Seed expulsion in fruits of Mesembryanthema (Aizoaceae): a mechanistic approach to study the effect of fruit morphological structures on seed dispersal

Most Mesembryanthema and some other members of the Aizoaceae have hygrochas tic capsules which open when moistened, releasing a portion of the seeds in rain. The mechanism of seed expulsion varies within the group depending on differences in morphological and anatomical details of the inner construct ion of the capsules. The aim of the present study is to analyse the effecti veness of the various structures by removing different parts of the fruits. The distance of seed expulsion and the percentage of expelled and retained seeds are taken as parameters for the effectiveness of the structures. 52 species from 25 genera were exposed to water drops falling from a height of 175 cm. The mean distance of seed expulsion varied between 7 and 65 cm in the. non-manipulated fruits. In 13 species, no seeds were expelled although several were present in the capsule. In no case, all seeds were expelled. With manipulations, i.e. removing of closing bodies, covering membranes and /or funicles, the mean distance of seed expulsion was reduced. The percenta ge of expelled seeds was higher in most cases in the manipulated fruits as compared to the non-manipulated fruits. The results of this study show that the distance of seed expulsion and seed retention were strongly C, affecte d by the manipulations, but the differences of sizes and consistencies of t hese structures in the different species do not seem to play a role as the differences in the distance and the percentage of expelled seeds showed sim ilar trends in most species. Any kind of manipulation, no matter whether fu nicles, covering membranes, closing bodies or a combination of them were re moved, led to a decrease of the effectiveness of the expulsion mechanism. W hen the complex structures of the capsule are disturbed by manipulations, t he optimization of the fruit construction and the efficiency of the mechani sm for seed dispersal by water are decreased, as shown by the retention of less seeds. Release of only few seeds at once may confer advantages, for th e presence of many seeds for subsequent rain events is an effective dispers al strategy in regions where erratic rains predominate and the same trigger (water) is used for dissemination and germination.