EN
The effect of different durations of exposure to 2,4-D on hypocotyls and cotyledons cultured in vitro was studied in Brassica napus L. cv. Kana. Organogenesis or callogenesis depended on the duration of explant exposure to MS medium supplemented with 2 mg/1 2,4-D (2,4-dichlorophenoxyacetic acid). Short treatment (1 and 3 days) with 2,4-D resulted in rhizogenesis on hypocotyls (100% and 98% of explants, respectively) and cotyledons (80% and 54% of explants, respectively). Adventitious shoots formed sporadically, with the highest frequency (14% of explants) on hypocotyls cultured 3 days on MS supplemented with 2,4-D and then transferred to hormone-free medium. Histological analysis clearly indicated that the basal part of hypocotyls is involved in root formation and callus production, and the apical part for shoots. Meristematic sites originated from groups of cells in the cortex layer (including cells of the endoderm), but the procambium, phloem and pericycle also showed meristematic activity. The present study indicated that the response of explants cultured on media containing 2,4-D at constant concentration depends on the duration of explant exposure to growth regulator.