In the process of apical dominance, lateral buds are unable to grow in the presence of the apical bud. This is due to:

The end of a shoot contains an apical bud, from where the shoot growth occurs. The lateral buds are more sensitive to auxin than the apical bud. There is a concentration of auxin at which the growth of the apical bud is stimulated, they are unable to grow above that concentration. In the process of apical dominance, lateral buds are unable to grow in the presence of the apical bud. The apical bud produces auxin (IAA) that inhibits the growth of the lateral buds further down on the stem towards the axillary bud and promotes the growth of apical buds.