Manuel Peinado Lorca and Luis Monje
As the days get colder and the first frosts appear, deciduous trees and shrubs begin the autumnal display of red tones , yellow, purple and brown that characterize the temperate forests of both hemispheres. To understand the design process of the fantastic autumn spectacle of the deciduous trees, it is important to understand what pigments are and what they are for.
The chemical and light magic of pigments
Plants are experts in capture the energy of light and use it to create sugars through photosynthesis . This process begins with the absorption of light by specialized organic molecules, pigments, which are found in chloroplasts and cellular vacuoles (Figure 1).
Light is a form of electromagnetic radiation, a type of energy that travels in waves. Together, all types of radiation make up the electromagnetic spectrum, whose lengths between 400 and 700 nm make up the light visible to the human eye (Figures 2A and 2B). Each particle, called a photon, has a fixed amount of energy that can excite a pigment (Figure 2C).
An excited pigment is unstable and has several options available to it to become more stable. In plants, the energy of photons is used to split water molecules within chloroplasts. In addition to oxygen, electrons and hydrogen ions are released in the process. These electrons and ions are used to generate energy in the form of adenosine triphosphate ( ATP ), which is used in the Calvin cycle, which aims to take CO₂ and use the energy generated to transform carbon molecules into chains of organic molecules.
The color of a pigment is the result of the reflected wavelength. The green color of the leaves is due to the presence of chlorophylls, pigments found within chloroplasts, which absorb most of the radiation in the visible spectrum, and reflect the greens (Figure 3). When abundant in cells, such as during the spring and early summer growing season, the color of chlorophyll dominates and masks the colors of any other pigment that may exist in the leaf.
Chlorophyll has a vital function : captures the sun's rays and uses the resulting energy in the manufacture of plant food: simple sugars. During the growing season, chlorophyll, which deteriorates with exposure to sunlight, is constantly made, degraded and replaced in the leaves, tireless manufacturers of new chlorophyll during the burgeoning growing season.
The life cycle of deciduous trees
Like most plants, the circadian rhythms of deciduous trees are sensitive to the photoperiod, that is, to the length of the daily period of darkness. In late summer, the days start to get shorter and the nights get longer. As the start time of the whole process depends on the length of the night, the fall colors appear at approximately the same time each year in a certain place, regardless of whether the temperatures are cooler or warmer than normal.
When the nights reach a threshold value and are long enough, the cells at the point of attachment of the leaf petiole and the stem that support it divide rapidly, but do not expand. In this way, an abscission layer of callosa is formed, that is, an impermeable layer of cells that little by little closes the phloem and, with it, blocks the transport of sap from the leaf to the branch. It also blocks the xylem and therefore the flow of minerals from the roots to the leaves.
As this layer develops, the amount of chlorophyll in the leaf begins to decrease. As the chlorophylls degrade, the hidden pigments of oxygenated derivatives of carotenoids, yellow xanthophylls, and orange beta-carotenes are revealed (Figure 3). These pigments are present throughout the year inside the cell, but the red pigments, anthocyanins, are synthesized de novo once approximately half of the chlorophyll has degraded.
Although they exist in the leaves throughout the year, the yellow-orange colors of the carotenoids remain masked by the green chlorophyll. As autumn approaches and the total supply of chlorophylls gradually decreases, the masking effect slowly wears off. When this happens, the yellow, brown, orange, and many intermediate hues provided by carotenoids begin to be seen.
Carotenoids are the dominant coloration pigment in approximately 15-30% of tree species. Their bright yellows and oranges stain the leaves of walnut, ash, maples, poplars, birches, cherry trees, bananas and alders, among many others.
Reds, purples and their combinations come from another group of cellular pigments, anthocyanins. There are two important differences between these pigments and chlorophylls and carotenoids. The first is that they are not found in chloroplasts. They are water-soluble pigments that are stored in vacuoles. The second is that, unlike carotenoids, they do not exist in the leaf during the growing season, but are actively produced towards the end of summer.
As autumn approaches, anthocyanins are made from the sap accumulated in cells. Its de novo synthesis depends on the decomposition of sugars in the presence of intense light as the phosphate level is reduced in the leaf. During the summer growing season, the phosphate level is high, because it plays a vital role in the breakdown of the sugars made by chlorophyll. In autumn, phosphate, along with other nutrients, moves from the leaf to the stem of the plant. When that happens, the sugar breakdown process changes, leading to the production of anthocyanin pigments. The brighter the light during this period, the greater the anthocyanin production and the brighter the resulting color.
In temperate regions, anthocyanins are present in approximately one in ten species of trees, although in some places up to 70% of tree species can produce the pigment. In autumn forests they stain maples, oaks, dogwoods, cherry and plum trees. These same pigments are often combined with carotenoid colors to create the most intense oranges, vivid reds, and tan typical of many hardwood species.
As fall progresses, cells in the abscission callous more and more. Like chlorophyll, other pigments degrade in light or when frozen. The only pigments that remain are the tannins accumulated in the cell walls, which are brown. The connections between the cells are weakened and the leaves end up falling.
Winter has arrived and with it falls the wonderful multicolored curtain of the autumn season which, in essence, like all the life that surrounds us, is literally built from zero for the particle that elevated Einstein: the photon. Pretty incredible, don't you agree?
About the authors: Manuel Peinado Lorca is a university professor in the department of life sciences and researcher at the Franklin Institute for North American Studies and Luis Monje is a biologist and professor of scientific photography, both at the University of Alcalá
The article Pigments and photons: the science behind the autumn colors of the forest has been written in Notebook of Scientific Culture .