Monday, December 2, 2013

The Color of Onions

A friend passed me a link to an EBay vendor selling the "Romanian Rainbow Onion" seen at right.

He and I have on several occasions talked about biologically plausible mechanisms that might result in interestingly colored plants, so he was curious if I thought this onion was a real thing.

I responded strongly to the true blue color depicted in some of the inner rings in particular, as being evidence of fakery. Blue is a fairly rare color in biology and I was quite certain that there had never been an onion with such a true blue color... let alone the beautiful color gradient depicted.

At this point I remembered that modern search engines let you search using an image as the query. The right half of the image at left was found in a Turkish news article talking about European onion imports. The left half of the image is the original rainbow-onion image (after being scaled/rotated/cropped to match the placement of the unmodified onion).

Unfortunately, it seems this rainbow onion is the figment of someone's imagination and modern image editing software.

However, there are biologically plausible mechanisms by which such lovely color gradients could be generated.[1]
The simplest relies on the interaction between pH the common anthocyanin pigment found in onions, cabbage, and many other plants. Onions generally have a pH on the range of 4-6[2], which corresponds to the purplish shades in the anthocyanin:pH-gradient

There are numerous examples in biology where chemical gradients are generated. Such gradients are critical in developmental biology. It is not inconceivable that an onion could at some point be found (or made) that produced a gradient of pH across the many petiole bases which make up the onion bulb. The gradient that would be produced by this mechanism would not result in the color gradient of the rainbow (ROYGBIV) shown in the original image.

If you want to produce the color gradient of the rainbow, you would have to convince the plant to produce yellow pigments (such as carotenoids) at the appropriate stages of the pH gradient. This more complicated color-gradient system is perfectly plausible in a biological sense, but would be much harder for an agronomist to produce.

It would be far simpler to just make an onion that was entirely blue, by giving it a pH on the range of 8-10. Not only would this be a dramatic color to add to a salad or burger, it would most likely have a very distinct taste. The sharp flavor we experience from onions is due to the presence of sulfuric acid, so by converting the onion to an alkali pH, we would be removing the major flavor component. Without the strong acid component, perhaps more subtle flavors would be revealed.

To breed for a blue onion, you would only have to select the most alkali bulbs each year to produce seeds from. To keep the project from taking a few more centuries than your life will last with modern medicine, I would advise you to find a way to increase the mutation rate of your seeds.