Plant in Sunlight

Why Plants Grow Towards Light (And the Role of Auxins)

Plants are more than a gift from Mother Nature. They have their own life force and raise many questions about plants, trees, and how any woodland flowers manage to survive in the wild without any human intervention.

How and why plants grow towards light is one of the many fascinating questions that’s baffled botanists for centuries. It’s been extensively researched and various experiments have been done to prove without a shadow of a doubt that plants do indeed need light to grow, and more than that, they’ll go out of their way to get the light the need.

One of the earliest successful experiments was from Charles Darwin, who with his son Francis (a botanist) tested light theory on the tips of grass seeds. The results showed that when the tips were exposed to light, they’d grow towards it, and when the tips were covered, growth was hindered.

The theory was proven, but the question remained as to why that happened. What Darwin and his son did was popularize the theory first introduced by Henri-Louis Duhamel who in 1758 first theorized about how plants grow towards sunlight.

The Darwin publication influenced future research, for which an entire field of study started with global influence.

Thanks to Darwin’s theory published in The Power of Movement in Plants‘, more experiments were done to answer the question of why plants gravitate towards light.

A lot of researchers were involved, but it was later when Nikolai Cholodny, a  Russian microbiologist, and Frits Warmolt Went, a plant physiologist first discovered the hormone ‘Auxin’.

Auxin is the missing component that Darwin and Francis couldn’t explain. It’s the hormone in plants that cause them to grow towards light.

The Role of Auxin in Plants

Auxin is a hormone in plants responsible for positive or negative phototropism.

Phototropism is the term used to describe a plant’s response to light. If it’s positive, it grows towards the light, whereas negative phototropism causes plants to grow away from light.

When you look at outdoor plants, they will always grow upwards towards the sky, reaching for the sun. The same thing is noticeable on plants perched on a windowsill. The shoots and stems will grow towards the sunlight.

In the case of shaded areas of the plants, they are smart enough to distribute auxins to parts of the plants that need them to respond to light.

Plants will distribute more auxin hormone cells to parts of the plant that are in the shade. Auxin is what seeks light, which is why when more is channeled to a shaded area, that part of the plant will then start to elongate towards the light. It’ll bend out of shape to get the light too.

This is why when you have plants on a windowsill and you don’t turn them regularly, the uneven distribution of the light source will cause the plant to grow lop-sided.

Instead of the shoots growing up, they’ll grow in the direction where the light is the strongest.

How Auxin Responds to Light Direction

Flower in Sunlight

Unless you’re growing plants with artificial lights, it’s going to be near impossible to distribute an even amount of light to every part of the plant.

Auxin responds to light by distributing the hormones where it’s needed. That’s in the shaded areas of the plant.

When using light directly beaming evenly to the tip of the plant, auxin is distributed evenly causing the plant to grow straight upwards.

When the light source is reaching the plant from one side only, auxin is transported to the darker side, causing the elongation you see in plant stems when they’re growing towards the light source. 

When plants grow towards light, it’s called positive phototropism.

Negative phototropism, also called skototripism, happens with the roots of plants because they don’t need the sunlight. The shoots they produce do.

This is why roots will grow into the soil, whereas shoots will grow out of the soil and reach for the light source.

However, within dense forestry, the climbing plants that need trees at ground level will use negative phototropism to survive because they need to find a dark area like the base of a large tree, so they can latch onto it and then climb they’re way up to reach sunlight.

Plants are smarter than they’re given credit for.

Manipulating Plant Growth by Removing Auxins

As auxins are primarily responsible for plant growth, the last thing you’d want to do on a growing plant would be to reduce the number of auxins, as that should, in theory, stop plant growth.

It doesn’t though. Auxins travel through the plant stems to the tips where they’ll then be distributed to shaded areas, causing the shaded parts to reach towards light, stimulating growth.

However, when you pinch the tips of plants, you remove some of the auxins. Most experienced gardeners know that by pinching back flowering stems, it encourages bushier growth and in certain types of plants, more flowerheads to develop.

The increase in flower production by pinching the stems of plants back happens as a result of manipulating the auxins in the tips of plants.

By disrupting auxin flow in the tips of plants, it forces the plant to expend its energy elsewhere putting more energy into the nodes beneath the tips, which can double up the stems, which in turn can double the flowerheads produced.

The flowers are usually smaller, but you’ll have more of them rather than one large flowerhead surrounded just by the green foliage.

If you want bushier foliage from your plants with an increase in flowerheads, pinch the tips before they flower.

The increase in auxin distribution in the lower part of the plant will cause more stems and flowers to develop. The more auxins there are distributed throughout the plant, the more stems will reach for light.

Pinching is a handy technique to use on outdoor plants as there’s more sunlight for each stem to reach for, or on plants grown indoors under grow lights where there’s plenty of light to reach all parts of the plant, rather than having uneven light distribution to different parts of the plant.

If you notice any of your plants growing lop-sided, there will be a part of the plant that isn’t getting enough light. Introduce more light to the parts of the plant in the shade for auxins to distribute more evenly in all of the stems.

The direction of light reaching your plants will influence the direction of growth.

Why Plants Grow Towards Light (And the Role of Auxins) was last modified: July 8th, 2019 by The Practical Planter

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