Wednesday, August 3, 2011

Pigment Pondering

No pictures today, which is ironic considering it's a post about plant pigments.  Think of it as an exercise for the imagination.  I promise pictures for next time, but today interesting fonts and colors will have to suffice to spice up the text. 

The previous post (which had lots of pictures), described the amazing plant pigment called betalain, found in Celosias and beets (and bougainvillias and cacti).  Very strangely, betalains have never been discovered in the same plant as today's pigment, anthocyanins.  Anthocyanins are the workhorse, common pigments, and betalains are the superhero pigments.  But anthocyanins are not without some amazing characteristics.  Come along and find out.

Anthocyanins are a group of molecules that are usually red, but sometimes they are blue, orange or even yellow.  They share a similar chemical structure and method of production in plants, and they are everywhere.  Every time you look at a plant and see red, unless you're angry or it's one of the plants with betalains mentioned above, you're looking at anthocyanins.  Red fruit, red leaves, red stems, and red flowers are all due to anthocyanins.

In plants, anthocyanins have many functions.  They can act as sunscreen, which is why immature leaves are often reddish.  Some trees' leaves produce anthocyanins in the fall to protect the dying leaves while their nutrients are recovered by the parent plants.  Anthocyanins are major antioxidants in plants, and they protect the plants' DNA from other types of radiation in addition to UV light.  Flowers use anthocyanins to attract pollinators.  Fruits use anthocyanins to attract dispersers.  Plants that grow amongst snow produce anthocyanins because they help the plant stay warmer and grow faster. 

Some anthocyanins change color with a change in pH.  The one in red cabbage turns red in acidic solution, blue at a neutral pH and greenish-yellow at a basic pH.  You can experiment with this at home with a red cabbage, some vinegar, some water and some baking soda.  The anthocyanins in some hydrangeas are red when the soil pH is around 6.5, and they are blue when the pH is a little lower, around 5.5.  The reason for the hydrangea color change has to do with the increased solubility of aluminum in very acidic soil.  The aluminum is used to make anthocyanins when it's present, which accounts for the blue color.  When aluminum is not available at a higher pH, the anthocyanins are made using iron.

All pigments are molecules that selectively reflect light.  Light coming from the sun, or from a light bulb, is white light, which contains light of all wavelengths.  Pigments absorb white light and hold on to most wavelengths, but a few specific wavelengths are bounced off the pigment.  Whatever those reflected wavelengths are are what your eyeballs detect when you look at the pigment.  Something that's red reflects light with only wavelengths of approximately 700 nanometers (very small).  Something that's violet reflects light with wavelengths of about 400 nanometers (even smaller).  Something that's white is not pigmented - it reflects all wavelengths.  Something that's black is also not considered a pigment - it reflects no light.  That's why looking at something that is black is like looking at a dark room - there is no light coming to your eyes from either.  Black clothes are hotter in the sun than white clothes because the black molecules in the clothes absorb all light that hits them.  The black molecules hold on to the light as heat.  And that is why I'll be wearing a white T-shirt tomorrow on the farm (and I'll bring my camera!).