Why do some leaves turn yellow and some turn red in autumn?

Figure 1: Autumn colors

(Image source: provided by the author)

Autumn is a warm season. The autumn winds paint the hills and forests with alluring colors, the ripe fruits reveal attractive colors, and the crops in the fields are all golden. Nature paints the colors of autumn on the earth with a stroke of a brush, bringing warmth and joy to people.

Figure 2: Autumn scenery of rice fields

(Image source: provided by the author)

The colors of autumn have both the emerald green inherited from spring and summer, and the golden color of late autumn with abundant fruits. Under the foundation of green and yellow, it will eventually transform into a fiery red, which is quite moving.

Part 1

Autumn is coming, and the changing colors and falling leaves are the way plants survive

Figure 3: Aerial photo of autumn scenery at Nanjing Zhongshan Botanical Garden

(Photo source: Nanjing Zhongshan Botanical Garden)

The color of plant leaves changes with the seasons. When the autumn wind blows, white clouds fly, the grass and trees turn yellow, and the wild geese fly south. The coming of autumn also heralds the beginning of the journey of plants changing their appearance, and the changes in deciduous broad-leaved forests are more intense. The rustling of the leaves of the Chinese parasol trees sends away the cold. Autumn not only brings cold air, but also reduces the length of daylight in autumn. After millions of years of evolution, plants have already evolved their own survival strategies in order to adapt to changes in the environment.

Figure 4-5: Creeper on Ailanthus tree

(Image source: provided by the author)

Plants change color and shed their leaves in autumn, which is also a measure taken by plants to cope with adverse environmental factors before the arrival of winter. Plant growth slows down in autumn, and at this time, both the light and temperature, as well as the nutrients and water they obtain from the soil, are reduced. Therefore, plants undergo a series of physiological reactions, causing their leaves to gradually change color and fall off, thereby effectively reducing water evaporation, reducing the consumption of plant nutrients, maintaining the balance of water and nutrients in the plant body, and storing the nutrients obtained in leaf buds, flower buds or root systems to cope with the dry and cold winter.

Figure 6: Aerial photography of autumn scenery

(Image source: provided by the author)

Figure 7: Autumn scenery of Ziwei Road in Nanjing Zhongshan Botanical Garden

(Photo source: Nanjing Zhongshan Botanical Garden)

During this process, in order to reduce the transmission of water and nutrients to the leaves, the plant's abscisic acid and other substances in the body change, forming an isolation layer at the base of the petiole, causing the leaves to fall off the branches , thus ensuring that the plant survives the winter. It should be noted here that leaf shedding is the plant's choice, not the temptation of the wind, which at most is just a matter of course. In other words, the leaves fall because the tree does not want to keep them, not because of the wind's request. The leaves that fall back to the ground will turn into spring mud to protect the flowers, especially the nitrogen in the leaves, which will serve as a source of nutrients for plant growth after natural degradation.

Figure 8: Autumn scenery of Hongfenggang in Nanjing Zhongshan Botanical Garden

(Photo source: Nanjing Zhongshan Botanical Garden)

It is a normal physiological phenomenon for plants to change color and shed leaves in autumn. New green leaves will sprout in the spring of the following year, and we will witness the beauty of the blooming mountain flowers together.

Figure 9: Pink muhly grass

(Image source: provided by the author)

When plants encounter diseases and pests, the yellowing and falling of leaves are abnormal. The fallen leaves do not distinguish between old leaves and new leaves, and the falling season is irregular. Usually the fallen leaves are accompanied by curling and spots. At this time, what we have to do is to remove the diseased branches and leaves as soon as possible and dispose of them properly to prevent the diseased branches and leaves from affecting the growth of the plants, and at the same time prevent the insect eggs on the diseased branches and leaves from causing new harm.

Figure 10: Liquidambar

(Image source: provided by the author)

Of course, abnormally high temperature, drought and cold weather can also cause plants to change color and shed leaves prematurely, and may even cause physiological imbalances in the plants and lead to plant death.

Figure 11: Pine and Cypress Garden of Nanjing Sun Yat-sen Botanical Garden

(Photo source: Nanjing Zhongshan Botanical Garden)

As global warming continues, people once believed that plant photosynthesis would be enhanced, absorbing more CO2 to increase carbon fixation. However, the latest research shows that although plant growth is affected by the environment, the annual carbon fixation amount is certain. After a long period of evolution, plants have their own leaf-falling mechanism.

Figure 12: Aerial photo of the Pine and Cypress Garden of Nanjing Zhongshan Botanical Garden

(Photo source: Nanjing Zhongshan Botanical Garden)

In addition, if plants perform abnormal photosynthesis, their growth and development may be affected. For example, crops planted under streetlights grow normally during the day like crops in the field. When the streetlights are turned on at night, these crops under the lights continue to photosynthesize. When the crop plants in the field slowly turn yellow and the fruits begin to ripen, the plants under the streetlights are still lush and have no intention of blooming and bearing fruit. When winter comes, these plants continue to photosynthesize and the leaves continue to transpire water. However, the vitality of the root system has decreased and it can no longer obtain enough nutrients and water, which will eventually cause the plants to wither or die, which is obviously not conducive to the survival of the plants.

Part 2

Different habitats have different leaf color changes

Figure 13: Ginkgo

(Image source: provided by the author)

The planet we live on can be simply divided into tropical, temperate and frigid zones according to different climates. Different climate zones form different plant groups.

Figure 14: Blood maple

(Image source: provided by the author)

There are many evergreen trees in tropical areas. As the seasons change, the old leaves of these plants will continue to fall off in small amounts, and new leaves will grow in a suitable environment, always maintaining a green look. There are also differences, such as the leaves of the kapok will turn yellow and gradually fall off in winter, and bloom in the spring of the following year before the leaves sprout, which is a typical plant that flowers before leaves.

Figure 15: Beautiful silk cotton tree

(Image source: provided by the author)

In the frigid zones, not all plants can withstand the test of severe cold. Most of the tree species distributed here will shed their leaves before the arrival of the cold winter, while some coniferous and cypress plants can stay in the wind and snow and remain evergreen all the year round.

Figure 16-17: Coniferous forest in Xinjiang

(Image source: provided by the author)

The amazing thing is that their leaves are not renewed every year. The leaves of some species have to go through years of wind and frost from growing to withering.

Figure 18: Cedar

(Image source: provided by the author)

Also in the Coniferaceae family, many pines and cypresses will turn from green to golden in autumn, and finally cover the ground, such as Baldcypress, Metasequoia, Pool Cypress and Golden Larch. Sometimes, they live by the waterside, facing the afterglow of autumn, and their reflections in the lake sway with the blue waves. At a glance, the moving autumn scenery comes to your face.

Figure 19: Golden Pine

(Image source: provided by the author)

In temperate zones, there are many plants that change color in autumn, such as the cotinus and torch tree in the north, the liquidambar, tulip tree and Chinese tallow tree in the south, or the ginkgo, creeper and sycamore that are commonly planted in the north and south. They are a beautiful sight in the colorful autumn.

Figure 20: Liriodendron tulipifera

(Image source: provided by the author)

Of course, there are also many evergreen plants in temperate zones, such as camphor (Cinnamomum camphora), tea, osmanthus (Osmanthus fragrans), magnolia, camellia and cedar, and they all have their own survival rules.

Figure 21-22: Chinese tallow tree

(Image source: provided by the author)

Friends who have been to Nanjing may have heard this saying: "No duck can swim across the Yangtze River." This is an interesting story about Nanjing people's love for ducks.

Figure 23: Camphor

(Image source: provided by the author)

There is a similar saying in the plant world: "Camphor trees cannot cross the river". Camphor trees are evergreen trees native to the south and southwestern provinces, but they are afraid of cold. Even if they are not frozen to death after being transplanted to the north, they will be dying. The camphor trees that we can see growing normally are probably planted on both sides of the streets in Xuzhou in the north. It is difficult to see them planted in large quantities further north. "The Spring and Autumn Annals of Yan Zi" says: "Oranges grown in the south of the Huai River are oranges, and those grown in the north of the Huai River are trifoliate oranges. Their leaves are similar, but their tastes are different. Why is this so? Because the water and soil are different." Although the ancients used orange trees as a metaphor, it can still be seen that in the Spring and Autumn Period, the ancients had noticed that plants would change due to different habitats .

Figure 24: New and old leaves of camphor tree in the same frame

(Image source: provided by the author)

As an evergreen plant, camphor trees are lush and green all year round. If you observe carefully, you will find that the leaves of camphor trees will change color and fall off in certain seasons. What is special is that this is in the spring when camphor trees bloom. At this time, accompanied by the unique fragrance of camphor trees, its old leaves will gradually turn red as new leaves sprout and grow. When a gust of spring breeze blows, the red leaves dance in the wind and fall down, laying a red carpet on the ground.

Figure 25: New and old leaves of camphor tree in the same frame

(Image source: provided by the author)

The leaves of camphor trees also change in the spring breeze and drizzle. Their new leaves are as thin as cicada wings, or tender green, or tender yellow, and sometimes light red. The ancients used the poem "Let the new leaves fall with a trace of lily, and the colorful algae cover the garden" to describe the beauty and fragrance of the old leaves of camphor trees when they fall. At this moment, if we pick up a fallen leaf and rub it with our fingertips, the fragrance of the camphor leaves will linger on our fingertips, refreshing our hearts.

Part 3

At its root, how do leaves achieve "color change freedom"?

Figure 26: Old leaves falling off from camphor tree

(Image source: provided by the author)

The color change process of plant leaves is actually the process of continuous changes in the pigments and other components in the leaves . Take autumn deciduous plants as an example. As the photosynthesis of the plants weakens, the nutrients they obtain are gradually decreasing. At this time, the chlorophyll in the leaves will gradually decompose and turn into nutrients to feed back to the plants. As the chlorophyll degrades, the chlorophyll in the leaves will no longer dominate, and the relatively stable lutein and carotene will take over the palette to mix the yellow and orange colors of the leaves.

Figure 27: Aerial photo of autumn scenery at Nanjing Zhongshan Botanical Garden

(Photo source: Nanjing Zhongshan Botanical Garden)

In the process of leaf color change, the most magnificent is red. Chairman Mao wrote in "Qinyuanchun Changsha": "Look at the mountains all red, the forests are dyed; the river is clear and blue, and hundreds of boats compete for the current." The red mountains and forests are another pigment playing the leading role in the color palette. It is anthocyanin produced by plants in response to environmental stress in autumn .

Figure 28: Aerial photo of autumn scenery at Nanjing Zhongshan Botanical Garden

(Photo source: Nanjing Zhongshan Botanical Garden)

Anthocyanins are water-soluble pigments that are widely found in plants. Most of the colors of various fruits and flowers are also related to anthocyanins. With the decline in temperature, the increase in the temperature difference between day and night, and the shortening of the sunshine duration in autumn, the content of soluble sugar and anthocyanins in the leaves increases. At this time, anthocyanins appear red in the acidic environment of the leaves , and the higher the content of anthocyanins, the brighter the red of the leaves.

Figure 29: Pistacia chinensis

(Image source: provided by the author)

The components of plants are complex and varied, and there is a substance that can change the color of leaves, which is tannin . Under the action of tannin, the leaves of plants sometimes become brown . Interestingly, tannin can not only change the color of leaves, but also improve the resistance of plants . Direct consumption of leaves or fruits rich in tannin is very bitter, which can effectively resist the feeding of birds or animals, and at the same time prevent the invasion of diseases and insects.

Figure 30: Fallen leaves of sycamore

(Image source: provided by the author)

In most cases, chlorophyll is the most abundant substance in leaves, which makes the leaves appear green. When faced with adversity, the chlorophyll in the leaves will gradually decompose, and anthocyanins will slowly form. Together with the participation of lutein and carotene, the leaves of plants will gradually change from green to yellow, orange, orange-red and deep red, creating a colorful forest.

Figure 31: The forest is full of colors

(Image source: provided by the author)

In another case, the same plant can show different colors in the same area. For example, many Chinese tallow trees are planted on the Crescent Dyke beside Yanque Lake in Nanjing Zhongshan Botanical Garden. Every year in mid-to-late October, the two large Chinese tallow trees in the middle of the Crescent Dyke always turn red first, and the other Chinese tallow trees change color one after another. This should be caused by their respective growth conditions and different microenvironments .

Figure 32: Aerial view of the Crescent Embankment in Nanjing Sun Yat-sen Botanical Garden in early autumn

(Photo source: Nanjing Zhongshan Botanical Garden)

Therefore, the color change of plant leaves is related to many factors. Sunlight, temperature, air humidity, temperature difference between day and night, soil environment and growth conditions will all affect the color change process of leaves.

In this autumn when the cold wind is rising, plants offer us the most quiet warmth and romance in their own way.

Editor: Sun Chenyu

Produced by: Science Popularization China

Author: Qin Yalong

(Institute of Botany, Chinese Academy of Sciences, Jiangsu Province, Nanjing Zhongshan Botanical Garden)

Producer: China Science Expo

The article only represents the author's views and does not represent the position of China Science Expo

This article was first published in China Science Expo (kepubolan)

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