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Response to Sinkkonen: Ultraviolet reflectance in autumn leaves and the un-naming of colours

2009; Elsevier BV; Volume: 24; Issue: 5 Linguagem: Inglês

10.1016/j.tree.2009.01.007

1872-8383

Marco Archetti, Thomas Döring, Snorre B. Hagen, Nicole M. Hughes, Simon R. Leather, David W. Lee, Simcha Lev‐Yadun, Yiannis Manetas, Helen Ougham, Paul G. Schaberg,

Plant biochemistry and biosynthesis

Ultraviolet (UV) vision, first discovered in ants more than a century ago, is a major area of interest for behavioural ecology. Because, unlike humans, many animal species can see UV light, spectrometry in the UV has revealed fascinating signalling systems that remain hidden to the human eye. In his letter to Trends in Ecology & Evolution, Sinkkonen [1Sinkkonen A. Ultraviolet leaf pigments as components of autumn colours: a constructive comment on Archetti et al.Trends Ecol. Evol. 2009; 24: 236-237Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar] argues that UV should also be considered in investigations on autumn colours, particularly when testing hypotheses that involve insect–tree interactions. Because UV vision is a generic feature of insect vision and has been confirmed for aphids [2Döring T.F. Chittka L. Visual ecology of aphids—a critical review on the role of colours in host finding.Arthropod-Plant Interact. 2007; 1: 3-16Crossref Scopus (174) Google Scholar], the main taxon of interest in the autumn colour debate, we agree with Sinkkonen that the role of UV needs to be explored in this area. Specifically, we need to know: (i) how much UV is reflected by autumn leaves and how much trees vary in UV reflectance; and (ii) how insects respond to UV leaf reflectance in autumn. With regard to the first question, it is important to consider spectral reflectance in actual leaves rather than the spectral properties of isolated phytochemicals. The presence of UV-reflecting compounds in a leaf does not mean that the leaf itself will be UV reflecting, given the number of other leaf compounds present that might absorb UV light. In senescent leaves of Gingko biloba and Quercus robur, which Sinkonnen mentions as containing UV-reflecting compounds [1Sinkkonen A. Ultraviolet leaf pigments as components of autumn colours: a constructive comment on Archetti et al.Trends Ecol. Evol. 2009; 24: 236-237Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar], maximal UV reflectances were found of only 10% and 9%, respectively (unpublished from Ref. [3Döring T.F. et al.Autumn leaves seen through herbivore eyes.Proc. R. Soc. Lond. B Biol. Sci. 2009; 276: 121-127Crossref Scopus (76) Google Scholar]). Moreover, the UV-reflective compound in senescent G. biloba does not occur widely, especially in woody plants, and it would be misleading to extrapolate to trees as a whole. Indeed, an analysis of 2409 autumnal leaf spectra reveals that 99% have a maximal UV reflectance of 30% [4Arnold S.E.J. et al.FReD: the floral reflectance spectra database.Nat. Prec. 2008; https://doi.org/10.1038/npre.2008.1846.1Crossref Google Scholar]. By contrast, the low overall UV reflectance in autumn leaves means that the necessarily small differences among leaves would be relatively hard for insects to detect. In fact, a colour choice model developed from trapping migrant aphids in autumn found no effect of UV reflectance, when trap colours, mimicking the situation in leaves, reflected little UV [3Döring T.F. et al.Autumn leaves seen through herbivore eyes.Proc. R. Soc. Lond. B Biol. Sci. 2009; 276: 121-127Crossref Scopus (76) Google Scholar]. Further studies are needed to test more thoroughly how aphids respond to UV, but for low UV reflectance (ca. <10%), the blue and green channels have the only significant role in the response of aphids to colours, according to the findings in Ref. [3Döring T.F. et al.Autumn leaves seen through herbivore eyes.Proc. R. Soc. Lond. B Biol. Sci. 2009; 276: 121-127Crossref Scopus (76) Google Scholar]. As well as the UV issue, Sinkkonen's comment raises another important question: should leaves be classified by human colour names? Should we just add 'UV' as another category to the range of 'red,' 'brown' and 'yellow' leaves [1Sinkkonen A. Ultraviolet leaf pigments as components of autumn colours: a constructive comment on Archetti et al.Trends Ecol. Evol. 2009; 24: 236-237Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar], or is there another, more appropriate way of describing leaf colours? Although it has been common practice in plant colour studies to use human colour names, we believe that a more quantitative and objective description is crucial for progress in this field. Using human colour classification, even if supplemented with a 'UV' category, is inaccurate and ambiguous [5Archetti M. et al.Unravelling the evolution of autumn colours: an interdisciplinary approach.Trends Ecol. Evol. 2009; 24: 166-173Abstract Full Text Full Text PDF PubMed Scopus (166) Google Scholar], and is based on the visual system of a species that has no biological connection with the evolution of autumnal leaf colours. Instead, multivariate statistical analyses of leaf spectra would be a more objective way to arrive at a classification system. Here it would certainly be important to include UV data. In conclusion, UV might not be as important for insect–tree interactions in autumn as suggested by Sinkkonen [1Sinkkonen A. Ultraviolet leaf pigments as components of autumn colours: a constructive comment on Archetti et al.Trends Ecol. Evol. 2009; 24: 236-237Abstract Full Text Full Text PDF PubMed Scopus (2) Google Scholar]; for different reasons, however, it might well be worth exploring UV reflectance in leaves further.