In the fall, why leaves change color?

In the fall, the colder days and hours of sunshine decreasing causes a slowdown of the rise of sap to the leaves. Chlorophyll production will stop. The tree will produce ethylene in larger amounts than auxin. Ethylene and auxin are two phytohormones. During spring and summer auxin stimulates the growth of the tree. In the autumn a  larger concentration of ethylene will cause the destruction of chlorophyll and the protein degradation of the leaves. The minerals and amino acids from this degradation will be directed to the rest of the tree and will be stored  for the winter and  re-use next spring.

Color Québec forest

The leaf chlorophyll in the process of photosynthesis absorb light in the red range (645 nm to 660 nm) and blue (430 nm to 445 nm). This is why a healthy leaf is  green.

There are in the leaves of some trees two other pigments. They will reveal their presence in the destruction of chlorophyll. Carotene is an orange pigment that absorbs light in the range of blue-green and blue (400 nm to 500 nm). It will give the yellow color of our autumn leaves.

Canada Color leaves

Finally, anthocyanins are pigments that absorb light in the range of the blue, blue-green and green (475nm to 560 nm). They play a role in DNA protection of plants against UV rays. They would also have a role in attraction of pollinating species. They are found in the vacuoles of plant cells. They change color depending on their nature and degree of acidity or alkalinity of the liquid vacuole. They give us shades of red, blue and purple. When they are present with the carotene, we get shades of red and orange. So that the color of anthocyanins occurs, we need three things. A more or less acid pH, a very sweet vacuolar liquid  and light.