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Why do the leaves turn yellow?
19 October 2020

The leaves on trees and bushes are green from spring to late summer. The basis of green color is the content of the main pigment in the leaves - chlorophyll. Plants use this pigment to harness solar energy to convert water and carbon dioxide into nutrients called glucose. Glucose is essential for plant growth and development. Chlorophyll is continuously used in the synthesis of nutrients, while itself is destroyed. The plant restores chlorophyll reserves during the growing season, and this helps the leaves to retain their green color.

The plant growth season slows down in late summer. Days get shorter, there is less sunlight, and the production of the green pigment needed for photosynthesis decreases. The capillaries of the veins, along which the juices move from leaf to branch, gradually close. So, at the base of the leaf, a cork cell layer gradually forms and the amount of moisture that the leaf receives decreases. This leads to the final destruction of chlorophyll and allows other pigments to appear.

Indeed, there are other pigments in the leaf tissue, which, with the active growth of the plant, are "interrupted" by abundant chlorophyll. Xanthophyll pigment gives the leaves yellow tints, and carotene - orange color (it also predominates in carrots). Vibrant reds and purple hues give the leaves the anthocyanin pigments. Maple and aspen leaves are often covered with crimson paints, birch leaves turn light yellow, oak, ash, linden, hornbeam and hazel leaves acquire brownish-yellow shades. Poplar leaves are just beginning to turn yellow and fall off immediately.

The autumn color of foliage depends not only on the species of the tree, but also on how much pigment has been developed during the summer. Pigment production is dependent on weather conditions. In a dry warm autumn, the foliage on trees and shrubs is brighter and more saturated than in a cool one. But during rainy autumn days, the leaves turn dull yellow or brown.

It is also believed that the color of autumn leaves depends on the quality of the soil on which they grow. So, on poor soil, red leaves prevail in trees, and in trees of the same species on rich soil, yellow leaves.