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Sintu Rongpipi1 2 Dan Ye1 2 Sarah Kiemle2 3 Cheng Wang4 Daniel Cosgrove2 3 Esther Gomez1 2 5 Enrique Gomez1 2 6

1, The Pennsylvania State University, University Park, Pennsylvania, United States
2, The Pennsylvania State University, State College, Pennsylvania, United States
3, The Pennsylvania State University, State College, Pennsylvania, United States
4, Lawrence Berkeley National Laboratory, Berkeley, California, United States
5, The Pennsylvania State University, State College, Pennsylvania, United States
6, The Pennsylvania State University, State College, Pennsylvania, United States

Cellulose is the most abundant source of organic polymers on earth and holds promise for efficient conversion into biofuels. Its largest source is the plant cell wall which is a complex structure of cellulose microfibrils forming a heterogeneous mixture with biopolymers like hemicellulose and pectin. The structure and assembly of these constituents in the plant cell wall is still not well understood. Many structural characteristics of cellulose microfibrils like preferred orientation and crystallinity are important for its role as source of biofuels. We attempt to study these characteristics of cellulose microfibrils through Grazing incidence wide angle X-ray scattering (GIWAXS). It is a morphological characterization technique that can probe not only the surface but also beneath it by tuning the angle of incidence. The grazing incidence X-ray beam geometry yields high signal-to-noise ratio and lesser radiation damage, making it ideal for plant cell wall samples that are fragile and weakly scattering. In our work, we find that GIWAXS can decouple scattering from cellulose crystals and epicuticular wax in onion epidermal cell walls. It also reveals a preferred orientation of cellulose crystals. The GIWAXS data from different scales of onion also reveals the dependence of degree of orientation on the age of tissue.

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