Staining tomato fruit cuticle and exocarp tissues

Immature fruit of tomato, Lycopersicon esculentum (Celebrity), was examined to observe the cuticle, its interface with the epidermis, and the general histology of the outer exocarp. Paraffin sections were stained first with Bismarck brown Y. Structures already stained in various hues of brown were stained again with either azure B, aluminum hematoxylin and alcian blue SGX, or the periodic acid-Schiff (PAS) reaction. Bismarck brown-azure B displayed the cuticle in strong contrast with subjacent tissue; however, nuclei were not easily identified at low magnification. Bismarck brown-hematoxylin-alcian blue produced a sharply contrasted combination of yellow cuticle, bright blue cell walls and purple nuclei. Nuclei stained purple with hematoxylin were easily identified at x100. Bismarck brown-PAS stained the cuticle golden brown and subjacent tissues mageta red. Surprisingly, epidermal cells stained specifically and intensely with PAS while pretreatment with an aldehyde blockade and omission of periodic acid prevented staining of all other tissues.