1980 American Rose Annual
Pigments and Petal Colors
Lidwien A. M. Dubois

Institute for Horticultural Plant Breeding
Wageningen, The Netherlands

Part I

One of the merits of the American Rose Society has been the classification of colours of rose flowers into 16 basic colour groups.

Although the Horticultural Colour Charts are much more detailed, the ARS system has the advantages of easy applicability. Moreover, owing to the relative wide limits of a colour group, a variety retains its classification under most environments.

For many rosarians any classification of their beloved subjects into uniform colour groups must be a gruesome affair, but what of people who want to investigate the underlying pigments of these colour groups?

We are sorry to confess we have done so out of scientific curiosity. In this paper we have classed the flowers of several hundreds of varieties and seedlings into the ARS colour groups and subsequently, by means of paper chromatography and colour comparison, determined the relative quantities of the flower pigments.

In an experiment in high school chemistry, dots are put with various coloured felt tip pens on blotting paper. When the blotting paper is allowed to suck up water, it can be seen that the proceeding water front decomposes what seemingly was one colour, into two or more different pigments.

This method, called chromatography, is also applicable to water soluble pigments in flower petals. Main pigments present in rose flowers are water soluble flavonoids and non-water soluble carotenoids.

Rose flavonoids consist of the anthocyanidins: cyanidin and pelargonidin [and peonidin] and the flavonols: quercetin and kaempferol. Cyanidin is deep red, pelargonidin is orange-red and quercetin and kaempferol are white or creamy. Flavonols give body to red and yellow pigments.

At the Institute for Horticultural Plant Breeding (IVT), we have developed a method in which, by means of paper chromatography, the relative quantities of each flavonoid can be determined in seven classes (0 equals absent ... 6 highest quantity) from its area on the chromatogram.

Carotenoids were extracted with petroleum aether and acetone and the intensity of the yellow solution was compared with a range of six standard solutions of potassium chromate (0 equals colourless ... 5 equals very intensive yellow).

It should be noted that in the living petal, sugars are attached to the flavonoid molecule, which give rise to slightly different colour impressions. This means our method somewhat simplifes an intricate situation.

Our experiments showed that quercetin and kaempferol occurred together in all petals analyzed. When occurring without other pigments, flowers were white. In all flowers with pink, red, orange or blend petals, cyanidin was found in many varieties accompanied by pelargonidin.

Pelargonidin was never found to occur without cyanidin. Carotenoids were found to determine the colour in yellow varieties, but appeared to be present in orange, orange-red and sometimes even in medium or dark red flowers.

It is thus seen that in our simple model, rose colours are determined by at least two (quercetin and kaempferol) and at the most five pigments (carotenoids, cyanidin, pelargonidin, quercetin, kaempferol). The pigment delphinidin, giving rise to blue coloured flowers in many plants, was not detected in roses.

In the Table, the ARS colour groups are indicated by figures 1-14. Groups 15 and 16 are omitted because in these we had only two representatives. For each colour group, the mean pigment content of at least 25 component varieties was calculated. Owing to the relative wide limits of each colour group, pigment composition of the individual varieties may show some deviation from these means.

Table

In the petals, the combined effects of the pigments can best be imagined by visualizing a paint mixture on an artist's palette: e.g., red and white give pink; red and yellow, orange. We have attempted to present the combined effects of the five pigments diagrammatically.

For the sake of simplicity, we have omitted the flavonols, keeping in mind they only give body to the other pigments. [Editor's note: In Dahlias, orange-red pelargonidin + flavonols gives purple.] Figure 1 shows how the relative quantities of the anthocyanidins: cyanidin and pelargonidin, contribute to the ARS colour groups.

White and yellow varieties (01, 02, 03) have no anthocyanidins. In light, medium and deep pinks (08, 09, 11), both pigments increase about equally reaching a maximum of orange-red (07) varieties. Yellow blends (04), apricot (05) and pink blends (10) tend to contain more cyanidin than pelargonidin.

Also in red blend varieties (14), cyanidin dominates over pelargonidin and even more in the medium red (12) and dark red (13) varieties. In orange or orange blend varieties (06), pelargonidin predominates.

Figure 1
 
Figure 2

A much better picture is obtained when the yellow carotenoids are also taken into account. To prevent an intricate three dimensional picture, the sum of the red pigments cyanidin and pelaronidin were used. The role of their relative quantities should be kept in mind, though.

Figure 2 represents the combined action of carotenoids and anthocyanidins, which allows accurate plotting of the ARS colour groups. It can be seen that each colour group moves within the relative wide limits of both carotenoid and anthocyanidin contents.

Dividing the varieties roughly into low and high carotenoid categories, the figure shows that white (01), medium yellow (02), light, medium, pink blend and deep pink (08, 09, 10, 11), medium and dark red (12, 13) varieties fall in the first category, while deep yellow (03), yellow blend (04), apricot (05), orange (06), red blend (14) and orange-red (07) varieties fall in the second.

Owing to differences in the relative quantities of cyanidin and pelargonidin, the medium pink (09) and pink blend (10) groups slightly ovelap the adjoining ones.

Our trials have shown that seemingly uniform flower colours consist of at least two and at the most five individual pigments. These pigments, occurring in various quantities, give rise to the unending colour range of our beautiful rose flowers.

Given a certain pigment constitution, the flower colour can be approximated, the reverse is difficult owing to the wide limits of the colour groups.

Table
ARS colour groups and mean pigment content of varieties in each group, as determined by paper chromatography

Anthocyanidins: Flavonols:
Color Group Code Carotenoids Pelargonidin Cyanidin Kaempferol Quercitin
white 01
0.5
0
0
4.1
1.5
medium yellow 02
3.8
0
0
3.5
2.9
deep yellow 03
4.3
0
0
2.6
3.1
yellow blend 04
4.2
0.1
1.4
2.1
2.9
apricot blend 05
5.0
1.0
1.9
2.6
1.6
orange (+blend) 06
2.3
3.8
2.6
3.6
1.4
orange-red 07
1.9
4.1
3.5
4.1
2.2
light pink 08
2.0
1.1
1.1
3.4
1.0
medium pink 09
1.7
2.1
1.7
3.6
1.7
pink blend 10
2.5
0.8
2.3
2.0
2.8
light red, deep pink 11
0.6
3.0
3.1
4.2
2.3
medium red 12
1.1
1.5
4.7
3.2
3.7
dark red 13
0.7
1.7
5.0
3.0
3.6
red blend 14
2.8
0.5
3.5
0.4
2.8

Part 2