cell in the fruitlet. The lignin deposition characteristics of stone cell were analyzed by the fluorescence microscopy and Wiesner and Mäule method. The chemical-functional group types of lignin in the stone cell of C. oleifera shell were examined by the ultraviolet spectrophotometer and transform infrared spectroscopy. The stone cells, vessels, parenchyma, and vascular tissue had existed during the young fruit growing period. The anatomical characteristics and the cell tissue ratio inverse relationship between stone cell and parenchymal cell suggested that stone cells developed from parenchymal cells. With the growth of shell, the stone cell wall thickened, and thickness-to-cavity ratio from 0 to 3.6. The fluorescent results showed that lignin content increased continuously; during shell development, the mean brightness of stone cell wall from 0 to 77.9 sections was stained with phloroglucinol-HCl, and Mäule revealed the presence of G-S-lignin in stone cells, and ImageJ results showed that G-lignin was distributed in the entire stone cell wall, while S-lignin deposition accounted for 48.59% of the cell wall area. In the FTIR spectra, the shell was identified as containing G-S-lignin.