The 2012 ASHS Annual Conference

It has been hypothesized that the stem of dwarfing apple rootstocks reduces basipetal transport of indole-3-acetic acid (IAA) to the root, thereby limiting root growth, biosynthesis of root-produced hormones (cytokinins and gibberellins), and their consequent transport in the xylem vasculature to the scion for growth. This hypothesis still remains largely untested experimentally; therefore, we applied the auxin transport inhibitor “1-N-naphthylphthalamic acid” (NPA) to the graft union of composite ‘Royal Gala’ apple trees to elucidate whether restricting shoot-root IAA transport reduced root growth (experiment 1) or cytokinin concentration in scion xylem sap (experiment 2). In experiment 1, ‘Royal Gala’ scions were grafted in early spring onto ‘M.9’ (dwarf), ‘MM.106’ (semi-vigorous), ‘M.793’ (vigorous), and ‘Royal Gala’ (‘R.G’; very vigorous, self-rooted control) rootstocks. NPA in lanolin was applied to the graft union (5 mg·ml^–1 per tree) of half the experimental trees in early, mid, and late summer; the remaining trees were untreated. Root dry weights were measured in winter. In experiment 2, ‘Royal Gala’ scions were grafted onto ‘MM.106’ in late winter. In the second spring following tree grafting, half the trees were either untreated or treated with NPA applied to the graft union as previously described. Zeatin (Z), zeatin riboside (ZR), isopentenyladenosine (IPA), and isopentenyladenine (2iP) concentrations were quantified in the scion xylem sap 96 hours after treatment. Rootstock and NPA interacted (P < 0.001, LSD = 12) to modify final root dry weights. Without NPA, root dry weight of ‘M.9’ was less than ‘MM.106’, ‘M.793’, and ‘R.G’ (41, 98, 114, and 85 g, respectively). With NPA, root dry weights were 20, 35, 35, and 22 g, representing a 52, 64, 69, and 74% reduction in root growth for ‘M9’, ‘MM.106’, ‘M.793’, and ‘R.G’, respectively. Root dry weights of ‘M.9’ and ‘R.G’ were similar only when NPA was applied. NPA lowered ZR in scion xylem sap from 0.49 to 0.32 ng·ml^–1 (P < 0.01) and 2iP from 0.43 to 0.17 ng·ml^–1 (P < 0.05), but did not affect Z or IPA. We conclude that the smaller root system of M.9 could result from limited shoot-root transport of IAA within the rootstock stem. Lowering shoot-root transport of IAA also appeared to down-regulate the biosynthesis of some root-produced cytokinins.