Why are some plants red?

A few years ago I would have been spending these snowy days in a couple of (minimally) heated greenhouses, surrounded by thousands of small growing plants. At the time, I was doing research on salad green production to see if it is feasible to do throughout a New England winter. (Sometimes is the answer, but that’s not today’s story.) Salad greens are a crop that includes a range of plant species and varieties. 



The thing is, not all of these salad greens are green. 

Many of them are not green at all, instead they fall more towards the red end of the spectrum. Think of bulls blood beets, spicy ruby streaks and even red leaf lettuce. All these salad ‘greens’ are colored with pigments other than just green chlorophyll. The red, pink, purple and blue colors in plants are due to anthocyanin molecules. Last fall, Sabah wrote about these molecules in relation to leaves changing color in the fall. 

Why do plants produce these pigments? Why aren’t all leaves just green powerhouses of photosynthetic activity?



Plants produce compounds called secondary metabolites. They are not necessary for plant survival, but do aid in growth and development by attracting pollinators, inducing seed dispersal, providing UV protection, and defense from herbivory and pathogens. Anthocyanins are just one kind of a secondary metabolite, and their major function is to make the plant area where they occur colorful.

Anthocyanins pigments absorb green and yellow waves of light, and reflect red wavelengths. (Chlorophyll absorbs blue and red, which is why most leaves are green.) Seasonality can affect the amount of anthocyanin pigments produced within leaves. Sabah used autumnal leaves as an example.  Throughout my winter-time salad green studies, seasonal temperature and light changes had visible effects on the color of red plant varieties. During dark cold months, like November, December and January, ‘Outredgeous’ lettuce leaves had a pale blush of red to them. Once warm, sunny spring days had arrived these leaves were deep, dark maroon. As temperature and light increase anthocyanin pigments also increase within plant tissues.

'Outredgeous' lettuce planted at different times of year. Images by  Claire Collie

'Outredgeous' lettuce planted at different times of year. Images by Claire Collie

This answers how plants are red to blue, but not why. Like fiddler crabs with their large claw, or UV nectar guides on flowers, anthocyanins are in plants to draw attention.

So, if you were a plant would you want your fancy colors to say “come eat me” or “stay away”?

Spilled wine.

Spilled wine.

If your goal was to disperse your seeds to new areas, you might want your colors to say “come eat me” to attract small furry or feathered beasts. Having brightly colored berries like grapes, blueberries, strawberries - pretty much anything with ‘berry’ at the end - are examples of fruits that contain anthocyanins to attract animals to aid in dispersal of seeds.  To a hungry scavenger a colorful berry sends a signal of ripe, tastiness (as long as it isn’t poisonous). Once the fruit is digested, any seeds will be deposited to grow further afield. For sedentary organisms like plants this is a pretty ingenious way to spread your progeny around without moving. 

Conversely, red leaves can deter nibbling insect herbivores. Researchers experimented on the young leaves of Quercus coccifera, a Mediterranean species of oak tree, to see if high anthocyanin concentration in young leaves acted as a deterrent for hungry insects. When young red and green leaves were compared, the green leaves were more likely to have been damaged and have more area lost due to insect herbivory. They hypothesized that bright green leaves act as an optical cue for insects, and the red pigment acts as a mask. So, anthocyanins appearing in different plant parts - berries and leaves - have totally opposite goals. 


My days working with greenhouse salad greens (and reds) may be a nostalgic memory right now. Anthocyanin molecules, on the other hand, are all around in the red and purple and blue fruits and vegetables I eat. Currently, their colorful attraction is luring me towards a glass of red wine.