October 10, 2014

It’s not easy being blue (for plants)

by Eric Dillalogue

Scientists have found tropical plants that perform “delayed greening,” a process of postponing the development of chlorophyll to protect young leaves from predators and potential UV damage. Interestingly, researchers have also found a few trees with blue leaves performing this delayed greening. This brings up an interesting point: if you think about the world around you, there are many plants with green leaves and, especially during these autumn months in the Northern Hemisphere, many trees with red, yellow, and orange foliage, but very few blue-leafed plants.

Blue leaves from a tropical tree performing delayed greening. Courtesy of Courtesy Beth King (STRI).

Since we see such a range of plant colors, there must be a reason why we rarely see blue plants.  Part of the explanation for this is how most plants convert sunlight into energy for growth. This process, called photosynthesis (Greek for “putting together light”), involves plants using chlorophyll molecules and chemically converting the light energy, carbon dioxide, and water to create sugars. (See our Photosynthesis I module for more details.) A chlorophyll molecule absorbs the violet-blue and red-orange areas of the light’s visual spectrum, but not the green wavelengths. When we look at these plants, we see the color green because that is the part of the visible spectrum being reflected back, whereas blue and red are being absorbed.

The absorption spectrum of chlorophyll. There are spikes in absorption in the violet-blue and red-orange ranges.

Another part of the explanation for why we see so few blue colors in plants is their natural pigmentation. Many plants have pigment molecules (called carotenoids and xanthophylls) that result in an orange or yellow color. The autumn color change happens when the chlorophyll molecules stop working and break down chemically, allowing these “true” colors to appear. There is also another pigment molecule called anthocyanin, which creates a pink, red, or purplish color, and it develops from a reaction between the plant’s sugars and other compounds, most often in the autumn.

Decreasing chlorophyll revealing the yellow color of a leaf.

 So, though we normally see green leaves during the growing season due to the green chlorophyll, we get to see orange, yellow, and red colors in the fall. But since none of these pigments are blue in color – that is, they still absorb the blue wavelength of light and reflect back other colors – we don’t typically see blue foliage.

Blue is so infrequent in the plant kingdom because “[t]here is no true blue pigment in plants, so plants don’t have a direct way of making a blue color…. Plants tweak, or modify, red anthocyanin pigments to make blue” notes David Lee, author of Nature’s Palette: The Science of Plant Color. Of course, we can all name many blue flowers or blue fruits. There are also blue-leafed plants in nature and many more in cultivation. For example, there are several conifers, like the evergreen blue spruce, that have a blue color to their needles; there are blue hostas, blue agaves, and blue grasses; and there is even an iridescent blue lycophyte, a plant related to ferns, called Selaginella and found in tropical forests. But you are more likely to see plants in any other color besides blue.

The iridescent blue leaves of Selaginella. Courtesy of The Royal Society.

So the next time you take a walk in the garden or wander through a forest, look for any blue plants in the mass of other colors. Those plants will be among the most unique in the world.

 

LEARN MORE

– Learn about photosynthesis in leaves that aren’t green at Bay Nature.
– Find out more about “delayed greening” at the Smithsonian Tropical Research Institute.
Science is Fun has more information on the chemistry of autumn colors.
– The iridescent leaves of Salginella, which are structurally similar to butterfly wings, are discussed at The Royal Society Colours blog.
– On a related side note, NPR did a story called How Animals Hacked the Rainbow and Got Stumped on Blue.

Eric Dillalogue

Written by

Eric Dillalogue holds a MS in Library and Information Science and a BA in English. He has worked in a variety of roles from service industry management, academic libraries, and grant administration. He has taught courses on information literacy, web research, and developmental reading. Eric joined the Visionlearning team as a project manager in 2014.