DISTRIBUTION OF [C-14] DICHLOROPHENOXYACETIC ACID IN CULTURED ZYGOTICEMBRYOS OF ZEA-MAYS L

The uptake of 2,4-dichlorophenoxyacetic acid (2,4-D), necessary for th e in vitro induction of callus formation and somatic embryogenesis in cultured immature maize embryos, was quantified after culture on nutri ent medium with [C-14]2,4-D. The identity of the C-14 label in the emb ryos was determined by high performance liquid chromatography (HPLC), and its distribution within embryos was visualized on sections of plas tic embedded material. Quantification of the C-14 label after a pulse label of 16 h showed a hundredfold accumulation of 2,4-D in the embryo s with respect to the initial medium concentration. During tissue proc essing for in situ detection of C-14, however, up to 70% of the label disappeared because of the embedding process. The best structural pres ervation was obtained after ethanol-mediated infiltration of Technovit 7100. Water-mediated infiltration of Technovit 7100 gave the highest retention of C-14. HPLC analysis showed that more than 95% of the resi dual C-14 label found in embryos was still 2,4-D. Autoradiography show ed that the embryogenic inbred line A188 contained C-14 label in disti nct regions of the scutellum, coleoptile, and suspensor. The nonembryo genic inbred line A632 contained more label after 16 h of culture in a different distribution compared with A188. Subculture of the embryos for 24 and 72 h and histologic analysis showed that cell proliferation and callus formation were restricted to specific regions of the embry o in both inbred lines. The pattern of 2,4-D distribution did not codi stribute with regions of proliferation, indicating that 2,4-D is not t he only trigger for proliferation.