Apical Dominance

a condition where vertical growth supercedes lateral growth in a plant. this is controlled by auxins, where in high concentrations can inhibit growth but on the whole promote it. Removal of the apex can induce lateral growth. Typically a shoot has leaves attached to a stem. The apex of the stem has an actively growing apical bud. It produces additional nodes and internodes to add to the length of the shoot. Additional leaves appear at the nodes as they are produced. In some plants, the lateral bud located in the axil of each leaf does not grow to form branches, especially at first. This condition is known as strong apical Dominance. apical Dominance is thought to be caused by the apical bud producing IAA (auxin) in abundance. This auxin is transported basipetally from the apical bud. The auxin causes the lateral buds to remain dormant. How could a lower concentration cause lateral buds to remain dormant and a higher concentration cause the apical bud to grow? This is explained in the following graph: The difference in response between the two kinds of buds is explained in their sensitivity to the auxin concentration. Clearly the lateral buds are more sensitive to auxin than the apical bud. There is a concentration of auxin at which the apical bud is stimulated to grow while the lateral buds are inhibited. That concentration would be near the letter "C" of "Concentration" in the graph above. When the apical bud is removed, the source of IAA is removed. Since the auxin concentration is much lower, the lateral buds can now grow. In fact their growth will be stimulated by a relativley small drop in auxin concentration. Thus, decapitating (pruning) a shoot will cause it to branch!


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