Investigating Interfaces in the Cell Wall of Fast-Growing Plant for Next-Generation Composites

<b>Overview</b>: Interfaces and interphase zones are vital for natural composites’ functionalities, such as improving toughness and ductility in bone and nacre and maintaining structural form and water conductivity in plants during growth. Insight into the mechanics of such biological interfaces is crucial for designing next-generation structural composites for a wide range of applications. Our work focused on the cell wall interface region in the stem of fast-growing plants during their rapid growth period to identify underlying mechanics for flexible composite design.<br/><b>Background</b>: The cells walls are effectively<b> </b>the skeletal system of plants and crucial for their structural adaptability. During the growth stage, the cell walls are flexible and called primary cell walls. The PCWs of vascular tissue provide structural rigidity to the plant while simultaneously adapting to multiple demands from accommodating large growth strains, effective water conduction, adhesion to adjacent cells, and adaptability to external stressors. The PCWs of vascular tissue thus provide an excellent design template for polymer-based flexible multifunctional structural composites and hence the focus of our work.<br/><b>Methods and Results</b>: Two sets of soybean plants were grown, one with complete nutritional support and hydration and the other with 50% reduced hydration. The stem cross-sections of these plants were investigated during rapid growth using stained laser imaging and confocal Raman spectroscopy. Raman microscopy provides a powerful tool for non-destructive compositional analysis of plant cell walls at high resolution to identify the presence, distribution, and form of plant polymers present within the cell wall. Stained optical imaging of plant structure provides an efficient method for internal structure and composition analysis and correlation with Raman data. Together, they provide cell wall composition and structural data for the current study. We investigated the cellulose-pectin interface and the pectin-dominant middle lamella of the vascular tissue of the two sets of plants to identify parameters such as hydration, cellulose crystallinity, the structure of pectin and cross-linkage between different polymer chains along growth stages and hydration. The nature of cellulose-pectin interactions at these interface regions was identified and compared with that within the core cell wall to address the fundamental questions of the role of interface zone for large strain sensitivity and their broader application to composite design.