To understand this, first we must remember why leaves are green. The simple answer is because they contain chlorophyll, the pigment (from the Latin pigmentum – paint), which is green in color. But let’s dig deeper. Why is chlorophyll itself green?
Why objects have different colors
In order to see an object, you need light to fall on it. For example, sunlight, or light bulb light, or fire light. It may seem that sunlight is white-yellow and firelight is orange. But in fact their light is not composed of one color, but of several at the same time. For example, sunlight is made up of the colors of the rainbow-red, orange, yellow, green, blue, blue, violet. And also 50% infrared and a small amount of ultraviolet. But we can’t see them because of the structure of our eyes. But the spectrum of incandescent light bulbs is almost entirely infrared. And visible light doesn’t get much of it. Infrared actually takes up several times as much of the spectrum as all the visible colors put together. And it could be broken down into several colors. But since we can’t see it anyway, it’s more convenient to perceive it as one color.
Different objects absorb certain colors from the light that falls on them. And those that don’t absorb – reflect. For example, a lemon absorbs all colors except yellow. Yellow reflects. That’s why the lemon looks yellow to us.
Why leaves are initially green
Objects can absorb certain colors from all light and reflect other colors. Plant chlorophyll is green because it absorbs rays of all colors except green. Green rays are reflected and come to our eyes. Rays regardless of color contain energy. Leaves do not absorb the green color because compared to other rays of the solar spectrum, green rays carry much less energy. It is more energetically efficient to make the rays that have more useful energy be absorbed instead.
What causes the leaves to change color
Chlorophyll, in addition to coloring the leaves, plays an important role in storing energy from the sun. Thanks to it, the energy of sunlight + carbon dioxide + water are converted into oxygen + carbohydrates. It is in carbohydrates that the sun’s energy is stored.
The most significant component is sunlight. During the cold season – autumn, winter – there is less sunlight. Therefore, plants have no need for so much chlorophyll. Most of it will not be able to recycle light energy anyway, because there is not enough light. And since a plant spends a considerable amount of energy to create chlorophyll, it is much more economical to stop the creation of green pigments.
Also the destruction of chlorophyll allows important elements – nitrogen and phosphorus – to return from the leaves to the branches. This way the plant won’t lose them when it sheds leaves in winter, and can use them the next spring.
But if the green color is gone, then should the leaves stop reflecting any light at all and turn black? No. Because chlorophyll is not the only pigment the leaves have. Carotenoids, the yellow and orange pigments, don’t stop being produced. In fact, they are in the cells of the leaves even when chlorophyll is being produced. But behind the intense green color, they are not easily visible.
Red color comes from an additional mechanism. Another type of pigments, anthocyanins, are responsible for this. By the way, they are responsible for the red and purple color of many fruits. For example, raspberries, blackberries, blueberries, cherries, eggplants, grapes, sweet and hot peppers and many other products. Unlike carotenoids, the yellow and orange pigments, anthocyanins are not originally found in leaves. It is produced as an additional defense – when nutrients are “taken out” of the chlorophyll, some parts of this apparatus continue to absorb energy from the sun. But because of the inability to produce photosynthesis completely, chlorophyll gives its energy to oxygen, which because of this becomes very active and destroys everything, including the systems involved in the evacuation of nutrients. Red pigments cover the chlorophyll molecules to block its energy absorption.
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