phototropism | Bonsaiplace

Photosyntheses — the process through which plants use energy from the sun, water from the soil and carbon dioxide from the air, to produce food for itself and then also oxygen. From this, we see that it is really important to get the light requirements of your Bonsai trees right.

Phototropism – the phenomenon where plants grow towards light. This is mainly caused by hormone stimulation that forces plants to grow towards the light source.

Too little light can cause issues and then you also have the other side of this continuum, the heat caused by direct sunlight, that can also cause harm.

Leaf scorch can also be caused by dry winds.

Let’s look at situations where the tree is in the shade too much. This could cause a deficiency in energy production, unless it is a plant adapted to grow in shade. Shady conditions can also cause leaves to grow larger and then also for branches and especially new growth to become spindly with very long internodes. Both of these growth patterns are not very good things for Bonsai where you need smaller leaf sizes and also more compact growth.

A more open aspect with no large structures providing shade.

Another light consideration is where you place your Bonsai trees in relation to the light source, i.e. the sun. Plants tend to grow towards the sun and if placed against a wall, it could be that the tree will grow away from the wall. It is also possible that you will have very little growth on the shady or wall side of the tree. The solution to this problem is to turn your trees often. Some of my trees, that are on stands / monkey posts with no wall near them, quickly show me that they need to be turned as well. It could be slight yellowing of the leaves or needles on the southern side (I am in the Southern Hemisphere) or denser growth on the sunnier side than the shadier side.

A Wall and even an open, but solid structure will influence growth patterns.

What is the most important here is to know more about the natural habitat of your trees. If it is a natural shade lover, you could get the opposite to what is described in the previous paragraph. Also look out for burn or scorching of these shade lovers on the sunny side.

Think about the placement of your trees. Study the different microclimates that can be caused by high walls or fences as well as trees and other plants. The construction of your display stands and where these are placed in your garden are all very important aspects of your Bonsai cultivation. When it comes to the regular turning of trees, I have a fixed day twice per month and I turn the tree through ninety degrees, always in the same direction (for me that is clock-wise). Sometimes I will keep it longer in a specific position due to the fact that there could be an undeveloped branch that needs the light source for longer to get its development up to speed.

On the left is a Pine with some yellowing on one side. On the right uneven growth pattern due to being too close to a wall.

You also need to think about the light requirements when it comes to specific maintenance tasks. After root pruning or repotting it is also best to keep your tree away from direct sunlight for a few days to a couple of weeks. The opposite when you get into the different grades of defoliation.

One of the reasons for defoliation, whether it is fully or partial, is to stimulate back budding and for this, more light is needed.

If you are heavily invested in Maples as Bonsai, it will be worth your efforts if you look at different light requirements of Maples as it can influence stunning Autumn colours and even new colours in Spring. That is a topic for another day.

Same tree at different times of the year. Temperature plays a big part here, but so is light exposure.

It is not complex, but certainly something to think about when you position your trees and every time when maintenance tasks are undertaken.

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.