10 POINTS!!! If Auxins promote plant growth, then why do they inhibit branching and the effects of Cytokinin?

Question

Pretty basic question.

Since, cytokinins stimulate cell division, why are they hampered by auxins in their attempt to encourage the branching of a plant. Shouldn't auxins actually help cyokinins instead of inhibiting them, since they do promote cell elongation and cell division?

Answer 1

Auxins are concentrated in the dominate buds of a tree while cytokinin is found in lower buds. Let me try to explain it like this. I'm sure that you are familiar with conifers such as pine trees or other such evergreens. The term conifers refers to

the shape of the trees in relation to their limbs. Wider at the bottom tapering to a point conifers=cone. Auxins are found in the entire lateral area of a tree (between the roots and the dominate buds at the top) and prevent lateral growth. This is achieved via apical dominance. What this means is that the auxin wishes the tree to grow upwards while the cytokinin wishes the tree to grow outward. Auxin overrides the cytokinin by diffusing through the tree from the terminal bud. Trees with a strong cone shape are said to have strong apical dominance while trees with little or no cone shape are said to have poor apical dominance. If you were to remove the terminal bud of a conifer then auxin would be removed from the system and the tree would become "leggy" meaning it would start growing laterally and look quite scraggly. This link may help you with its explanation on apical dominance and its illustrations. http://plantphys.info/apical/apical.html

I have just read your additional details and I don't think that I have answered your question thoroughly enough so let me elaborate a little. Ignore the fact that Auxins inhibit Cytokinin for a moment. As you know Auxins promote upward growth on the other hand Cytokinins promote lateral growth such as branching and branch elongation. OK now where are both of these hormones found and where do they originate from? Auxins originate from the terminal bud and it is here that they are in greatest concentration. As you move down the tree (both down the trunk and through out the branches) Auxins occur in weaker concentrations. Cytokinins on the other hand originate in roots, young fruits, and seeds. Lets concentrate on the roots. Cytokinins are in greatest concentration in the roots and their concentration lessens as you move upwards.

When a higher concentration of Auxins conflicts with a lower concentration of Cytokinins the influence exercised by the Auxins "over rides" that of the cytokinins by pure numbers. Only so much growth can happen in a single growing season. So the weaker concentration of Cytokinin produces a little lateral growth while the higher Auxin concentration produces much more vertical growth.

Answer 2

I think so but i think that they  should.

Answer 3

Are you asking for a quantitative or qualitative answer?

I'll throw out a qualitative one:

Auxins are responsible for PRIMARY plant growth. They are produced by the apical meristem, and get slowly broken down as they travel down the stem, so as you get farther away from the meristem, the concentration of auxin goes down.

By inhibiting branching, auxin prevents branches near the meristem from forming. However, since it is less concentrated farther down the stem, branching occurs and the plant leafs out.

Auxin also prevents the activation of lateral meristems. Ever wondered how pruning works? By cutting off the apical meristem, you're killing the flow of auxin. WIthout the inhibitor, the buds directly below the meristem can now function - and the plant gets bushier.