Analysis of heterogeneity and dynamic variations of recalcitrance in maize straw to enzymatic hydrolysis

As an important sugar crop, maize is widely cultivated around the world. During planting and processing, large quantities of agricultural residues (straw) are generated, which are considered valuable feedstock in biomass conversion and biorefining. Different organs or tissues exhibit notable heterogeneity in chemical composition and physical properties, introducing complexity and uncertainty to the high-value utilization of maize straw. In this study, a systematic experiment was conducted on different parts of maize straw at various growth stages to determine the chemical composition: including neutral detergent solubles, cellulose, hemicellulose, and lignin and the physical properties, such as the degree of polymerization and crystallinity of cellulose, and to evaluate their effects on reducing sugar yield from enzymatic hydrolysis. A multi-dimensional analysis was performed to investigate the resistance factors that hinder enzymatic hydrolysis and their dynamic changes throughout growth, with the aim of elucidating how chemical composition and physical structure influence enzymatic digestibility.The results showed that the composition of enzymatic resistance factors in maize straw was consistent across different parts, yet the distribution of these components exhibited distinct evolutionary patterns among tissues. Throughout the growth period, leaves and stem pith displayed relatively minor changes in chemical composition, whereas stem rind and leaf sheaths underwent more pronounced variations. Lignin content increased across all tissues, with the most substantial change observed in the stem rind, rising from 1% to 15%. In the late growth stage, lignin content in the stem rind (9%-13%) was higher than that in the stem pith (9%-13%), leaves (5%-10%), and leaf sheaths (1%-13%). Cellulose content showed distinct tissue-specific trends, decreasing in the stem pith from 30% to 25%, while continuously increasing in the stem rind, ranging from 20% to 35%. In contrast, hemicellulose gradually increased in the stem pith (28%-30%) but decreased markedly in the stem rind, falling from 28% to 17%, a pattern potentially related to the different functional roles of cell walls in these two tissues. Neutral detergent solubles were highest across all tissues in the early growth stage (35%-45%) but declined significantly in the stem rind and leaf sheaths during the later stage. Ash content remained the lowest and relatively stable throughout, ranging from 0.1% to 0.5%. The overall content followed the order: neutral detergent solubles、 cellulose、 hemicellulose、 lignin、 ash.Crystallinity index across different tissues ranged from 18% to 48% and showed an overall increasing trend with plant growth, reflecting cell wall maturation and structural reinforcement. The degree of polymerization ranged from 350 to 900, initially increasing before stabilizing, suggesting that during straw growth, the number of terminal reducing ends of polymer chains increased, thereby providing more anchoring sites for cross-linking with hemicellulose and lignin.The anti-degradation barrier in maize straw is synergistically constructed by multiple resistance factors throughout the growth process, with the dominant enzymatic resistance factors exhibiting heterogeneity among different tissues. Crystallinity served as the primary anti-enzymatic factor in the stem rind and pith, whereas degree of polymerization and hemicellulose played key roles in the leaf sheath. The anti-enzymatic effects of these factors on leaves were not pronounced, which may be attributed to the relatively uniform and stable composition and structure of leaf biomass. In addition, neutral detergent solubles also negatively affected reducing sugar yield from enzymatic hydrolysis, indicating that the anti-degradation barrier results from the mutual constraints and synergistic interactions among various resistance factors.These physicochemical properties exhibited a high degree of heterogeneity across different growth stages of maize. Specifically, leaves and leaf sheaths showed similar heterogeneity patterns, whereas pronounced differences were observed between the rind and pith. In summary, the recalcitrance of maize straw is characterized by pronounced heterogeneity and complexity, which in turn contributes to its more intricate properties. This study provides insights into the mechanisms underlying this recalcitrance and lays a theoretical and technical foundation for the high-value utilization of maize straw based on its specific recalcitrance characteristics.