Comparative morphological and anatomical study of self-repair in succulent cylindrical plant organs

During evolution plants have evolved a multitude of adaptations to harsh environments lacking liquid water. Selectively advantageous structural adaptations include the storage of water in the parenchymatous tissues of diverse succulent plant organs, the formation of effective evaporation barriers and the minimization of water loss by rapid sealing and subsequent healing of injuries. The investigated succulent species, namely Euphorbia tirucalli, Rhipsalis baccifera ssp. mauritiana and Sansevieria cylindrica, belong to different phylogenetic lineages. They were selected based on the similar cylindrical geometry of their succulent organs and their occurrence in different ecological sites all characterized by periods of (temporary) drought. Comparative morphological and anatomical analyses were carried out on virgin control samples and on test samples after artificial mechanical damage in the longitudinal and transversal directions of the respective plant organ. Wound reactions were studied over a time span of 21 days, with various functional principles being found. Immediately after damage, mainly injury-induced physical reactions occur causing rapid self-sealing, which included: (1) tissue deformation in the wounded region resulting in wound closure and (2) discharge of mucilage/or latex. During the subsequent self-healing phase, chemical reactions and more complex biological responses dominated: (3) coagulation of latex, (4) formation of a ligno-suberized boundary layer and (5) development of a wound periderm. The results provide fundamental knowledge concerning the similarities and differences with regard to self-repairing mechanisms in succulent plant organs.