What a plant sees

What is the difference between what we see and what do plants see? And what if I told you that plants know what color of shirt you're wearing?

In this and few next articles we will be sourcing from work published by Professor Daniel Chamovitz, Ph.D. and his popular science book “What a plant knows”, which also served as a base for a course with the same name on Coursera. We will be discovering and explaining basics of plants sensing, what they know or how they are similar to us, and I will be focusing mainly on information that is relevant for me as an urban farmer. If you have enough time, I highly encourage you to enroll in the course and watch it all by yourself instead of reading this article, because it is really well made and certainly more entertaining in video form than my writing. If you prefer the latter, let’s dive straight into the first part!

While plants don’t see in pictures like we do, they do see in colors in many ways. They can see a UV light that gives us sunburns or an infrared light that heats us up, which we are blind to. They can differentiate between very little light from a candle and a very strong light from the sun in the middle of the day. Plants also know the direction from where the light is coming from. You can try this on your own – place a light source next to a plant, watch how it turns in its direction, then reposition the light on the opposite site – the plant will bend towards the light again. Actually, one of the first people to study this behavior was Charles Darwin (yes, famous mainly for his work on evolution) in his book The Power of Movement in Plants. We as human use photoreceptors in the retina to absorb the light. Darwin was curious to locate the similar point of perception in plants. With his experiments, he found out that tip of the plant senses the light and somehow transfers this light energy or signal to bottom part where the bending then occurs – similar to human system, as we have perception, signal transduction and response too.

Interestingly enough, plants also react to a changing day length, we call it a photoperiodism. With this information, we are able to split plants into groups based on their actions. There are short-day plants which flower only when the day gets shorter than night, and there are long-day plants, which are the exact opposite. There is also a third type, day-neutral plants, which don’t show photoperiodism and flower only when they get to a certain body size. What is important is that plants are not really sensing how long the day is, but how long is the night. This is important from an agricultural point of view since we can manipulate plants to flower in controlled environment regardless of conditions outside.

However, even more important aspect is that plants know colors of the given light and have a memory of them. For example, a flash of red light in the middle of the night would induce flowering in a long-day plant and it would inhibit flowering in a short-day plant. If you would use a red light on a long-day plant followed by far red light (~730 nanometers), it would inhibit the previous flash of red light – as it would not even happen, you cancel the effect of the red light. That all means plants remember the last light and therefore have some kind of memory.

The scientist have been able to isolate a molecule, working at the level of the protein in the cell, that lets the plant know when to flower. We call it a phytochrome and it has two different components, one is a protein component and the other is called chromophore – a part that absorbs the light and reacts to it. The chromophore can be found in one of two conformations in the cell: a phytochrome red (Pr), which absorbs red light and once the red light is absorbed it becomes the second type, and a phytochrome far red (Pfr), which absorbs far red light and once that is absorbed it becomes Pr again. This means that if we see Prf in the plant cells, the plant is flowering, if then the far red light shines on the plant, Prf absorbs it, changes to Pr and it becomes inactive and inhibits the flowering.

The light regulation is essential from the very beginning. If you grow plants under a light, the seedlings (hypocotyl) will be short, with opened and extended leaves (cotyledons), and this type of development will continue to grow a plant and eventually begin a flowering – this is called photomorphogenesis. On the other hand, you can have a plant grown without a light source, with very long hypocotyl and closed cotyledons. That is because the plant will think it is situated under the ground and it will try to find a way back to the surface – this is called skotomorphogenesis. Therefore a plant grown under a constant light will grow faster through photomorphogenesis since it doesn’t need big body size in the initial phase.

There are more than 15 photoreceptors to detect light, mainly in the blue and red portion of the humanly visible color spectrum (and other photoreceptors even beyond that). Each of them enables plant to adapt to various conditions – there is a receptor for bending in direction of a light source (tropism), a receptor responsible for additional growth in order to gain more light (shadow avoidance), a receptor for transition to flowering, a receptor for regulation of seedling size and structure, and many more.

So we can agree on that seeing is crucial for plants same as for a human. While we can move and get energy from food, plants are stuck in a place and in order to maximize their gain of energy they have all these amazing abilities and mechanisms. In fact they are fighting for survival the same way human and animals do, only slower and not so visibly.