Studies on Auxin-membrane Permeability Relations in Fruit and Leaf Tissues

In a previous paper (22) I reported on the relationship between auxin and membrane permeability in bean endocarp tissue and in abscission phenomena in Coleus. Segments of the pods of Kentucky Wonder pole beans, prepared in the manner described for the Wehnelt bean test (4), remain rigid and plump up to 17 days when stored in the dark at 25? C in solutions of 4 to 50 ppm naphthalene acetic acid (NAA) or indoleacetic acid (IAA). In contrast, control segments in distilled water become soft and flaccid in 3 days. A study of hand sections discloses that in auxin-treated segments the intercellular spaces of the endocarp are filled with air (a normal situation), while in the control segments the spaces are filled with liquid. This suggests that the increased longevity of bean segments in vitro results from an effect of auxin on maintenance of membrane properties. In the absence of added auxin movement of solutes and water into the intercellular spaces results. This water-logging of the spaces is followed by dissolution of the middle lamella (cellular dissociation). Permeability changes occur only in the endocarp (fig 1) of the bean segments, not in the exocarp. The different responses of adjacent tissues suggest that permeability changes could be involved in abscission phenomena. Experiments with Coleus show that 2 to 3 days after deblading, water-logging of the intercellular spaces and cellular dissociation occur in the abscission zone, followed by abscission. IAA applied distally to debladed petioles prevents these phenomena, supporting the hypothesis that auxin inhibits abscission by maintaining normal membrane properties.