Why rose petals change colors

This is something I’ve always wondered about, ever since I saw purple petals developing a rather harsh red edge in the heat of a Houston summer.

Now, a paper (well, for me an abstract until our library gets the book in) that describes the development of anthocyanins in petals of the HT ‘Charleston’ with the maturation of the bloom. (If you cut and paste the link below, you can see Charleston on HelpMeFind.)

As I read the abstract, the genes’ expression levels of anthocyanins are actively changing in the petals as the bloom matures. And it’s UVB radiation that’s responsible.

Link: www.actahort.org/books/760/760_91.htm

Ann, this sounds completely plausible because the same effect is seen in many other species and not just in flowers. Rainbow’s End is only yellow/orange in my greenhouse in winter when there is little uV allowed though by the Kalwall plastic roof, for instance. And Kordes Perfecta shows little pink if opened inside. I have some pink gladioli where the buds that open indoors are very pale compared to those that open outside. Frequently the same is true for chrysanthemums. The red color on apples is very definitely controlled in that way and in Washington State there is a major research program to optimize the color for fruits being shipped to Japan.

We did a lot of studies with fast-growing brasssicas called Wisconsin Fastplants where radiation stress from various sources controlled how much of the red pigment they produce. For instance a turnip or radish is red above ground and white below quite typically. Just like a leaf is green in sun and colorless in dark, they make the anthocyanins as a sunscreen. The intensity of color is proportional to the intensity of radiation exposure in many cases, over some range of color and radiation exposure. An anthocyaninless mutant of the brassica (anl) is much more sensitive to uV damage than the wild type.

Obviously the most intensely red roses are making a ton of anthocyanin in response to very little radiation cues, while the color-changing ones are making very little color unless they get a relatively large dose of some kind of light or other radiation. The extremely red Delicious apple you see these days are selections that go red without much sun, in contrast to the original Delicious

which had very little red at all. I don’t know if the mutation is in the response element controlling anthocyanin, or in the signal detection element.

You can test these effects by placing the right kind of glass or plastic over a flower bud, (or a developing apple). I’m guessing that’s what was done in the study you cited.

Fluorescent lights may have a lot of uV B, depending on what “color” they are. But I have a purple gloxinia that needs more of it than they can provide to get a good strong purple.