Why Plants Grow Towards Light: & Other Ways Auxin Affects Plants

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We understand that our plants need sunlight to survive – it’s basic biology one-o-one. But why do plants grow towards the light? What exactly is the makeup and biology of plants that causes their stems and shoots to curve so perfectly towards any light source? The answer lies in their cells, auxins, and tropisms.

In this article, we’re going to look at the biological reactions in plants that help them survive and thrive. Beyond light, we’ll also discuss the effect of gravity, water, and oxygen, all of which are needed to ensure your plants grow well.

Understanding our plants will help us care better for them, and it’s always fun to know what is going on inside our favorite flower’s biology.

TropismsTropisms

We’ll start with the physical reaction, called tropisms.

Tropisms are the physical plant growth responding to outside stimulus. The stimulus can include water, gravity, and of course, light.

Phototropism is the specific reaction of tropisms to light; this can be split into two distinct categories; negative and positive. Positive tropisms will curve towards light sources. You will find these in the top part of your plants, the stem, and the branches, and the flowers. Negative tropisms, in turn, will instead curve away from light sources. You will find these in the plant’s roots; they don’t like the sun a whole lot.

Overall, all types of tropisms can be split into these two categories. The branches, for example, will fight against gravity, whereas the roots will allow gravity to pull them into the earth. Oxygen also players apart, and a root system will physically dig itself out of the ground should the ground not support enough oxygen to survive.

A plant responds quite intensely to its environment; it has to be adaptable if it wants to survive – and it is designed to adapt.

Auxins (why the plant curves)Why Plants Grow Towards Light

Now we’re going to look at the plant cells and how it reacts to the stimulus.

Auxins are the cells in the plant which regulate the growth of stems and roots. They are a family of hormones and make up a wide range of different types. But their primary function is to ensure the plant grows in the right direction.

Auxin starts at the tip of the shoot and will be distributed throughout the plant over its lifetime. The cells can elongate as needed to ensure the plant can reach the maximum amount of light, but only if the light source is direct.

A plant in a shaded area with indirect sunlight will grow straight up, the same as a plant in direct sunlight. Should a plant grow in a shady area, but light pours in from another spot, the shaded part of the plant’s auxin cells will elongate, and the plant will curve towards the light.

In the early stages of growth, a plant is most sensitive to light. Proteins in their makeup allow them to ‘sense’ light, and this enables the plant to figure out how to push itself and acquire maximum light. Mature plants can also easily twist themselves to reach for the light, but they’re not as quick to do so like a shoot.

Other factors for plant growthOther factors for plant growth

Apart from twisting to reach sunlight, different types of auxin hormones will affect the plant from other stimuli. Two of which are Hydrotropism and gravitropism. Oxygen also plays a significant role in keeping your plants happy, but we’ll get to that in a bit.

  • Gravitropism – fighting against gravity

A plant can physically fight gravity. Charles Darwin discovered this when he placed a potted plant on its side and watched as the plant physically curved to continue to grow upwards. Auxins once again affect the plants. By compacting at the bottom (against the stimulus), the plant can have the freedom to curve and push itself upwards. This is called positive gravitropism.

Roots, on the other hand, work on negative gravitropism. They seek to push themselves into the earth to ensure the plant is rooted and secure. The auxins will cluster on the top of the root, away from the stimulus, giving the plant the freedom to push itself towards gravity.

What is even more interesting is that plants can physically sense gravity. They create a specific protein that senses the side where gravity is. So no matter how you try and fool a plant, it will always figure out which side is up.

  • Hydrotropism – seeking water

Hydrotropism is the fantastic ability of roots to seek out water physically. Much like phototropism which spends all its energy seeking sunlight, roots can identify the moisture and grow towards it in the ground. They have a specific protein for sensing water as well. Much like gravity, plants always know where the water is at.

Auxins play much the same role here as they do in the stems, bending the roots towards the source it needs.

Another factor for your roots will always be oxygen. A plant cannot survive without a source of good clean air. The top part of your plant, stems, shoots, and flowers, can’t physically seek out oxygen, but beware, your roots can.

  • Oxygen – the final factor

Much like seeking water, a plant’s entire system will desperately grow anywhere it can to find oxygen. It will override all other functions of its cells to ensure that it can breathe. In the case of the root system, it will even go so far as to sprout a secondary root system to push up and out of the dirt to take a breath. Even if your plant is hearty and healthy in all other aspects of its life, it will die if the soil is too condensed.

What is tragic is many people will create ‘volcano mulch’ underneath their trees because they think it looks pretty and neat in the garden. Unfortunately, this has the horrible effect of suffocating your tree’s primary root system. Be sure to keep your tree’s primary root open and clean from debris to ensure it can take a proper breath every morning.

In closing

Plants are beyond fascinating. They are a marvel to study and a joy to understand. Although they are pretty simple to keep, water, sun, earth, and air are mightily complex under those unassuming branches and stems. It helps to precisely understand why they need what they need and appreciate how adaptable they can be in harsh conditions. Unlike animals, they can’t simply lift themselves and find a new place to live, so it’s up to us to ensure they get what they need and that they get the best of it in perfect amounts. 

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