Effect of light and riboflavin on indolebutyric acid-induced root formation on apple in vitro

Maximum root formation on apple ( Malus ) shoots cultured in vitro occurred after an incubation in the dark on medium containing 3.2 or 10 μ M indolebutyric acid (IBA) plus riboflavin. Omission of riboflavin or culture in light resulted in a significant decrease in the number of roots formed. About 95% of the absorbed IBA was inactivated by conjugation, ca 4% was extracted as the free IBA acid (IBAH) and only 1% as IAAH. It was investigated whether the decrease in root formation caused by exposure to light or omission of riboflavin during culture was parallelled by a shift in the concentrations of the physiologically active auxin compounds (IBAH and IAAH) in the stem base, i.e. the location where the roots emerge. At least 90% of the absorbed ³H-IBA was located in the stem base. Omission of riboflavin, either in the dark or in the light, had no effect on the IBAH and IAAH concentrations, whereas root formation decreased significantly. Incubation in the light on medium containing 10 μ M IBA with or without riboflavin and culture in the dark on medium containing 3.2μ M IBA plus riboflavin resulted in similar IBAH and IAAH concentrations. However, the number of roots was significantly lower after culture in the light. Therefore, we conclude that the synergistic effect of riboflavin and the antagonistic effect of light on IBA-induced root formation are not solely based on changes in the concentrations of the active auxin components resulting from IBA uptake.