Q: How does photosynthesis occur in plants that are not obviously green, such as ornamental plum trees with deep purple-colored leaves? [Paul, Santa Cruz]
A: Photosynthesis (which literally means “light put together”) is that very elegant chemical process that jump-started life as we know it some 4 billion years ago. So to answer your question, we’ll need a short chemistry lesson. Basically six molecules of water (H2O) plus six molecules of carbon dioxide (CO2) in the presence of light energy produce one molecule of glucose sugar (C6H12O6) and emit six molecules of oxygen (O2) as a by-product. That sugar molecule drives the living world. Animals eat plants, then breathe in oxygen, which is used to metabolize the sugar, releasing the solar energy stored in glucose and giving off carbon dioxide as a by-product. That’s life, in a nutshell.
All photosynthesizing plants have a pigment molecule called chlorophyll. This molecule absorbs most of the energy from the violet-blue and reddish-orange part of the light spectrum. It does not absorb green, so that’s reflected back to our eyes and we see the leaf as green. There are also accessory pigments, called carotenoids, that capture energy not absorbed by chlorophyll. There are at least 600 known carotenoids, divided into yellow xanthophylls and red and orange carotenes. They absorb blue light and appear yellow, red, or orange to our eyes. Anthocyanin is another important pigment that’s not directly involved in photosynthesis, but it gives red stems, leaves, flowers, or even fruits their color.
Many plants are selected as ornamentals because of their red leaves— purple smoke bush and Japanese plums and some Japanese maples, to name just a few. Obviously they manage to survive quite well without green leaves. At low light levels, green leaves are most efficient at photosynthesis. On a sunny day, however, there is essentially no difference between red and green leaves’ ability to trap the sun’s energy. I have noticed the presence of red in the new leaves of many Bay Area plants as well as in numerous tropical species. The red anthocyanins apparently prevent damage to leaves from intense light energy by absorbing ultraviolet light. There is also evidence that unpalatable compounds are often produced along with anthocyanins, which may be the plant’s way of advertising its toxicity to potential herbivores. So red-leaved plants get a little protection from ultraviolet light and send a warning to leaf-eating pests, but they lose a bit of photosynthetic efficiency in dimmer light.
Botanists have been wondering about red versus green leaves for the past 200 years and there is still much research to be done in this arena. So you are in good company, Paul.