AUXIN TRANSPORT CAPACITY IN RELATION TO THE DWARFING EFFECT OF APPLE ROOTSTOCKS

Basipetal auxin transport in shoots obtained from dwarfing (M.9) and less-dwarfing (MM.111) apple rootstocks was compared by measuring the distribution of C-14-[carboxy]-labelled indole 3-acetic acid supplied to the ends of shoot segments. Auxin was transported at a greater velocity in MM.111 than in M.9 in both woody and non-woody segments, and was effectively blocked by 2,3,5-triiodobenzoic acid. The velocities of transport were approximately 8 mmh-1 in M.9, and 13 mmh-1 in MM.111. There was relatively little transport in the acropetal direction, and there were no significant differences between the two rootstocks in total auxin uptake. Chromatographic analysis suggested that the metabolism of the auxin supplied followed similar patterns in both rootstocks. It is proposed that the slower auxin transport in M.9 results from a lower capacity for auxin efflux from transporting cells. A limiting auxin transport capacity could explain the reduced cambial activity and subsequent xylem formation in the dwarfing M.9 rootstock, and thus make an important contribution to the dwarfing effect.