Cutting and Pinching, part 1

The explanation for apical dominance is partly because of the plant's need to photosynthesize, e.g. convert the sun's energy into carbohydrate sugars. The terminal bud is growing towards the light and the development of more buds could result in the shading of lower stems causing unnecessary drain on the plant's resources. Instead the energy is put into one primary growing point. The inherited genetic instructions of the plant primarily control whether one or more growing points develop naturally.

Triggers of Change.
Auxins and Cytokinins perform a couple of other functions in the plant, too, which we must know a little about to understand the responses.

Auxins have the ability under certain conditions to induce root formation in meristem tissue (buds or leaves).

On the other hand, Cytokinins have the ability to stimulate bud development (make buds develop into new growing points).

The trigger for any of these to happen is the ratio of Auxins : Cytokinins. In a normal healthy plant where the Auxins : Cytokinins ratio is balanced these abilities are not triggered.

Putting It All Together.
If suddenly the growing point is pinched out the Auxins : Cytokinins ratio will change.

The Cytokinins concentration are still as high as when the growing point dominated, but the high Auxins levels of the growing point has now been removed. This shifts the hormonal balance to a high Cytokinins : Auxins ratio. Remember that a high ratio of Cytokinins : Auxins can prompt dormant buds to grow.

When the large concentration of Auxins was removed by pinching, so was the suppressing effects on the auxiliary buds. Together with the higher Cytokinins ratio the auxiliary buds will now be stimulated to develop.

In other words, when the growing point is pinched out, the previously dormant auxiliary buds further down the stem will start to elongate and develop into growing tips.

In horticulture this is used successfully to provide certain plants with ideal traits. Florist's chrysanthemums are a good example. These plants would be long winding plants if the growing tips were not continuously pinched during the development period. Growers can improve the appearances of many bedding plants in this manner.

A Look at The Other End.
If instead of the growing tip, the plant was severed from its roots, it would experience a high Auxins : Cytokinins ratio because the Cytokinins supply, produced in the roots, would be removed. The high levels of Auxins traveling down the stem will now stimulate the formation of adventitious roots.

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