The apple rootstock and its influence on endogenous hormones

The mechanism whereby a rootstock controls the growth of an apple scion grafted on it is not fully understood but there is evidence to suggest that an influence on the hormone system of the tree is involved. The results presented here are based on examination of the effects of a range of tree size-controlling rootstocks (M.27, M.9, M.26, MM.106, MM.111 and MM.104) on hormone relations. Uptake of [H-3]-IAA and movement of this through rootstock shoot segments were examined during June, July and August. On all three monthly occasions the invigorating rootstocks showed a higher uptake, and greater polar transport through the explant to an agar receptor, than did dwarfing rootstocks. Where [3H]-IAA was applied to the surface of a mature leaf of cv Fiesta on a range of rootstocks there was a greater movement of label to the young roots of invigorating root systems compared with levels in the roots of dwarfing stocks. Analysis of shoot xylem sap and root pressure exudate indicated a higher concentration and absolute amount of cytokinins in the shoot sap of unworked invigorating rootstocks or in sap from Fiesta grafted on these rootstocks. Likewise, there was a higher concentration of cytokinins in root pressure exudate from the root systems of invigorating stocks compared with that from dwarfing ones. Abscisic acid in the shoot bark was also influenced by the rootstock. In this case, ABA levels were consistently higher in the bark of dwarfing stocks, sampled on several occasions over the growing season. These results are discussed in terms of the possible involvement of hormones in the rootstock effect.