Brillouin light scattering microscopy reveals micro-mechanical inhomogeneities of the apple fruit cuticle

The cuticle is a polymeric membrane covering all plant aerial organs of primary origin. It regulates water loss and defends against environmental stressors and pathogens. Despite its significance, understanding the micro-mechanical properties of the cuticle (cuticular membrane; CM) remains limited. In this study, non-invasive Brillouin light scattering (BLS) spectroscopy was applied to probe the micro-mechanics of native CM, dewaxed CM (DCM), and isolated cutin matrix (CU) of mature apple fruit. The Brillouin frequency shift (BFS) decreased significantly with wax extraction from the CM and further decreased with polysaccharide extraction from the DCM, consistent with tensile test results. Spatial heterogeneity was observed by BLS microscopy of the CM, with BFS of the anticlinal region being significantly smaller than that of the periclinal region. In the DCM, BFS was higher in the periclinal than in the anticlinal region, while in the CU, BFS was similar in both regions. The key conclusions are: (1) BLS is sensitive to micro-mechanical variations, particularly stiffness, offering novel insights into the CM’s micro-mechanical behavior and underlying chemical structures; (2) CM exhibits spatial micro-mechanical inhomogeneity, with periclinal regions being stiffer than anticlinal regions, likely due to the heterogeneous distribution of wax and polysaccharides.