Organs of gibberellin synthesis in light-grown sunflower plants

The sites of gibberellin (GA) synthesis in light grown sunflower plants were studied. The results of organ excision and the exogenous application of indole acetic acid and gibberellic acid indicated that gibberellin synthesis occurred in the young leaves of the apical bud. This was substantiated using a combination of diffusion and extraction techniques. Diffusion of sunflower apical buds on agar for 20 hours revealed a level of gibberellin greater than that obtained by solvent extraction of a similar number of apices, indicating that synthesis of gibberellin was occurring in those apices during the diffusion period. The gibberellin level of apices extracted following a 20 hour diffusion period was the same as that obtained from buds extracted immediately following excision from the plant, again suggesting that apical buds are sites of gibberellin synthesis. A similar experiment was conducted with young internodal sections, the results indicating that they were not sites of gibberellin synthesis.