Exploring Plant Surface Chemical Variability: Lettuce Leaf as Model

The plant cuticle has been traditionally believed to be a continuous lipophilic layer covering most aerial plant surfaces, but recent methodological advances based on Atomic Force Microscopy (AFM) enabled mapping the distribution of hydrophilic and hydrophobic areas in the papillae and pavement cells of rose petals and olive leaf trichomes. Since previous investigations associated the occurrence of foodborne diseases in leafy green vegetables with a high water wettability and stomatal traits, we hypothesised that lettuce could be a suitable model for a plant surface having a higher frequency of nano-hydrophilic areas. Hence, a wettable Romaine lettuce variety with water contact angles between 64° and 75° and a high degree of polarity for both leaf sides was selected and characterised by electron microscopy, AFM, Fourier Transform Infrared (FTIR) and Raman Spectroscopy. Leaf samples were analysed fresh and after critical point drying (CPD), and no major structural or chemical differences were recorded. Leaves are amphistomatous and were found to have nano-scale chemical heterogeneity in both leaf sides, with hydrophilic nano-areas predominantly occurring in stomatal regions. These hydrophilic nano-areas could facilitate microbial adhesion and affect transport phenomena across leaf surfaces, but the functional significance of hydrophilic nano-zones in the cuticle of aerial plant organs will need to be addressed in future multidisciplinary investigations.