In spring and summer the leaves of the plants are green thanks to clorophyll, a substance capable of transforming water and carbon dioxide into glucose using the energy of light. It, in fact, produces the fuel that feeds the vital function and allows the growth of the plant. There are various type of clorophyll but all of them absorb the luminous bands of blue and red and reflect green, coloring of this color the plants.
With the arrival of autumn many species of temperate climes get ready for winter sleep and their leaves gradually change in color. The appearance of autumn colors is a tipical phenomenon of deciduous trees, that is of those that lose leaves before the arrival of the winter. This does not happen in evergeen plants, like many conifers, where there is a gradual replacement of leaves that never leaves the crown unprotected.
The color change is due to the fact that the clorophyll molecules are no longer produced in the leaves and the other pigments, called “accessories”, are no longer masked and therefore can emerge. These have strange names (xantophylls, caotenoids, anthocyanins, tannins) and make the leaves assume the yellw, orange, brown, red, purple and blue shades typical of autumn. These substances perform various functions, such as protecting the leaf from harmful ultraviolet rays, acting as antioxidans or making the plant unpleasant and indigestible towards animals.
The change in leaves colour is a phenomenon induced by the decrease of the lights and temperatures and is preparatory for photosyntethetic hibernation. It is followed by the fall of the leaves.
All of this is fundamental for plants survival; leaves would not survive the rigors of winter and therefore the most important nutrients contained must be recovered and stored in the stems. If the leaves died with such substances, the plant would suffer a strong damage; from the timely “suicide” of some parts therefore depends on the life of the entire organism.
The same phenomenon can happen during very hot and dry summer, in which trees and shrubs are induced to lose their leaves to survive.
With the return to favorable climatic conditions the plant awakens and nourishes the buds from which new leaves are born. These will resume the photosynthetic activity that allows to restart the growth.
For some time now the observation of autumn phenomena has become an exciting new activity that has also tourist and economic implications in some areas. This is called “fall-foliage” which literally means “autumn foliage”. There is no poet who has not mentioned this natural phenomenon in his works or painter who has not expressed it in its thousands shades.