Why are some plants naturally yellow?

Question

Some plants are naturally yellow. Or rather, they have yellow leaves. This is shown in the following picture, taken from my neighborhood.

I found a Quora article that explains why, but since that is not a reliable source (as far as I know), I would like to have more evidence to this claim. In short, the article claims that plants are yellow due to artificial selection (they look good, so we breed them more). Is this true? Or is there a more plausible answer to why plants are naturally yellow?

In the case of this particular plant, it is not looked after very much, and perhaps only watered once in a while. Won't the lack of chlorophyll affect the plant's ability to perform photosynthesis? I have an alternate theory as to why this plant is yellow, but I'm not sure if that is correct. This plant is very close to a backwater which is highly contaminated with pollutants (the green in the background of this photo is actually weeds and algae in the river). Could that be a valid reason why the plant is yellow?

I was not able to find any other links to this question (they were all about dying plants no matter how I phrased my Google search), but I will edit the question if I get anymore links.

Answer 1

There simply can't be one answer to this question, especially since I couldn't identify the plant from the photo. However, looking at the different possibilities can be helpful, even if it's just scratching the surface of a very large topic.

Introduction

First of all, why are the plants green? It is common knowledge that the photosynthetic pigment, chlorophyll, gives this color to leaves. This substance has maximum absorption in the red and blue parts of the spectrum, so the spectrum of light transmitted and reflected from the leaf is dominated by the green part of the spectrum. In most plants, the amount of chlorophyll present is far bigger than the amount of any other pigments, and hence we perceive the leaf/plant as "green". However, some plants have other pigments that is the dominant pigment in their leaves, and this causes the leaf to be a different color, "yellow" in this case. Let's take a look at some hypotheses as to why this can be the case.

Hypothesis 1

It should be remembered that chlorophyll is far from being the only pigment found in leaves. For example, carotenoids - which give yellow and reddish colors - are present in plant leaves. There are many carotenoids (according to Wikipedia there are over 1100 known, but that number will continue to grow). The biological roles of these carotenoids are also varied. In the course of the question, we may be interested, for example, in the photoprotective role of carotenoids. They are involved in the deactivation of reactive oxygen species (ROS). ROS can be formed during photosynthesis and can potentially be harmful to cells. Therefore, in conditions of excess solar radiation, plants can increase concentrations of carotenoids to prevent oxidative stress. It has already been pointed out to you in the comments that younger leaves look yellow - this is a common occurrence. The leaf is a very expensive organ, in the sense that the plant invests a lot of plastic substances in its development. So it makes sense that young, growing leaves get extra protection. That is, a young leaf that has not yet formed all the necessary structures (thick enough cuticle, efficient conductive system, etc.) is less efficient in terms of photosynthesis and therefore more susceptible to negative processes of photodamage. Increased concentrations of carotenoids, among other things, can reduce such risks. If you add to this the small thickness, it is understandable why young leaves often look more yellow.

Hypothesis 2

I have already said that leaves are expensive organs. They have a high protein content, which is very valuable to the plant. If a leaf is damaged or aged, there is a threat of irreversible loss of protein, which would be a great waste. Therefore, in such cases, plants trigger complex processes of removing valuable substances from the leaves. In particular, chlorophyll begins to break down, and the decomposition products are transported to the more durable parts of the plant. This is the reason why leaves change color in the fall, before defoliation. When the concentration of chlorophyll decreases, other pigments, such as carotenoids, increasingly affect leaf color. That's why damaged and old leaves often turn yellowish.

Although, I doubt that in the case of your plant, this process is often the cause for yellow leaves.

Hypothesis 3

Again about photodamage. We have to remember that the capture of photons by chlorophyll is only the beginning of a very long chain of chemical transformations needed for photosynthesis to take place. If the plant is under unfavorable conditions (low or high temperature, lack of moisture, etc), the biochemical cycles can be disrupted. In this case, chlorophyll will continue to absorb photons and enter the excited state, but it will not be able to release this energy further into the cycle. This can be due to many reasons, like the depletion of the plastoquinone pool for example. Under such conditions, the threat of heat shock (if the captured energy is released as heat) or oxidative shock through the formation of reactive oxygen species (ROS) increases. Some plants have developed interesting mechanisms to prevent such adverse effects. In particular, mottled leaf coloration, where green areas alternate with meteolated (chlorophyll-free) areas. Such meteolated areas act as a kind of buffer storage of substances which can be used when the resources of the green areas are exhausted (I am speaking in a very simplified manner here; in fact, these mechanisms are very complicated and it is not quite correct to describe them simply as storage of substances). What is important to answer the question is that some plants can purposely form areas of tissue without chlorophyll, including yellowish coloration. In general, the leaf may look "more yellow."

Hypothesis 4

Before that, we were talking about chemistry. But leaf color is also influenced by anatomical structure. The evolution of the leaf was aimed at maximizing its efficiency as an organ of light capturing and transformation. For example, in the structure of a typical broad-leaf, we can distinguish (conditionally) 4 basic layers of tissues: upper epidermis, columnar tissue, spongy tissue, lower epidermis. All these histological layers contain cells of different shapes. When light passes through the cells, it refracts. That is, each cell acts as an optical lens. For example, the upper epidermis can focus scattered light. Columnar cells often act as photoconductors (similar to an optical cable). The lower epidermis can also act as a reflective layer (so the underside of the leaf often looks lighter than the top). In this case, the light that has passed through the upper epidermis, columnar tissue, and spongy tissue is reflected from the lower epidermis, and is forced to pass through the spongy tissue again. Then it can reflect again from the lower edge of the columnar tissue. In general, geometry and optical properties of leaf cells are also very important for effective light energy absorption. It is important that, passing through the leaf, light is repeatedly refracted and reflected. We know that photons with different wavelengths refract and scatter differently. Therefore, the very anatomical structure and shape of the leaf cells may have an effect on how different parts of the spectrum will be reflected or transmitted by the leaf. One (potential) consequence is, the leaves of some species may be more likely to reflect the yellow portion of the spectrum (and hence be perceived as yellow).

Hypothesis 5

Selection. Indeed, leaves with unusual coloration are perceived as more ornamental. Therefore, artificial selection is often aimed at selecting plants that exhibit unusual leaf color. We have already looked at several natural mechanisms that give yellow leaf color. If artificial selection (by humans) was aimed at intensifying such mechanisms, you could very well get plants with bright yellow leaves. The same applies to mottled and variegated leaves.

Conclusion

The hypotheses I have described are only the tip of the iceberg. Even though it requires a lot of research and investigation to get an accurate answer to your question, I hope these hypotheses have provided you with a general intuition as to why some plants have yellow leaves.