Why do plant leaves change color?

When autumn comes, the leaves in the parks and on the streets turn red or yellow... Then some people can't help but ask why they change color?

Is it the season?

Many people believe that the Beginning of Autumn in the 24 solar terms marks the beginning of autumn. In fact, in the meteorological sense, autumn officially begins when the daily average temperature or sliding mean temperature is less than 22°C and greater than or equal to 10°C.

In terms of the external morphology of plants, the ancients said: "One leaf falling tells us that autumn has arrived." Indeed, the normal change in leaf color and falling is a typical sign of the rotation of seasons and the arrival of autumn. Usually, these trees have green leaves, which turn into a variety of beautiful colors in autumn, such as purple, yellow and red, or even gradient colors of red, yellow and green. Is the color change of plants related to the season?

But some careful friends have discovered that some plants' newly sprouted leaves in spring or leaves that grow after pinching in autumn also appear red and yellow. Why is this?

Reasons for plant leaf discoloration

1. Absorption and reflection of light by some pigments in chloroplasts

The main source of light in nature is sunlight. Sunlight has different wavelengths when it hits the earth, forming a wide spectrum. The wavelength range of light that we humans can see is generally between 440 nanometers and 700 nanometers. This wavelength range is also the color of the rainbow, red, orange, yellow, green, blue, and purple. There are chloroplasts in the leaf cells of plants. The chlorophyll, carotenoids and other pigments in the chloroplasts can absorb and reflect part of the visible light in the sunlight for photosynthesis. Among them, chlorophyll mainly absorbs blue-violet light and red light, and almost does not absorb green light. The green light is reflected by the leaves, and we see green leaves. Carotenoids mainly absorb blue-violet light and reflect yellow, orange or orange-red light. Chlorophyll is synthesized in spring and decomposed in autumn. The chlorophyll content is reduced, and other pigments appear, and the plants will show color changes.

2. Light and temperature conditions affect leaf color

Chloroplasts contain chlorophyll, carotenoids and anthocyanins, which are factors that affect the color of plants. Light is one of the important conditions for the development of chloroplasts. In the development of plant individuals, chloroplasts develop from protoplasts. Only protoplasts in meristems accompany cell growth and differentiation. Under light conditions, the inner membrane of the proplasts concave inward and gradually form chloroplasts. In many plants, the chloroplasts in young leaves are not fully developed, and the chlorophyll content is also low. Relatively speaking, other pigments are more dominant, and the leaves will show other colors; as the leaves gradually mature, the chlorophyll content increases, and the green color will gradually deepen.

In addition, light and temperature will also affect the coloring effect of leaves by regulating the synthesis and decomposition of chlorophyll, carotenoids, etc. in leaves. The temperature difference between day and night significantly promotes the accumulation of anthocyanins and the decomposition of chlorophyll. The appearance of anthocyanins will make the leaves turn red in autumn. The content of various pigments in the leaves will change with the changes in environmental factors such as temperature, humidity, and light, so the leaves will have different colors in different seasons and growth stages. Knowing this, we can explain why the same species of plants will have different color change time and degree in different locations or on the shaded and sunny sides of the same plant. Of course, these manifestations also come from individual differences in plants.

3. pH value affects the color of plant leaves

The vacuoles of plants also contain a variety of pigments (common ones include anthocyanins). Different pigments will show different colors under the same pH conditions, and the same pigment will also have different colors under different pH conditions. For example, anthocyanins are an unstable flavonoid compound that is red or purple under acidic conditions, nearly colorless when the pH is close to 7.0, and blue in alkaline solutions. In autumn, the light weakens and the temperature drops. Most of the vacuoles of plant cells in the leaves are acidic or alkaline, and the color of the leaves will change to red, yellow, or mottled colors. The depth and mottled color of the leaves are closely related to the different content, proportion, and distribution of natural pigments.

After the Frost Descent, cold air frequently moves southward, the temperature drops, the daylight hours shorten, and many changes occur in plants. We carefully observed that the green hue of the leaves gradually disappeared, and other colors that were covered by chlorophyll became visible. Most green-leafed trees turned into breathtaking colors. In fact, there are many reasons for the color change of leaves. It's just that changes in temperature, light, humidity, and acid-base transition are easier to perceive and capture.