A plant hormones, or phytohormones, are naturally occurring signaling molecules. These molecules move throughout plants regulating all physiological processes including growth, fruit development, responses to environmental stimuli and espalier fruit tree training.

Auxins and Cytokinins are the primary plant hormones that espalier fruit tree training techniques modify. Learning these two plant hormones control growth and interact will improve your espalier training sucess.

Auxins

Auxins are involved in many aspects of tree growth. Of particular interest when training espalier fruit trees is Apical Dominance.

First thing in the spring apical buds of vertical shoots and a few buds just below them on a vertical shoot start to grow first due to the Source-Sink effect. Auxin is produced in the basal area each of these buds starts once they start to grow.

Auxin is actively moved through downward through the cells of from the where it is produced downward in the shoot. Movement of Auxin is always downward.

As auxin flows downward in a vertical shoot it prevents lateral buds that have not started to grow from starting to grow. This is Apical Dominance.

Lateral buds that started to grow into lateral shoots before auxin production started in buds higher on that shoot will grow with no inhibition because the Auxin cannot move upward through the new shoot to reach the growing bud.

Cytokinins

The second plant hormone involved in apical dominance is Cytokinin. This hormone stimulates buds to grow, the opposite of Auxin.

Cytokinin is primarily produced in tree roots. In the spring the Source - Sink effect moves Cytokinin, water, nutrients and carbohydrates from the roots through the Xylem to the highest parts of the tree stimulating early season bud growth.

Auxin/Cytokinin Ratio

When the downward flowing Auxin meets the upward flowing Cytokinin, the ratio of the two at the location of a dormant bud determines whether it will grow or not. The higher the amount of auxin the lower the probability the bud will start growing. Many of the techniques used to train espalier fruit trees are specifically designed to modify this ratio.