Innate colour preferences of flower visitors

Freshly emerged flower visitors exhibit colour preferences prior to individual experience with flowers. The understanding of innate colour preferences in flower visitors requires a detailed analysis, as, on the one hand, colour is a multiple-signal stimulus, and, on the other hand, flower visits include a sequence of behavioural reactions each of which can be driven by a preferential behaviour. Behavioural reactions, such as the distant approach, the close-range orientation, the landing, and the extension of mouthparts can be triggered by colour stimuli. The physiological limitations of spectral sensitivity, the neuro-sensory filters, and the animals' different abilities to make use of visual information such as brightness perception, wavelength-specific behaviour and colour vision shape colour preferences. Besides these receiverbased factors, there are restrictions of flower colouration due to sender-based factors such as the absorption properties of floral pigments and the dual function of flower colours triggering both innate and learned behaviour. Recordings of the spectral reflection of coloured objects, which trigger innate colour preferences, provide an objective measure of the colour stimuli. Weighting the spectral reflection of coloured objects by the spectral composition of the ambient light and the spectral sensitivity of the flower visitors' photoreceptors allows the calculation of the effective stimuli. Perceptual dimensions are known for only a few taxa of flower visitors.     

The significance of flower colour change in eight co-occurring shrub species

Flower colour changes from white or yellow to various shades of red at or near the sites of harvestable pollen in Calytrix glutinosa, Grevillea pilulifera, Isopogon dubius and Petrophile biloba , and over most of the flower in Hypocalymma angustifolium, Verticordia chrysantha and V. huegelii and over the pseudanthium in Darwinia citriodora. All bee, wasp, beetle, fly, butterfly and moth visitors select flowers in the white/yellow phase rather than the red or intermediate phase.
Nectar is produced by five species, harvestable pollen by four species and detectable perfume by three species, all of which features are usually absent from the red phase. The timing of the colour change in all species also corresponds to loss of stigma receptivity, completion of pollination and onset of ovule seed) swelling. Six species also undergo minor morphometric changes which discourage visitation. In all species, colour change is non-inducible by pollinators, taking 2–30 days to complete. In three protandrous species, all available pollen may be removed in the first visit, requiring transport of non-self pollen to rewardless flowers during the 10 h period of the yellow phase.
These species are highly floriferous and occur in dense patches. Since only a small proportion of flowers may be receptive at any one time, it is concluded that retention of flower parts essentially serves to enhance long-distance attraction, while colour change maximizes pollination and foraging efficiency.     

The evolutionary adaptation of flower colours and the insect pollinators' colour vision

Summary The evolutionary tuning between floral colouration and the colour vision of flower-visiting Hymenoptera is quantified by evaluating the informational transfer from the signalling flower to the perceiving pollinator. The analysis of 180 spectral reflection spectra of angiosperm blossoms reveals that sharp steps occur precisely at those wavelengths where the pollinators are most sensitive to spectral differences. Straight-forward model calculations determine the optimal set of 3 spectral photoreceptor types for discrimination of floral colour signals on the basis of perceptual difference values. The results show good agreement with the sets of photoreceptors characterized electrophysiologically in 40 species of Hymenoptera.     

Greenish blue flower colour of Strongylodon macrobotrys

The flower colour of Strongyledon macrobotrys is luminous blue green and attracts bats for pollination. The chemical basis for development of the flower colour was investigated. The flower contained an anthocyanin (malvin) and a flavone (saponarin), approximately 1:9 (malvin: saponarin) in molar ratio. The pH of the pigmented epidermal cell sap of the jade vine petal was exceptionally high, 7.90, while the pH value of the colourless inner tissue was 5.60. Copigmentation test using the mixtures of malvin and saponarin (1:9 M ratio) at various pH values revealed that the characteristic blue green colour of the jade vine is developed by copigmentation of malvin with saponarin in slightly alkaline cell sap, pH 7.9. In the copigmentation in slightly alkaline condition, saponarin shows a strong yellow colour, which gives a greenish tone to the flower colour.     

Molecular genetic basis of flower colour variegation in Linaria

To identify transposons that may be of use for mutagenesis we investigated the genetic molecular basis of a case of flower colour variegation in Linaria, a close relative of the model species Antirrhinum majus. We show that this variegation is attributable to an unstable mutant allele of the gene encoding dihydroflavonol-4-reductase, one of the enzymes required for anthocyanin biosynthesis. This allele carries an insertion of a transposon belonging to the CACTA family (Tl1, Transposon Linaria 1) which blocks its expression thus conferring an ivory flower colour phenotype. Tl1 is occasionally excised in dividing epidermal cells to produce clonal patches of red tissue on the ivory background, and in cells giving rise to gametes to generate reversion alleles conferring a fully coloured phenotype. This finding may open the way for targeted transposon-mutagenesis in Linaria, and hence for using this genus in comparative genetic studies.     

Utilization of new attributes for evaluating a flower colour spectrum

Flower colour spectra were initially used in India to characterize the flora of urban localities (Nagrathna, 1968; Oommachan, 1973), but only on a presence-absence basis. In order to have a quantitative appraisal of the flower colour spectrum of the vegetation at a particular locality, the density of the species and the number of flowers on each plant should be taken into account. Only this gives the actual number of flowers of a particular colour per unit area: the flower density.     

Geographic variability of flower colour inCrocus scepusiensis (Iridaceae)

The distribution of four variants for flower colour ofCrocus scepusiensis in the northern part of the Western Carpathians is described. The frequency of white stigmata morphs declines from east to west. In the center of the area stigmata colour morphs show strikingly patchy distribution. White perianth morphs usually occur at low frequencies and their distribution with minor exceptions is restricted to the central part of the area, where patchy distribution of the morphs is a rule. These distribution patterns suggest that the founder effect has played a major role in determining the genetic composition of individual populations. The cline for stigmata colour may also be explained by the dynamics of population expansion. No influence of selection can be demonstrated, but the association between perianth and stigmata colour, and the excessively low frequency of white perianth morphs may imply that the polymorphisms are not selectively neutral.     

Relationship between the flavonoid composition and flower colour variation in Victoria

• Victoria (Nymphaeaceae), an annual or perennial aquatic plant genus, contains only two species: V. amazonica (Poepp.) J. C. Sowerby and V. cruziana A. D. Orb. Both species have large floating leaves and variable flower colour. Both Victoria species are night bloomers, which have white petals on the first blooming night that then turn pink or ruby red on the second blooming day. The mechanism of the colour change of Victoria petals during anthesis is still unclear.
• In this study, flavonoids in Victoria petals of both species were evaluated and quantified by high‐performance liquid chromatography with photodiode array detection (HPLC‐DAD) and by ultra‐performance liquid chromatography coupled with tandem mass spectrometry (UPLC‐MS/MS) for the first time.
• In total, 14 flavonoids were detected in Victoria petals, including 4 anthocyanins and 10 flavonols. The flavonoid compositions differed across the two species, resulting in different colours between the inner and outer petals. With increased anthocyanin content across blooming days, the colour of Victoria flowers changed over time.
• The results of this study will improve understanding of the chemical mechanism of colour formation and lay the foundation for selective colour breeding in Victoria.