Auxin is the leading factor of controlling apical dominance.The plant hormone auxin,synthesised in the shootapex,moves down along the stem and inhibits lateral branching,does not travel upward into the branches,so it must actindirectly;auxin may acts by regulating the production of a second messenger which is transported into the bud.The best documented candidates for such second messengers are cytokinin and a novel graft transmissible signal (MAX-dependent hormone).How ever,recent study on auxin transport sug gests a possible additional mechanism whereby auxin transport in the stem prevents the establishment of auxin transport out of the branches,thus inhibiting their grow th.Auxin acts in xylem-associated or medullary cells to mediate apical dominance.