Autumn Leaves Turn Red

BACKGROUND INFORMATION

Daylight time and temperature decrease during autumn as a warning for trees -and many other organisms- that winter is coming. Many trees lose their leaves; but before that is done the green color of the leaves turns into yellow, red, and orange. These changes do not happen randomly; behind it there is a transformation of the pigments in leaves.

One of the main pigments in leaves is chlorophyll and it absorbs the red and blue light from sunlight that falls on leaves; hence, the leaves reflect the "green light" and that is why leaves appear to be green. The chlorophyll is part of a bigger structure called chloroplasts. Photosynthesis, a process in which plants transform solar energy into chemical energy, takes place in the chloroplast; furthermore, it is thanks to this chemical energy that plats produce seeds, grow and flower. However, chlorophyll is not a stable compound because sunlight causes it to decompose; hence, plants have to synthesize it continuously. Nevertheless, this "synthesis" requires sunlight and warm temperatures and that is the reason why it is scarce during autumn.

Carotene is another pigment found in plants; it absorbs blue and blue-green light and hence the light reflected from it appears to be yellow. Carotene is found in the chloroplasts of many plants and when it is paired with chlorophyll both absorb many "lights" and therefore the plant looks green. The light carotene absorbs is transferred to chlorophyll which uses it during photosynthesis. This compound is a lot more stable than chlorophyll and it remains in the plant eve after chlorophyll is absent. At this point the plant appears to be yellow.

Anthocyanins is pigment that absorbs blue, blue-green, and green light and the light reflected by it causes for the plant to look RED. Anthocyanins, unlike chlorophyll and carotene, are not attached to the cell membranes but are dissolved in the sap. The pH of the cell's sap influences the color produced by this pigment; for example, under very acidic pH a bright red color is reflected, and under less acidic conditions a purple color is reflected. The red skin of ripe apples and the purple of ripe grapes is caused by the pigment found in anthocyanins. This pigment is produced after a series of reactions between certain proteins and sugars in the cell sap take place.

Why do leaves in trees turn red during autumn?

The conditions during summer are ideal for chlorophyll to synthesized and hence the leaves of trees act as factories producing sugar from water and CO2 after light hits on chlorophyll (the reactions

...