The effect of the graft union on hormonal and ionic signalling between rootstocks and scions of grafted apple (Malus pumila L. Mill.)

The transport of chemical signals between rootstocks and grafted scions is implicated in the dwarfing capacity of apple (Malus pumila) rootstocks. This study investigated whether the intensity of putative ionic and hormonal signals between a dwarfing rootstock (M.9) and a semi-invigorating rootstock (MM.106) grafted with 'Queen Cox' scions was altered by the graft union. The capacity of rootstocks with different dwarfing potential for polar auxin transport (PAT) was also compared. Split-top pressure chambers were used to collect xylem sap samples at a range of flow rates from above and below the graft union in composite M.9 and MM.106 trees. Concentrations of hormones, anions and cations were quantified in expressed xylem sap. The effects of the graft union, flow rate and rootstock on xylem sap solute concentrations and deliveries were compared. Rootstocks of different dwarfing capacities maintained in micro-propagation were used to estimate and compare acropetal auxin transport; the velocity and intensity of PAT were determined using radiolabelled auxin. Sap osmolality and Ca+ + concentrations were reduced by passage through the M.9 graft union at very low flow rates. At transpirational flow rates, Ca++ and Mg++ concentrations were increased, and those of Na+ and NO3− decreased, by passage through the graft union. Deliveries of anions and cations from roots into shoots of M.9 composite trees were always similar or greater than those of MM.106 composite trees. Sap zeatin and zeatin riboside concentrations were reduced above the graft in both rootstocks. No evidence was found that a sub-optimal supply of essential mineral ions was involved in the dwarfing effect of M.9. Root- and shoot- specific deliveries of ABA in M.9 composite trees were significantly higher compared to MM.106 trees, and the greater import of ABA correlated temporarily with earlier shoot growth termination in M.9 composite trees. Intensity of PAT through micro-propagated stem tissue was lower in the dwarfing rootstock compared to more invigorating rootstocks.