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Do all plants have a UV signature?

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#1 nfoto

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Posted 20 May 2013 - 10:15

Rørslett, B. 2013. Do all plants have a UV signature? http://www.ultraviol...a-uv-signature/

This short article is meant to initiate a discussion of terminology for UV documentation of botanicals. Contributions and arguments pro et contra are most welcome.

The terminology in dealing with ultraviolet studies on flowers is little standardised and many of the phrases used are pretty inaccurate in a scholary context. For example, using 'honey guide' instead of 'nectar guide'. This descriptor is misleading because the pollinators such as bees produce honey, plants only provide the raw materials (nectar).

If we do a Google search for various combinations of phrases commonly used to describe UV appearance of flowers, we come up with the following figures (as of 20 May 2013);
  • UV flower: 22 300 000
  • UV iridescence flower: 13 700 000 (likely including a lot of false positives)
  • UV signature flower: 13 400 000
  • UV pattern flower: 5 690 000
  • UV guide flower: 4 780 000
  • UV honey guide flower: 3 930 000
  • UV conical cell flower: 1 650 000 (682 000 without the 'UV' term)
  • UV fluorescence flower: 1 650 000
  • UV bull's eye flower: 863 000
  • UV nectar guide flower: 299 000
These figures emphasise the need for better or more appropriate terminology. I shall recommend using UV signature as a common denominator for the UV features of a flower. A UV signature encompasses and takes into account all aspects of the flowers as seen under UV light. Thus, even a UV dark flowers has a UV signature (the key ingredient being its UV darkness). Accordingly all flowers have a UV signature. You just need to consider the flower as a whole, including the UV reflectivity of its surroundings.

The question is now more whether it is feasible to arrange these signatures into major groupings. At present the available documentation may not be comprehensive enough to attempt doing this kind of classification, but we are getting closer to being able to do this by every new signature documented. Just ensure you have covered all bases of the UV appearance. In particular, whenever a UV-dark flower is registered, try to explore potential iridescence or specular reflections by varying the angle of incidence for the UV light source.

The phrase "a flower with no UV pattern" is often encountered in literature or on internet sites. While the exact notion frequently is lacking, I take this to mean 'a flower that does not reflect UV at all , or having very low UV reflectance'. I contend this is still a UV signature, and furthermore a pattern can emerge as well if conical cell distribution or iridescence behaviour is included in the signature. So, basically, we should think in terms of a UV signature rather than the ambiguous 'UV pattern' term.

Goldberg & Atsatt (1975) and Primack (1982) claimed that about one-third of all species in a temperate region strongly reflect UV. Probably they referred to these species as having a "bull's eye" pattern. A compilation presented by Kevan et al. (2001) indicated the range of UV-reflecting flowers to be 14-40% in different geological regions. Again, the exact definition of what constitutes a "UV reflective" species is lacking and they may think in terms of the "bull's eye" pattern. Since UV reflectivity is continuously variable from very low to very high, this kind of tabulation is not very informative at all.

Kevan et al. (2001) did point out that overall importance of UV features is to be assessed with other aspects of the flower in mind and basing an analysis of say flower-pollinator relationship on UV alone may be misleading. I agree with their assertion with the caveat that an UV signature should be based on the total appearance under UV illumination, not just the reflectivity. We have recently seen many examples of the intricacy in UV signalling brought about by conical cell patterning, iridescence, and fluorescence and all of these add another dimension to the total UV signature. In taxonomical terms the entire UV signature should be quite important as closely related species often can be differentiated on their UV signature alone.

Instead of playing down the imporrtance of a plant's UV features, I assert the UV signature is a significant aspect of its appearance to the world around it. As such a UV signature is the facilitator, the means by which the signalling of a flower to the external world is maximised in terms of brilliance and colour vibrancy. Just like adding an optical brightener to fabrics to make them look better and more colourful.

The last decade has seen a remarkable development in UV imaging and its fidelity. When I first set out doing UV shooting of flowers in the digital domain (around 2001-2), the chase was on for bull's-eye patterns of classic showcases such as Rudbeckia, Taraxacum, and Tussilago. At that time, file resolution was 3-6 MPix and any UV-dark flower was merely recorded and quickly forgotten as having nothing of interest to offer. Additional features such as conical cells or iridescence could not be shown by my technique before say 2005 or 2006, when file resolution moved upwards to 10 MPix.

At present, having file resolutions reaching 16 to 24 MPix, we can document not only conical cells but show they have different physiognomy and contribute towards iridescencve by acting as diffraction gratings. UV-induced florescence of nectaria apparently is wide-spread, whilst pollen or styles may or may not be fluoresceing. In some case, such as with Narcissus, hyaline cell structures around the nectaria are encountered which make specular reflections in UV.

I cannot free myself from the impression that some of the comments published by biologists on UV appearance would not be cast in the same manner were the authors familiar with what we now can demonstrate for UV features.

References:
Guldberg,L.D. & Atsatt. P.R.1975.Frequency of reflection and absorption of ultraviolet light in flowering plants.The American Midland Naturalist 93: 35-43
Kevan P.G., Chittka, L. & Dyer, A.G. 2001. Limits to the salience of ultraviolet: Lessons from color vision in bees and birds. J. Experimental Biol. 204: 2571-2580.
Primack, R. B. 1982: Ultraviolet patterns in flowers, or flowers as viewed by insects. Arnoldia 42,3: 139-146

[Published 20 May 2013 last update 21 Aug 2013]

#2 Nico

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Posted 21 May 2013 - 15:25

UV signature is fine for me. Alternatively, one could use the term UV reflection. It might be close to 0% it might be very strong and it can also vary on different parts of the flower. However, the term signature also works for me. – It’s just a convention, anyway.

#3 nfoto

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Posted 21 May 2013 - 16:03

Not entirely a convention, more like an all-encompassing concept. Thus 'UV reflection' alone would not convey the totality of what I refer to. Two flowers with near zero UV reflection can exhibit very different UV signature. Think in terms of all involved components.

Sometimes, the UV properties are not apparent even in an otherwise excellent UV capture. An example is Mexican Goldpoppy (Eschscholtzia californica ssp. mexicana; Papaveraceae), a plant of the Sonoran desert in USA. In UV the petals are virtually jet black. Closer examination will reveal traces of iridescence and there is also a stark contrast between the black flower and UV-bright environment. However, when I did a UV video of these flowers, I observed numerous extremely bright flashes making the flowers visible from a distance. These turned out to be specular reflections from the very soft petals that flexed back and forth in the breeze. So, the petals themselves did hardly reflect UV, but the epidermal structure still had capability to act like a mirror in UV.

#4 OlDoinyo

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Posted 31 October 2014 - 23:17

I think there is a desire for a term meant to convey the presence or absence of different spatial patterns in the UV, as opposed to what the naked eye sees. Your term "signature" seems overly broad to emphasize this concept, being the ensemble of all optical properties of any flower. I think many will continue to use "UV pattern" for this reason, rather than "differential signature" or some such.

#5 Andrea B.

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Posted 31 October 2014 - 23:38

But if you say, for example, that "a flower's UV-signature is uniformly dark", then that phrase conveys a lot more than saying that the flower has "no UV pattern". And a similar statement goes for "a flower whose UV-signature is uniformly bright". Both such flowers would be globbed together under the phrase "no UV pattern".

And there are all kinds of in-betweens I haven't even mentioned. As well as the "UV twinkle" we (and others) have seen from the conical cells of the Poppies. "UV twinkle" really isn't a "UV pattern" because it is not fixed. But it is part of the flower's UV-signature.
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#6 photoni

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Posted 22 March 2021 - 16:23

View Postnfoto, on 21 May 2013 - 16:03, said:

Two flowers with near zero UV reflection can exhibit very different UV signature.

sorry nfoto, a curiosity, is there a list of the different flowers and which nm does it correspond to? a kind of filter verification manual.

#7 Andrea B.

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Posted 22 March 2021 - 17:39

The Chittka paper linked below shows the general floral responses between 300 - 400 nm based on flower color and UV response. I think it will help you with what you are asking about. Scroll through and look for the "averaged" reflectance charts.

But remember that floral signatures are made up of more than just the petal or ray response. The reproductive parts, sepals, stems and foliage all contribute to the UV floral signature as does the background against which the flower grows and any iridescence which the flower petals/rays might exhibit. :grin:

I am providing two links to the downloadable PDF for this paper just in case there is any global variation in how they work. (There are actually more than just two links available.)

Ultraviolet as a Component of Flower Reflections and the Colour Perception of Hymenoptera
by Chittka, Shmida, Troje, Menzel
5 April 1993
LINK1 LINK2

Important Note: The knowledge of Bee Vision has increased and changed since 1993, so this Chittka paper may have some out-of-date info about bee vision. That, however, does not affect the utility of the flower color analysis.

If you do want the latest references on bee vision see any of the work by Adrian Horridge. It is quite fascinating.
Professor Horridge's website contains his book and links to several of his papers: Adrian_Horridge
(Sometimes you have to scroll down when you land on that website depending on what browser you are using.)



Professor Chittka has also collected some spectrometric information about floral UV absorption. Birna has some quibbles with his methodology. And of course there is no possible way that all flowers can ever be measured. But you might find it of interest.
FRED: Floral Reflectance Database
Andrea G. Blum
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#8 Andy Perrin

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Posted 22 March 2021 - 17:54

Interesting to revisit this thread in 2021. A GREAT deal more UV signatures are available to look at now, including just on this site.

2013 Birna:

Quote

The question is now more whether it is feasible to arrange these signatures into major groupings. At present the available documentation may not be comprehensive enough to attempt doing this kind of classification, but we are getting closer to being able to do this by every new signature documented. Just ensure you have covered all bases of the UV appearance. In particular, whenever a UV-dark flower is registered, try to explore potential iridescence or specular reflections by varying the angle of incidence for the UV light source.
It seems like the available documentation ought to be sufficient for that now?

Edited by Andy Perrin, 22 March 2021 - 17:55.


#9 Andrea B.

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Posted 22 March 2021 - 18:33

If you are referring to our collection of floral signatures, I think it is probably the lack of a useful characterization scheme and the TIME to go through all the flowers which prevents it from happening. :grin: I did once make an initial stab at this. Can't remember where I posted it.
Andrea G. Blum
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#10 photoni

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Posted 23 March 2021 - 07:40

Thanks Andrea, an ocean of information. hard to understand for me as a rookie.
The thing I "guessed" is that a single color cannot be represented or associated with a petal but it is multi-spectral.
they are letters of an alphabet that we do not know.
fortunately the different graphics make it easier to understand.