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CORVALLIS - Some ornamental plants have leaves that aren’t green. Rather, they have purple, red, yellow or variegated leaves. Ever wonder how these plants photosynthesize, since they don’t have a green color?
“There is no secret here,” said Sven Svenson, research horticulturist with Oregon State University. “The chlorophyll needed for photosynthesis is ‘hiding’ within the leaf color, whether it be purple, yellow or red. Our eyes lack the ability to see that chlorophyll is there.”
Plant leaves have three primary classes of pigments: chlorophyll, carotinoids and anthocyanins, explained Svenson.
Chlorophyll absorbs the red and blue light from the sunlight that contacts the leaf. Therefore, the light reflected from or transmitted through the leaf is deficient in red and blue light, so it appears green to our eyes. “Green” is the type of light used the least by chlorophyll. When a leaf has a high concentration of chlorophyll relative to other pigments, the leaf appears green.
Carotinoids absorb the blue-green and blue light from the sunlight that contacts the leaf. Light reflected by carotinoid pigments appears yellow or yellow-orange to our eyes. Generally, carotinoids assist chlorophylls in the process of photosynthesis. Carotinoid pigments are involved in forming the color of carrots. When a leaf has a high concentration of carotinoids relative to other pigments, the leaf usually appears yellow.
A third class of pigments found in leaves is the anthocyanins. Anthocyanins absorb blue, blue-green and green light. When leaves contain high concentrations of anthocyanins relative to other pigments, the leaves appear red or purple to our eyes. Anthocyanin pigments are involved in the red skin of apples, and the purple color of grapes.
Purple leaves usually have high anthocyanin concentrations relative to chlorophyll. Since the anthocyanin absorbs green light (chlorophyll reflects green light), and reflects reds and purples (chlorophyll absorbs these light colors), the leaves “appear” purple to our eyes. The chlorophyll is still there, but it is masked by the higher concentration of anthocyanins.
“If you look at the leaves of a “purple” plant that is growing in the shade, you will see the leaves look muddy-purple or even green,” said Svenson. “In the shade, the leaves produce more chlorophyll to assist in photosynthesis, so the purple color is not as strong by comparison. Similarly, many apples are reddish on the ’sun’ side, and green on the ’shade side.
“So, plants with leaf color other than green perform photosynthesis just like green-leafed plants (if they did not, they would not live). The chlorophyll needed for photosynthesis is masked among the colorful pigmentation.”
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