Pea (Pisum sativum L.) tendril-surface changes are correlated with changes in functional development

A scanning electron microscope (SEM) developmental study of tendrils of both conventional (Improved Laxton's Progress) and semileafless (Curly) cultivars of Pisum sativum was carried out in order to ascertain if there were changes in surface features that correlated with tendril functional development and to identify any potential thigmotropic receptors. These results were also compared with leaflet, rachis, and petiole surfaces at comparable developmental stages. Tendrils from the two cultivars did not differ, nor were positional differences found among tendrils from the same leaf. A number of dome-shaped protrusions were present on the dorsal tip cells of tendrils. These protrusions bear a superficial resemblance to the tactile bleps reported for other tendril species. However, they are unlikely to play any role in the thigmotropic response since they are not located at the site of greatest thigmotropic sensitivity and are also present on leaflet tips. A distinct pattern of epidermal ridges was found on the tendrils that changes from interwoven during the young expanding phase, to primarily radial when thigmotropic, to nearly absent at maturity. The composition of these structures was determined, with transmission electron microscopy (TEM), to be wall material and cuticle. Ridges were also variably present on the other cylindrical organs. These ridges may be related to the requirement by tendrils for directional wall extension during coiling and to similar requirements in rachides and petioles for rapid extension during leaflet unfolding and growth.