Pardon the title, but the fact that I’m focusing a bit this season on petal color changes due to light exposure (which I’m calling “photochromism”, but is probably not, because that definition implies reversibility) got me to wondering if there are any cultivars which are more susceptible to anthocyanin color shift based on soil pH. I have followed discussions which imply that, for example, reds may be “enhanced” with acidic soils, and yellows with alkaline, but I’m looking for more dramatic visual contrasts with pH. Anybody ever observe this?
I’m not sure that any “reds” (I’m going to use quotes because of the very loose definitions of colors in this case) are very significantly altered by soil acidity alone; do you happen to have any sources handy for that assertion? It might be useful to examine them first, and maybe there is something that could be tested. I imagine that the supposed enhancement of “red” coloration is actually supposed to be due to deepening from anthocyanins that add “blue” to the coloring, making the “red” appear deeper. I know that some people have claimed that “blue” coloration in roses is enhanced by soil acidity, but I am very skeptical. Technically, if vacuolar pH were being affected, that might make sense–but I don’t think there is any relationship between soil pH and vacuole pH in roses. Of course, cooler temperatures usually have a much stronger effect on anthocyanins and the tone of “red” coloration, certainly far stronger than anything being caused by soil pH. In our summer weather, many “red” roses that bother to flower at all appear more hot pink or magenta, but the same roses look very different under cooler spring and fall conditions.
In my experience, some yellow-flowered roses do seem to benefit from the addition of some lime, but that is not to say that they actually need alkaline soil, nor that the color of the flower is changed much as a result (usually, it’s the health of the plant and/or the color of the foliage that is most noticeably improved). Again, it would be helpful to examine the sources of those claims to see if there is a testable hypothesis.
The term “photochromism” as applied to roses actually has no strict, formal definition, much less one that implies reversibility–that might be something garnered from a definition regarding its use in the field of chemistry. In the context of roses, it was primarily a logical and less-inaccurate replacement for an older term (“phototropism”) that was once being widely (mis)used to indicate the effect that some types of of light exposure have on the development of petal coloration in certain sensitive clones. Since “phototropism” already has a very different meaning in plant physiology, not to mention the obvious problem with the “-tropism” part of the term, it could not stand.
Stefan
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