Leaves are typically green, but the reason why leaves have this color is not widely known. Let’s explore why leaves are green.
The green color of leaves is due to the presence of chloroplasts within them. Chloroplasts contain chlorophyll, which is essential for the process of photosynthesis. In reality, there are other pigments in leaves that appear yellow, orange, and red; however, due to their lower concentrations, the green color of chlorophyll prevails.
During photosynthesis, chlorophyll absorbs sunlight to produce organic compounds, with the strongest absorption occurring in the red and blue regions of the spectrum. The green light is absorbed very little and is reflected back to our eyes, making the leaves appear green.
The green color of leaves is due to chlorophyll, which has a structure consisting of a porphyrin ring with a magnesium atom at its center. The green color of chlorophyll is a result of this magnesium atom. Chlorophyll’s function in absorbing sunlight energy is accomplished through changes in the positions of double bonds within the chlorophyll molecule; thus, the green color does not relate to its function and is not directly tied to photosynthesis.
So, why does chlorophyll appear green? Chlorophyll is green to optimize the absorption of sunlight, specifically in the infrared and red parts of the spectrum. Remember a bit about spectral physics; the light we see emitted from leaves is actually reflected light that the leaves do not absorb. This means that other colors (especially red and blue) are absorbed while green is reflected.
Leaves.
However, for some reasons, the leaves of certain plants do not appear green.
Some species of seaweed have red or brown leaves to better absorb blue light because red light has difficulty penetrating seawater. Therefore, in shallow waters, we see seaweed appearing green, but as we move to deeper waters, it gradually shifts to brown and red.
Another common exception is the Begonia plant. The leaves of this plant display two colors; the upper side is green while the underside is brownish-red. This occurs because the plant typically grows in shady areas under the canopy of other plants. The upper side can capture a bit of remaining light filtered from above, while the underside is brownish-red to efficiently absorb the weak light reflected from the ground or from its other leaves.
Begonia plant.
Chloroplasts act like tiny machines that absorb sunlight to generate energy for the plant. Over time, these machines wear out and need to be replaced. There are two schools of thought on this. Evergreen plants replace chloroplasts in their leaves whenever they reach the end of their lifespan. In contrast, deciduous trees do not replace chloroplasts individually but shed all their leaves at the end of autumn.
At the beginning of autumn, leaves start to gradually turn yellow or red as chlorophyll is reabsorbed by the plant. The yellow or red pigments also slowly disappear as other compounds in the leaves are gradually reabsorbed. By the end of autumn, the leaves are left with just a brown color before they fall off. The dry brown leaves likely represent a mixture of the remaining pigments.
Red foliage.
Yellow foliage.
