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For the botanists - flowers response to UV (and lack of it)?


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I love taking UV pictures of the Buttercups in the garden at home, the dark centre of the flower looks amazing against the yellow of the outer parts of the petals. While I was looking at one the other day I got to wondering about the driving force for that dark centre. As a result I have a question for the botanists out there.

 

In humans we have our natural pigmentation driven by genetics, but we can also produce more UV absorbing melanin in response to UV exposure (and other forms of stress). Does the same thing happen in plants? If you were to grow Buttercups in the absence of UV (under say a solar lamp but with the UV filtered out), would the flower the produce less UV absorbing chemicals? Or would it be the same as one grown out doors in full sunlight - "I am a Buttercup and I produce XXX amount of UV absorbing chemicals"?

 

The interesting thing with humans, is that hyperpigmentation is produced not only in response of UV induced skin damage, but to other forms of oxidative stress, indicating a strong antioxidant role for it. Do the UV absorbing materials in plants also perform the same role as general antioxidants?

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complete [06 Nov 2018 11:10AM EST]

 

 

According to my general reading in botany, pigmentation in a flower is genetic. Like any genetic feature, expression may be absent or present and if present, may vary in degree of expression. What we have seen reported about UV floral signatures is that they can vary by flower age, flower location and other factors. Altitude and latitude are often important factors. This makes sense, of course, because altitude and latitude affect the amount of sunlight reaching the flower.

 

I haven't read about or seen any examples of "missing" floral signatures within a species. Size of UV-absorbing areas may vary, but they do seem to always be present in those flowers which have them.

 

 

We have some examples here on UVP of UV-absorbing pigment change due to floral age. Here is an example.

Myosotis sylvatica As the little Forget-me-not flower ages, its UV absorption fades. A bit of research might reveal whether this is due to pollination.

 

 

Here is an example of pigment variation which is known to occur due to pollination and does not seem to be related to UV-absorption/reflection changes.

Lupinus sparsiflorus

Look at Set 1, photos 3 (visible) and 4 (bee vision), to see that after the lupine flower is pollinated it expresses magenta pink areas on the upper petals, called the standards. The Set 1, photo 4, example in bee vision shows quite a dramatic change, but looking at the photos in Set 2 we can't see any particular change in the UV-absorbing areas after pollination. So it is not always UV-absorbing pigments which are relevant to floral signatures.

 

 

Here is a reference to a paper which discusses UV bullseye size variation due to altitude.

Altitudinal cline in UV floral pattern corresponds with a beharioral change of a generalist pollinator assemblage.

In the flower Argentina anserina, the UV-bullseye increases with increasing altitude. Larger UV-bullseyes attracted more pollinators then smaller UV-bullseyes.

 

Here is a more general paper (it's really fascinating!!) by Koski & Ashman which discusses various geographic and bioclimatic factors affecting genetic expression of UV pigmentation in the Potentilla tribe.

Macroevolutionary patterns of ultraviolet floral pigmentation explained by geography and associated bioclimatic factors There is some mention in this paper of UV-absorbing areas being protective against UV damage but no mention of that as an anti-oxidant effect. Still, I think that partly answers what you were asking. This paper has quite a lovely list of references.

((I note in passing that Koski & Ashman make the typical reference to UV "patterns" rather than UV "signatures" as we do here. Absence of a "pattern" is a "pattern". But this is a minor quibble.))

 

 

Jonathan, that's all I have. Hope it helps! You asked an excellent question. :D

It is nice to see curiosity about these things. If I were younger, I think that I'd go back to school and learn enough to do research in UV floral signatures and go on field trips and write nice papers about it all.

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I good nature vs nurture argument and the study of Lemarkian genetics.

Could be a fun experiment. I am not sure of the mutation rate in plants, but for yeast its 10 to the 6. That is in a population of a million you can select out a mutation. Thus you may need a lot of butter cups to select out one from the population that has no pigment or one with lots of pigment under lots of UV stress.

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Thanks Andrea, that's great. Not being a botanist I didn't know where to start (or if it had already been covered). With the larger bullseyes at increased altitude, that would also fit in with being exposed to more UV to. Probably a lot of factors involved with it.

 

David, yeah, I don't think I'll be selective breeding my Buttercups somehow :) I might try growing some indoors next year though.

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From one chemist to another I have pondered the question of what molecules produce the UV absorbing areas on flower petals with particular reference to Australian plants.

 

Here's a post I made some time ago http://www.ultravioletphotography.com/content/index.php/forum/44-uvir-techniques-tests-gear-talk/

 

The back story is that Australian acacia flowers are BLACK in UV. The paper referred to above was from the "heroic' days of natural product studies in Australian universities where the yellow pigment turns out to be a strong UV absorber. I suspect this is just one molecule amongst many which absorbs UV.

 

Dave

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Just a thought that occurred to me while reading this message: the UV-dark pigments, when present, are at the center of the flower, where the germline (or whatever its name is in plants) is located, so it would make sense that this evolved/was recruited as a protection mechanism. However, I'm not holding my breath until someone tests this experimentally...
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Just a thought that occurred to me while reading this message: the UV-dark pigments, when present, are at the center of the flower, where the germline (or whatever its name is in plants) is located, so it would make sense that this evolved/was recruited as a protection mechanism. However, I'm not holding my breath until someone tests this experimentally...

That was my initial thought too Renaud which prompted this - is the UV absorbing pigment there to reduce oxidative stress by absorbing UV before it can damage the parts of the plant responsible for reproduction, and does the plant respond to the environment it is in by producing more pigment in situations of high UV. But yes, proving it is another thing entirely.

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A lot of the spring flowers have UV dark to UV-black stems, abaxial side of the flowers/sepals and even foliage behaving in a similar fashion. I always associate these features with the ability to absorb energy and thus quickly thaw out of spring snow, as snow falls in spring can be quite treacherous for the plant as a whole.

 

The darling of the spring, Tussilago farfara, is a good example. It will emerge from fresh snow in a very short time, provided there is sufficient daylight present.

 

I0803226930.jpg

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Interesting Birna, so in this case absorbing more UV being absorbed would be seen as being a good thing, to give the plant a head start with growth then.
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I think many spring flowers have similar traits. Eranthis behaves similar although the foliage isn't as UV-dark as the Tussilago (which incidentally only has vestigal scales on the flowering stems).
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Shifting from botany to ornithology for a moment. The males of some species of birds have UV "honesty markings", feathers that change their UV reflectance with the age and/or health of the bird. Thus the female will know which bird is younger and healthier, despite other aspects, e.g., vocalization.

 

Is the same principle at work with the bullseye of flowers? Are the older flowers less appealing to pollinators?

 

 

[However, as the UV markings are structural, not pigment-based, the UV reflectance of feathers does not change after the death of the bird.]

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Sigh... Is it just me or do other people manage to hit the magic words sometimes and Google produces what you had been searching fruitlessly for.

 

Here's a very interesting paper from 2001:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC61112/

 

"Attractive and defensive functions of the ultraviolet pigments of a flower" and you can download the PDF for free. That's an order :rolleyes:

 

Dave

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That's a really good paper, Dave. Nice find! Thank you for this link.

 

I have often read about the flavonoid role in UV floral signatures, but the phloroglucinols are new to me. (Not that I have anything more than a superficial knowledge of floral pigments in the first place. :) )

 

I recall that Bjørn Birna and I have always encouraged the photography of the abaxial side of the flower because we noticed early on that there can be a difference in the UV-absorbing characteristics on the underside of the flower. So it certainly makes sense to me that there can be some sort of a protective effect for buds as Gronquist et al suggest.

 

Added Later: The paper was written in 2001. At that time the authors used Kodak EPY film to make the fotos. The fotos look quite nice and have a cyanotype appearence.

 

A very short summary:

 

The authors showed that a certain pigment(1) -- found at a remarkably high concentration in the ovary wall and also in lower amounts in the petals of the flower Hypericum calycinum -- served to deter certain moth(2) larvae from eating their favorite food(3) when the pigment was added to it. So this pigment is thought to protect the Hypericum calycinum flower seeds from insect attack.

 

(1) dearomatized isoprenylated phloroglucinols (DIP)

(2) Utetheisa ornatrix

(3) pyrrolizidine alkaloids

 

The DIP is UV-absorbing and also contributes to the pollinator attracting UV-signature of H. calycinum.

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I tried in vain to find any later papers by the major authors Thomas Eisner and Jerrold Meinwald on this UV related topic, both are now deceased alas. This paper confirms my guess that working out which pigments are responsible for UV absorption is a very non-trivial pursuit. You need a lab which was state-of-the-art in 2001 and the ability to use all the high-tech gear $$$$. The authors compared the extracts of UV-reflective and UV-absorbing petal parts and found essentially only one component in the UV-reflective areas but there were 6 components with generally related structures in the UV-absorbing areas including the DIPs above which had not previously been found in UV-absorbing petals. They also found that the anthers and filaments contained UV-absorbing flavonoids which is consistent with my UV images of all Australian native flowers that I have photographed with UV-black anthers and filaments. This is just one more little brick in my house-of-cards about what is really going on in UV signatures.

 

Going out on a limb, and fulling expecting it to break, leads to the proposal that UV absorbers evolved to protect the reproductive parts of the plants from UV radiation damage and slowly became expressed in the petals where they perhaps increased the fitness of those plants by attracting pollinators to the flowers. Any other components which served to protect the flowers by deterring insect attack would be a bonus.

Dave

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Dave: Going out on a limb, and fulling expecting it to break, leads to the proposal that UV absorbers evolved to protect the reproductive parts of the plants from UV radiation damage and slowly became expressed in the petals where they perhaps increased the fitness of those plants by attracting pollinators to the flowers. Any other components which served to protect the flowers by deterring insect attack would be a bonus.

 

I'm teetering out there on that limb with you Dave because there are too many unexplained aspects of UV-absorbing areas as pollinator signals. For example, what could my patch of Shasta Daisies* be signaling with their uniformly UV-dark non-patterned signature? The blooms are always covered with bees, flies, wasps and other insects. It seems that some kind of protective role for the UV-dark pigment is a better explanation. Of course, then you have to ask about those flowers which have no UV-dark pigmentation. Don't they need protection too? Well, Ma Nature is never consistent in her expressions. It's probably a little of this and a little of that along with a pinch of the other.

 

*Leucanthemum x superbum

 


 

 

Reed: Is the same principle at work with the bullseye of flowers? Are the older flowers less appealing to pollinators?

 

I haven't seen, so far, that the UV bullseye kind of UV-signature fades with age or changes with pollination. But there are lots of them out there that I have not seen.

 

The Forget-me-not mentioned in my Post #2 is one example of a flower whose UV-signature changes with age. I have read that pollinators don't instinctively recognize UV signatures, but that learn them quickly in a local area. So the conclusion could be drawn that the bees learn a faded Forget-me-not doesn't offer what the fresher flower does. However......

 

However, if we think about the possible protective value of UV-absorbing areas (referencing the paper which Dave linked to in Post #15), then any change in the UV signature with age (if present) could have something to do with that rather than being a pollinator signal.

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These questions are really fascinating, and really almost impossible to completely disentangle. We have been looking at phenolic metabolites in plant leaves for quite a long time now. Most of the UV-B literature repeats the idea that flavonoids accumulate when plants are exposed to UV-B and that they provide protection either as "sun-screens" or "antioxidants". In many cases they do accumulate to some extent upon exposure to UV-B, but the total concentration changes a relatively small, and this obviously is not enough to matter much as a sun screen... on the other hand flavonoids accumulate also (usually more) when plants are exposed to blue light. Furthermore, there is a mutant of Arabidopsis (tt4) that is defective in chalcone synthase (CHS). Plants carrying this mutation accumulate barely any flavonoids, but they do accumulate phenolic acids (rather more than in the wild type). They grown in full sunlight and tolerate UV-B, so flavonoids are not really required for "life under solar UV". The clearer response to UV-B radiation of flavonoids is in most cases are changes in the composition than in the total amounts of flavonoids.

 

Going back to flowers (not my subject) one important consideration is the contrast of flowers against the background given the visual system of the pollinators and the spectrum of natural illumination. This I think is sometimes used to explain differences in UV and visible colouration between flowers from plants growing in shade or in the open.

 

At the moment we are analysing the data from a small study done with the Nasturtiums I grew this summer in the balcony. I first photographed in UV, VIS and NIR the flowers from seven different plants (from two different cultivars of mixed colours). The next step was to measure the spectral reflectance of the petals with an OceanOptics Jaz spectrometer with a spectro-clip integrating sphere and a xenon light source. Third step was to ask the people at our campus' metabolomics unit to analyse with HPLC the extracts from the petals of flowers from the different plants. I do not know the details of the analysis results, but several flavonoids, phenolic acids, anthocyanins and carotenoids were found. I am eager to know how the photographs, in-vivo spectra and chemical analysis match each other. It will be, I think very interesting, as among the yellow flowers some were UV opaque and other UV reflective. I will report as soon as possible here some preliminary results and the intention is to write a manuscript for publication.

 

By the way, I hope to be photographing flowers in a bit over a week's time as I will be travelling to Adelaide and staying there for three weeks. Here in Finland there are not many flowers left at this time of the year...!

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Going back to flowers (not my subject) one important consideration is the contrast of flowers against the background given the visual system of the pollinators and the spectrum of natural illumination. This I think is sometimes used to explain differences in UV and visible colouration between flowers from plants growing in shade or in the open.

 

Yes. For example, when traveling through some of the Southwestern US desert areas, Bjørn and I observed how well some uniformly UV-dark desert flowers stood out against the UV-bright desert floor. It's natural to think that this kind of display might be a useful signal to some pollinators.

Example:

http://www.ultraviol...ve-desert-star/

http://www.ultraviol...ert-gold-poppy/ Scroll to 3rd photo.

 

****

 

Pedro, your nasturtium study sounds very interesting. Let us know when that write-up is done and perhaps tell us what you found.

 

****

 

I'd like to mention once again this nice book: Nature's Palette, the Science of Plant Color by David Lee. This is an enjoyable overview of colour in flowers, fruits, leaves and seeds with insights into the chemical, biological and cultural nature and function of plant colour. The appendix contains a nice classification of plant pigments and their molecules.

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eye4invisible
I've always doubted the "bullseye guide" reasoning behind UV signatures. After all, the patten is only a "bullseye" to a lifeform with rectilinear vision, definitely not one to bees and other insects with compound eye structures. There have to be other factors at play here.
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The bees seem to be able to distinguish some circuler shape and also color/contrast. Their vision is so complex that I've gotten somewhat lost in reading about it.

 

But I agree that there are very likely many factors which play into the UV-absorbing adaptations of insect pollinated flora.

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