Promotion of growth and invertase activity by gibberellic Acid in developing Avena internodes

Gibberellic acid (GA₃) induces invertase activity within 6 hours in Avena stem segments that are incubated in the dark at 23°. The maximum amount of promotion is about 5 times that of invertase activity in untreated segments. GA₃ causes significant promotion of invertase activity at concentrations as low as 3 × 10⁻⁵ μm GA₃. The increase in invertase activity elicited by GA₃ between 3 × 10⁻⁵ μm and 300 μm closely parallels the growth promotion that is caused by GA₃ over this concentration range. In control segments, invertase activity rises steeply during the first 6 hours of incubation, then decays slowly between 12 and 48 hours. In GA₃-treated segments, the invertase activity also rises during the first 6 hours, parallel to that in control segments and continues to rise during the next 42 hours. These changes in invertase activity during 48-hour incubation periods do not parallel the changes in growth that occur in control and GA₃-treated segments. Cycloheximide at 10 μg/ml abolishes all GA₃-promoted growth and invertase activity in these segments. Actinomycin D at 40 and 80 μg/ml decreases GA₃-promoted growth by 20% and invertase activity by 38 and 44%, respectively. The data clearly support the idea that protein synthesis is necessary for GA₃-promoted growth and invertase activity in Avena stem segments.