Histological responses of azalea to Off-shoot-O and Dikegulac as chemical pinching agents

Histological responses to chemical pinching agents, Off-Shoot-0 and Dikegulac, applied as foliar sprays, were observed in shoot apices of Rhododendron obtusum 'Hinodegiri.' Tissues for paraffin and plastic embedding were fixed in buffered glutaraldehyde, and for plastic were further fixed in osmium tetroxide. The non-treated control shoot tips were characterized by large populations of cytoplasmic globules in a great range of sizes, some larger than nuclei. These globules were especially prominent in the trichomes of the differentiating primordial leaves, but were also present in the differentiating young stem tissue. The observation of these unique cellular inclusions was apparently the result of non-coagulant fixation, since they have not been reported from other studies in which coagulant fixation was employed. The large number of globules suggests that they may have an important role in actively growing tissues. The composition of the globules has not been identified. In this experiment, however, their color reaction with methylene blue-azure A in plastic sections suggests a phenolic composition, possibly a precursor to lignin In the globules. In the treatment with Off-Shoot-0, the globules in the trichomes disappeared three minutes after spraying. Dikegulac did not destroy the globules or other cellular inclusions, but the cells in the young primordial leaves and apical meristems of the shoot tips taken two weeks after treatment showed some changes compared with the non-treated shoot tips. Changes were observed in the color of the stained cells and there was increased vacuolization. Therefore, Dikegulac could interfere with the activities of plant hormones, especially auxin and cytokinin, in relation to apical dominance and side branching.