Plant hormones have an affect on aspects of plant life. Every cell in a plant can produce plant hormones and these can act in that specific cell or it can be transported somewhere else. There are five major plant hormones; auxins, cytokinins, gibberellins, ethylene and abscisic acid.
Auxins are referred to as growth hormones. These chemicals stimulate plant cells to elongate. They are mainly found in the tips of stems and branches. with this accumulation of auxins in the tips, growth in other parts are subdued. During pruning of Bonsai these tips are removed and the excess of auxins in these areas are reduced. This in turn will allow growth to take place lower down on these branches. This is the so-called apical dominant display. When the auxins are not there, the dominance is reduced and back budding will be allowed to occur. It will also allow branches lower down the tree to develop more. Auxins are also produced in roots.
The phenomenon of phototropism is directly attributable to auxin concentrations. Auxin moves away from light. It accumulates in the cells further away from light and will stimulate those cells to lengthen. As this growth is only on one side of the stem, it “pushes” the stem towards the light. This growth of plants towards light is what phototropism is all about. When we wire stems and branches and bend them in a specific direction, no role is played by auxins. We manually do what auxins will normally do. This also allows us to do things against what the normal plant hormones were programmed to do. We can as an example, bend a branch to force it to grow away from light.
Another practical implication for Bonsai enthusiasts is to regularly turn the trees on the benches as it will stimulate growth around the plant and not only on one side.
Cytokinins promotes cell division, in other words, growth. Where auxins will inhibit lateral buds, cytokinins will promote growth all over the tree. In the absence of auxins, cytokinins will cause lateral buds and dormant buds to grow. Pruning, candle removing and pinching of new growth will all aid in reducing auxin levels and promoting more lateral growth.
Gibberellins are responsible for shoot elongation, seed germination and fruit and flower development.
Absisic acid promotes seed dormancy. It also assists with the opening and closing of stomata.
Ethylene is a gas produced by ripe fruits.