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

Basipetal auxin transport in shoots obtained from dwarfing (M.9) and l ess-dwarfing (MM.111) apple rootstocks was compared by measuring the d istribution of C-14-[carboxy]-labelled indole 3-acetic acid supplied t o the ends of shoot segments. Auxin was transported at a greater veloc ity in MM.111 than in M.9 in both woody and non-woody segments, and wa s 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. T here was relatively little transport in the acropetal direction, and t here were no significant differences between the two rootstocks in tot al auxin uptake. Chromatographic analysis suggested that the metabolis m of the auxin supplied followed similar patterns in both rootstocks. It is proposed that the slower auxin transport in M.9 results from a l ower capacity for auxin efflux from transporting cells. A limiting aux in transport capacity could explain the reduced cambial activity and s ubsequent xylem formation in the dwarfing M.9 rootstock, and thus make an important contribution to the dwarfing effect.