The influence of an interstock of the dwarfing cultivar M9 and the nondwarfing cultivar MM115 on the distribution and metabolism of labeled gibberellic acid A(4) ([(3)H]GA(4)) of high specific radioactivity (5.18 x 10(10) becquerel per millimole) applied to the xylem of the rootstock in grafted apple (Malus x domestica Borkh.) trees was compared. Free [(3)H] GA-like metabolites of [(3)H]GA(4), including putative GA(1), GA(2), GA(3), and GA(34), as well as various (3)H-putative GA glucosyl conjugates were detected in stem segments from both cultivars. M9 interstocks reduced the total uptake of [(3)H]GA(4) and decreased the proportion of (3)H metabolites transported to the shoots and leaves of scions. The M9 interstock tissue and adjacent rootstock and scion tissue retained a much greater amount and a higher proportion of the label than did comparable tissue of the nondwarfing MM115 interstock. In addition, the amount and proportion of free [(3)H]GAs was higher, and the proportion of putative [(3)H]GA glucosyl conjugates lower, in M9 interstocks compared to MM115. These effects of the dwarfing interstock on GA distribution and metabolism indicate a significant role for GAs in any satisfactory explanation of the dwarfing mechanism in apple.