Subsurface image analysis of plant cell wall with atomic force microscopy

The hypothesis of this paper is that atomic force microscopy (AFM) is not just limited to imaging topography, but, with appropriate image processing, it can give important and quantitative subsurface information. The technical approach was to use high-resolution imaging of cellulosic structures with AFM, then use image processing with specially developed software. The example chosen here was a hydrated plant cell wall. The novelty of this work was that with the new software, it was possible to image and analyze four layers of plant cell wall laminates below that of the surface layer. In particular, the structure of primary celery (Apium graveolens L.) epidermis cell walls was characterized at the nano-scale using AFM in the Peak Force Tapping Mode. The plant cell wall micro-fibrils were found to be well separated with spacings of up to almost 50 nm and it was possible to identify and evaluate five layers in terms of fiber thickness, angular orientation and spacing. We concluded that the micro-fibril structure is weakly anisotropic and shows evidence of both horizontal and vertical bundling of micro-fibrils. The results are significant in that they provide information about cell wall characteristics several layers below the surface.