Changes in the starch content during organogenesis in in vitro cultured Begonia rex stem explants

Stem explants, excised from greenhouse‐grown Begonia rex plants, were cultured on basal medium (T. Murashige and F. Skoog, Physiol. Plant. 15: 473–497, 1962) contained in sterile Petri dishes. The medium was supplemented with benzyladenine (0.1 mg 1−1) naphthaleneacetic acid (0.01 mg 1−1) and, according to experimental requirements, with either sucrose (3%) or mannitol (3%). Histochemical and biochemical examination of the starch content of the explant was carried out over several days. There was no starch deposition or organogenesis in tissue cultured on mannitol and carbohydrate‐free growth medium. The most dramatic finding was the heavy accumulation of starch in tissue cultured on sucrose medium. This copious accumulation preceded any organ formation and was mainly in regions which ultimately gave rise to shoot primordia. The heavy build‐up of starch preceding organogenesis was also observed when explants previously cultured on mannitol medium were transferred to medium containing sucrose. During shoot primordia development there was a decrease in the starch content of the cultured tissue indicating the utilization of the polyglucan in the organogenic process.