The Caruncle of Ricinus communis L. (Castor Bean): Its Development and Role in Seed Dehydration, Rehydration, and Germination

Abstract

The cytophysiology of the caruncle of Ricinus communis was studied from the fifth day after pollination until seed maturity. The caruncle consists of epidermal and parenchyma cells. The surface area of the epidermal cells increased between 10 and 15 d after pollination (DAP) but does not change significantly after 15 DAP, whereas the size of the parenchyma cells increases and their walls thicken during seed development. The inner tangential and radial walls of epidermal and parenchyma cells have numerous pits. During development many starch grains appear and gradually decrease in number, being almost completely absent at maturity, when reserves consist mostly of lipids. The epidermis of the caruncle does not have cuticle, unlike the rest of the seed. The role of the caruncle during dehydration, rehydration, and germination was tested experimentally using whole seeds and seeds from which the caruncles had been removed. All processes were facilitated by the presence of the caruncle, which absorbed and temporarily retained water, passing it to the rest of the seed. This enabled seeds with caruncles to germinate under conditions that were too dry for seeds without caruncles.