The plant pigments that ‘paint’ autumn

With the arrival of autumn our parks become a symphony of colors, on the ground there are green, yellow, red, ocher and burnt sienna leaves. The whole looks more like a palette abandoned by a painter than a garden.

The explanation for all this must be sought in chemistry. In autumn the hours of daylight decrease, temperatures drop and plants stop photosynthesizing. In terms of energy consumption, the leaves are no longer profitable, since their maintenance exceeds productivity, so the smartest option is to drop them.

Before the wind breaks them, they change color because the chlorophyll degrades and

the other pigments, which have always been there, begin to make an appearance. The color change is, in itself, a defense system, since the purple and reddish tones act as sun filters to protect the chlorophyll while oxidation takes place.

Non-photosynthetic pigments

Thus, autumn hues are produced by pigments, a word used in biology to describe any molecule that absorbs light and translates the existence of a color. Plants have a huge variety of pigments, among the non-photosynthetic are the anthocyanins and the betalaínas.

The anthocyanins –Which literally means blue flower- are water-soluble flavonoid pigments that take on a color that ranges from red to blue, depending on the pH at which they are found. These pigments are more visible in flower petals and have a protective function against ultraviolet radiation. They are responsible, for example, for the typical red color of the Euphorbia pulcherrima or poinsettias, the famous poinsettias.

For their part, betalaínas They are water-soluble pigments that contain nitrogen in their composition and whose presence is restricted to a small group of botanical families, which are ‘painted’ with reddish and yellowish tones. Among the seventeen families that boast of betalains are the Cactaceae (cactus), the Nyctaginacea (bougainvillea) and Beta vulgaris (beetroot). The incandescent red color of beets is precisely due to betalains.

photosynthetic pigments

In addition to anthocyanins and betalains, land plants can have photosynthetic pigments, molecules that have the ability to transform light energy into chemical energy. There are only two types of photosynthetic pigments in plants: chlorophylls and carotenoids.

Chlorophyll is green in color and is synthesized in chloroplasts, where glutamic acid is converted into delta-aminolevulinic acid (ALA), from where several metabolic phases are produced that end with the formation of four pyrrolic rings (tetrapyrroles) and one magnesium atom in the center.

Plants usually have two forms of chlorophyll, called ‘a’ and ‘b’. The difference between them is that chlorophyll a has a methyl group (-CH3) attached to the tetrapyrrolic ring, while chlorophyll b has oxidation and the formation of a formyl group (-CH = 0). This biochemical difference explains that each of these pigments absorb light of different wavelengths within the visible spectrum.

The carotenoids, in turn, can be of two subtypes: carotenes (alpha and beta) and xanthophylls. The former generate yellow, orange or red tones, and are responsible for the bright colors of the fruits and seeds, which act as a “hook” for the animals during the pollination process. These pigments also have a screen effect, preventing the destruction of chlorophyll.

The color of the carotenoids, together with their fat solubility, is responsible, for example, that in a tomato sauce the oil takes on a red-orange color. This pigment is also ultimately responsible for the color of paprika, the fruit of Capsicum anuum.

Finally, the xantophilas (from the Greek, xantos, blond or yellow, and phyllon, leaf) are always yellow in color and they are responsible for the characteristic hue of corn and egg yolk, since pigment is a common element in the diet of birds .

Pedro Gargantilla is an internist at the Hospital de El Escorial (Madrid) and the author of several popular books.

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