Tl;dr Plants may contain multiple pigments, with different colours. While other-coloured plants also have chlorophyll, they have other pigments in greater quantities than chlorophyll, so even though they appear red, they can do photosynthesis.
Have you noticed that while most grass, bushes, trees and such are green – there are a few that didn’t get the memo? Red and purple plants are not uncommon but can still be rare. So, comes the question – why are they red/purple? Can they still photosynthesize? Are there any other differences between these and green plants?
Let’s start from the basics,
Photosynthesis Doesn’t Only Go Green.
Plants are green due to the presence of certain photo-active pigments present in their photosynthesis factories. Chlorophyll is the name given to a range of green related pigments and it is found in most organisms that photosynthesize.
Pigments are molecules that absorb specific wavelengths (energies) of light and reflect all others. Pigments are colored: the color we see is the net effect of all the light reflecting back at us. Thus, chlorophyll is a green pigment that absorbs red and blue light, reflecting, and thus appearing, green. Other plants, like a red-leafed tree, have plenty of chlorophyll, but the green color that results from the chlorophyll molecule is masked by the relative abundance of another pigment.
What other pigments?
Carotenoids are usually red, orange, or yellow pigments, and include the familiar compound carotene, which gives carrots their color. These compounds are composed of two small six-carbon rings connected by a “chain” of carbon atoms. Due to this structure, they do not dissolve in water, and must be attached to membranes within the cell. Carotenoids cannot transfer sunlight energy directly to the photosynthetic pathway but must pass their absorbed energy to chlorophyll. For this reason, they are called accessory pigments. (Here is an article that describes the mechanism of this energy transfer).One very visible accessory pigment is fucoxanthin the brown pigment which colors kelps and other brown algae. Psidium cattleyanum, also known as Strawberry Guava, is quite unique as it tends to grow both green and purple leaves. The purple and green pigments are almost equally abundant which results in these dual-colored leaves.
Here’s a great article on why green plants tend to turn yellow before falling off. During the end of the leaf’s life, chlorophyll production gradually reduces which allows the colors reflected by these accessory pigments to be seen (Hence, yellow/red.)
Anthocyanins are water-soluble pigments that, depending on their pH, may appear red, purple, or blue. If you’d like to read more about anthocyanins, this page contains a lot of up and coming research using these versatile pigments
Phycobilins are a group of red or blue photosynthetic pigments present in some algae. They are water-soluble pigments, and are therefore found in the cytoplasm, or in the chloroplast. They occur only in Cyanobacteria and Rhodophyta.
If you’d like to observe the various pigments yourself, there’s a simple experiment you can conduct.
Why other pigments?
The evolutionary advantages are not fully understood. One theory is that extra anthocyanins provide an alternative with which chloroplasts (structures within cells that contain chlorophyll) can break down their chlorophyll, helping the plant reabsorb its amino acids for valuable nitrogen. Another theory is that anthocyanins, which are powerful antioxidants, protect the plants in training for winter. If you’d like to read a bit further into this: Genetic and Evolutionary Analysis of Purple Leaf Sheath in Rice