Yellow color appears in the foliage of nowhere at the wave of a magic wand with the onset of autumn, it is always present in it. The yellowness give the leaf colour substances called carotenoids. Their impact on leaf color is shown only when it begins to weaken the "green" chlorophyll, the substance which is produced in large quantity only by the action of abundant sunlight.
When the summer passes, the sun is getting smaller, and the chlorophyll is gradually destroyed and ceases to absorb sunlight. The role of the main "colors" of the leaves is transferred to the carotenoids, pigments, orange and yellow colors.
Not only the lack of sunlight can cause yellowing of the foliage. This can occur during a drought when the leaves are not able to get the necessary amount of water. Plant shedding of leaves to conserve moisture. Drought affects wild plants, in the first case due to the lack of rain and the second because of the dry air, inadequate watering, tightness pot, etc.
Yellow color can cause the lack of mineral salts so necessary for the synthesis of chlorophyll. It is nitrogen, iron, magnesium.
Another reason for the yellowing of the leaves – the parasites. The leaves or roots of plants are attacked by viruses, bacteria, mites, etc. in addition, a houseplant may receive mechanical damage during transplantation or from accidental or deliberate actions of the little inhabitants of the house, children or Pets.
Why do leaves turn yellow before fall, and not fall off the green? The answer is simple: a tree or plant takes all the remaining valuable substances in the roots or branches, and "pumps" in senescent leaves food selection. Thus, we can conclude that carotenoids are not for the sustenance of plants are important substances.
Leaves of different plants turn yellow in different ways. For example, birch leaves and limes begin to change color in August, in early September, the yellow maple, then aspen. Each plant has its own biological clock, which occurs with seasonal changes.