Auxin, a plant hormone, makes a plant grow toward the sun.

Auxin makes a plant grow toward the sun. One of the very first things I learnt about biology in school was that plants grow toward the sun. We even did an auxin experiment, and I have an idea it is probably the most frequently performed biological experiment in schools. First, we grew bean seeds in damp cotton wool and then transferred them to small pots. Then we placed a cardboard box with a small hole cut in one side over the pot and within a few hours the plant began growing toward the hole and the light. The way plants react to light is called Phototropism, photo meaning light, and tropism meaning to change.

Everything that grows above the soil grows toward the light. This is called positive phototropism. The roots grow away from the light. This is called negative phototropism.

Plant hormones – Auxins

So, why do leaves grow toward the light? The answer is a plant hormone called auxin. Auxins are produced by plants, and work like a signalling mechanism. They are responsible for controlling a number of subtle but vital functions in a plant. One of the things auxins do is they make plant cells grow in length so regions of a plant that have more auxins grow longer. They also inhibit lateral growth, side shoots. Auxins are eradicated by sun light, so plants that grow in full sun have fewer auxins than those growing in shade. As a result, plants that grow in sun still produce as many cells as those in shade, but the cells are smaller. This results in more compact growth which is what we are always trying to achieve. This is why bonsai need to grow in full sunlight. Trees that grow in shade have an overabundance of auxins. As a result, trees that grow in shade have long thin branches with very few sideshoots.

Sunlight eradicates auxins

Sunlight eradicates auxins, but plants are constantly creating more. Auxins are created at the very end of growing shoots, an area called the terminal bud. They are also created in the root tips. As terminal buds create auxins, the sun, falling on the sunny side of the shoot eradicates them. However there will still be auxins on the shady side of the shoot. This imbalance will cause the cells on the sunny side to be slightly shorter in length and the cells on the shady to grow slightly longer. This makes the new shoot bend toward the light.

Nipping buds creates bushy growth

When we nip the end of a shoot, we temporarily stop auxin production in that shoot. As a result, the shoot stops growing in length. However, because there are fewer auxins, the plant can now grow in width, ie produce sideshoots. Over the space of a few days nutrients will build up in the shoot resulting in new shoots emerging from the nodes of the main shoot. Each new shoot will have its own auxin producing factory and these new shoots will all grow in length until the grower nips these new shoots. This is how we control growth and develop bushy foliage.