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What Do Plants 'See'?

Can plants really “see” the light?

Have you ever noticed how plants bend toward light? How are they able to tell which direction the light is coming from, and how do they bend toward it? They don’t have eyes or muscles, after all.

Phototropism

Darwin experiment on phototropism

Scientists call a plant’s ability to bend toward light phototropism. Even as far back as ancient Greece it’s been a big puzzle about how plants are able to do it. People experimented with how plants accomplish this amazing feat, but no one really figured out how it worked—until Charles Darwin came along.

While you're all familiar with Darwin's work on evolution, what you're probably less familiar with is that the last 20, 30 years of his life, most of Darwin's research had to do with plant biology. And one of his most important works was published in 1880, together with his son, Francis Darwin. It's called The Power of Movement in Plants. What Darwin noticed was that if you put a plant (canary grass) in a room with only a very dim candle on one side of the room the plant would bend towards the candle. Well Darwin came to the following conclusion plants are sensitive to light, and will bend towards the light.

Through these experiments, they found that light was perceived at the plant's tip (coleoptile). However, the response, bending, at a cellular level, unequal elongation of cell, took place well below the tip. They concluded that some kind of signal must be sent downwards from the coleoptile’s tip towards its base.

Phototropins and Auxin

Today, we know that proteins called 'phototropins' are the main photoreceptors responsible for light detection during phototropism. Like other plant photoreceptors, phototropins are made up of a protein bound to a light-absorbing organic molecule, called the 'chromophore'. Phototropins absorb light in the blue range of the spectrum. When they absorb light, they change shape, become active, and can change the activity of other proteins in the cell which is is transducing a signal to the lower part of the plant.

When a plant tip (coleoptile) is exposed to a source of light, phototropin molecules on the illuminated side absorb lots of light, while molecules on the shady side absorb much less. These different levels of phototropin activation cause a plant hormone called auxin to be transported unequally down the two sides of the coleoptile.

More auxin is transported down the shady side, and less auxin is transported down the illuminated side. Auxin promotes cell elongation, causing the plant to grow more on the shady side and bend in the direction of the light source.

Graphic showing phototropism process

What Colors Do Plants 'See'?

plants can differentiate between colors. Human eyes have 4 types of photoreceptors: red, green blue and monochrome. Plants actually are much more complex than we are, when it comes to light signaling. Plants have upwards of 13 photoreceptors. They have one that's called UVR8 which allows the plant to detect UV light. Light that we're blind to. Plants can see this and respond. Plants have upwards of five photo receptors that detect blue light. They have two, what we call phototropins. A photo receptor for flowering in blue light and another photoreceptor for letting a plant know about seeding development. Plants also can detect green light. They have multiple photoreceptors for red and for far red light.

So that we can see that, from a plant's perspective, humans actually may be somewhat visually dysfunctional. 'because plants can see much more.

Photoperiodism

The term "photoperiodism" was coined to describe a plant's ability to flower in response to changes in the photoperiod: the relative lengths of day and night. Because flowers produce seeds, flowering is crucially important for the plant to complete its life cycle. Some types of plants require particular day or night lengths in order to flower — that is, to transition to the reproductive phase of their life cycle.

Plants that flower only when day length drops below a certain threshold are called 'short-day plants'. Rice is an example of a short-day plant.2

Plants that flower only when day length rises above a certain threshold are called 'long-day plants'. Spinach and sugar beets are long-day plants.

By flowering only when day or night lengths reach a certain threshold, these plants are able to coordinate their flowering time with changes in the seasons.

Although we classify plants as short-day or long-day, in some cases, plants may actually be measuring the length of the night. That is, it can be the length of the period of continuous darkness, not the length of the period of continuous light, that determines whether or not the plant flowers.

The Light Switch

What controls photoperiodism? What we actually have here is a light activated switch. Red light is turning the switch on but far red light is turning the switch off.

When does a plant see red light? And when does it see far red light? In the morning as the sun is rising we see is that the sun has is low and the light waves have a longer path through the atmosphere. Under these conditions there's more far red light, than there is red light. But as the sun rises, up in the, to the, to its zenith, the ratio of red to far red changes, such that there's much more red light, than there is far red light. So for the majority of the day, a plant is seeing primarily red and not far red light.

Now what happens though, at the end of the day as the sun goes down? The ratio again changes such that the longer and longer wavelengths are reaching the Earth. We see this in the change of color as the sun as it's setting. And the ratio of red to far red becomes so low, that the plant is only seeing at the end of the day, far red light.

Further Information

The Life Cycle of Plants: Fertilization
The Spruce: How to Make Your Own Fertilizer
Pollination and Fertilization
4 Ways To Use Eggshells For Your Plants
10 Ways to Use Banana Peels in Your Garden