Comparison of lignin distribution, structure, and morphology in wheat straw and wood

Abstract

Agricultural fiber sources, such as wheat straw, are promising lignocellulosic feedstocks for the generation of renewable substitutes for synthetic materials (e.g., plastics, construction materials, biofuel, and other biorefinery products). The interest in the utilization of lignin has increased rapidly during the last years; the number of publications increased more than five times between 2000 and 2020. The number of publications concerning lignin from wheat straw follows the general trend with an increasing scientific interest in lignin but comprises less than 5% of the total lignin publications. The structure and morphology of lignin in straw and wood differ between the different species. The monolignol composition and spatial location in plant tissue are notably different, as well as the nature and abundance of lignin-carbohydrate linkages involving p-coumaric acid (pCA) and ferulic acid (FA) units in wheat straw lignin. To further enable the utilization of wheat straw as a resource for bio-based materials, a solid understanding of the wheat lignin structure and composition is required. This review aims to consolidate the state-of-the-art in wheat lignin and focuses on lignin and its distribution, fundamental chemical structures, and morphology in wheat straw and compares these features with lignin in wood cell walls. • The state-of-the-art in wheat lignin is consolidated. • Wheat straw lignin has primarily β-O-4 linkages and fewer C-C than wood lignin. • Wheat straw and wood differ clearly in lignin distribution, structure, and morphology. • Main units in wheat straw lignin are (H,G,S), (G) in softwood, and (G,S) in hardwood. • Lignin-carbohydrate linkages are of great significance to biomass refinement.