GC-MS-SIM analysis of abscisic acid and indole-3-acetic acid in shoot barkof apple rootstocks

Concentrations of abscisic acid and indole-3-acetic acid were measured by G C-MS-SIM in the shoot bark of clonal apple rootstocks (M.27, M.9, MM.106 an d MM.111) when the rootstocks were growing actively in the UK. These rootst ocks are known to exhibit a wide range of control of tree size when grafted to a common scion. Shoot bark of the dwarfing rootstocks (M.27 and M.9) co ntained higher concentrations than the more vigorous rootstocks (MM.106 and MM.111) of ABA. Concentrations of ABA increased from May to July, followed by a decline in August. Only the month of sampling showed any significant influence on the concentration of IAA in shoot bark; however, there was a g eneral increase, although not significant statistically, in IAA concentrati on with the increasing invigoration-capacity of the rootstock. At each samp ling date the dwarfing rootstocks showed greater ratios of ABA:IAA than the invigorating rootstocks and generally the ratio for each rootstock increas ed from May to July, except for M.27 which showed the smallest ratio in Jun e and the largest ratio in August. The results are discussed in relation to the generally accepted control exerted by the rootstocks on tree size and the possible influence of ABA on polar auxin transport.