J Genetics 21: 125–159 (1929)
The Genetics and Cytology of Dahlia Species
W. J. C. Lawrence

THE GENUS DAHLIA.

From inspection of the botanical descriptions of Dahlia species it is evident that, with the exception of D. variabilis, they can be assigned to one of two groups for flower colour: Group I (ivory-magenta-purple); or Group II (yellow-orange-scarlet).

GROUP  I (ivory-magenta-purple).

D. Merckii* D. Maximiliana D. pubescens
D. Maxoni* D. imperialis* D. platylepis
D. excelsa D. dissecta D. scapigera
D. Lehmanni    

GROUP  II (yellow-orange-scarlet)

D. coccinea* D. coronata* D. gracilis D. Tenuis

*I have grown and observed these species

DAHLIA VARIABILIS DESC.

(1) MORPHOLOGICAL OBSERVATIONS.

The position of D. variabilis with regard to the colour groups is unique in that it unites both series within itself. Whites, ivories, yellows, scarlets and purples, with many intermediate colours and intensities, can be seen in various patterns combined with various flower shapes.

These flower colours and their distribution in the garden dahlia are the result of the presence or absence of two series of soluble pigments: (1) flavones, (2) anthocyanins.

The flavone series is expressed in an almost continuous gradation of colours ranging from ivory to deep yellow (Plate XII, figs. 2-6), all of which give the characteristic flavone reaction when fumed with ammonia, i.e. the ivory flowers turn a bright lemon yellow; the yellow change to an intense orange. Flowers intermediate between ivory and yellow change, when fumed, to shades intermediate between yellow arid orange.

Among the flowers which, upon visual examination alone, might easily be classed as ivories, certain kinds are found. which give no reaction with ammonia. In this account such forms are called "whites" as distinct from ivories which, though visually similar, always give a positive reaction for the presence of flavone.

The anthocyanin colours may be arbitrarily classified in three groups: (a) magenta to deep purple, (b) pale orange to deep scarlet, (c) colours intermediate between (a) and (b).

Given a wide enough range of present-day varieties for inspection it quickly becomes evident that all the colours of the magenta-purple group are invariably associated with ivory flavone grounds; and that all the colours of the orange-scarlet group are invariably associated with yellow flavone grounds. (It will be noticed that this association corresponds with that of the two colour groups previously referred to in which Dahlia species may be classified.) The intermediate flower colours are associated with intermediate flavone grounds.

Visual examination of the flower colours as to their kind, association and constitution is much facilitated by the occurrence of (a) mosaic flowers in which the anthocyanin is distributed in flecks and streaks, (b) patterns where the anthocyanin covers a portion only of this petal, and (c) relatively frequent somatic mutation of both anthocyanin and flavone pigments.

In mosaic flowers the distribution of anthocyanin is sharply discontinuous, forming flecks and streaks—the flavone ground colours showing where no anthocyanin occurs. Mosaic flowers in time magenta-purple colour group always have ivory grounds; in the orange-scarlet group they have yellow grounds. Another type of colour distribution is that in which the anthocyanin occurs as a flush across the middle of the petals exposing the flavone ground at the tips and bases, and again the same association of flavone and anthocyanin colours is to be seen. Further evidence as to this relation is afforded by somatic mutation involving each series of pigments.

(a) Somatic mutations of anthocyanins and flavones.

(i) Anthocyanins. Apart from mosaicism, plants with coloured flowers occasionally sport sectors without. anthocyanin—usually quite narrow, rarely so much as 1/4 in. wide, and running the length of the petal. Orange and scarlet flowers have yellow sectors; magenta-purple flowers, ivory sectors (Plate XII, figs. 10, 11).

Not uncommonly dilution or intensification of the anthocyanin occurs. All of the pigment may be missing from a sector, and immediately adjacent on one side another sector, usually corresponding in size with the uncoloured portion, will have a double close of anthocyanin (Plate XII, fig. 11).

In such a case it is probable that a mitosis has occurred such that one of the daughter cells has received all of the chromosomes or factors for colour—the other receiving none—and that these cells have by division given rise to the uncoloured and doubly-coloured segments respectively. Attention is drawn to the similarity of this type of somatic mutation to that observed by Eyster (1924) in variegated maize cobs1.

1Miss de Winton who is working on Primula sinensis at this Institution tells me she has observed similar variations in flower colour, and that they always occur in plants heterozygous for the factor concerned, (See also footnote to p. 138 of this paper.)

A similar condition may occur without the entire loss of anthocyanin, dilute and deeper stripes being found side by side (Plate XII, fig. 12). More rarely a single, solitary deep or dilute stripe occurs unaccompanied by any other visible change.

Petals have been seen where several of these mutations have been present side by side.

(ii) Flavones. Yellow dahlias have been observed to sport ivory and whit sectors. Similarly, ivory flowers have sported white sectors. In the ivory sporting to white, it is much more difficult to recognise where a change has occurred. In all these experiments the final criterion of the identity of a white or ivory flower has been the fuming of the petal with ammonia. With practice however it is possible to distinguish white from ivory by eye—the white having a peculiar translucent appearance and seeming to be very faintly tinged pink or mauve. This is dire to the reflection of light and is merely an optical effect.

The question arises as to what results if the yellow and ivory flavones sport in the presence of anthocyanin. Variations of this kind have been observed—clearly showing the association of particular flavones with particular colours.

(a) If the yellow flavone ground of a crimson-scarlet flower sports to ivory, then the flower colour will be purple, not scarlet (Plate XII, Fig. 14).

(b) If either yellow or ivory sports to white, then no anthocyanin develops.

Confirmation of (a) is provided by flowers which have yellow flavone in the basal half of their petals, shading off almost to ivory at the tips. In these flowers it would he expected  that anthocyanin on the basal half would appear scarlet shading off to purple at the tips, and such is in fact the case (Plate XII, fig. 8). Moreover, the mosaic variety "Dorothy'' has a yellowish ground with crimson flecks, which frequently sports an ivory ground. On the ivory ground the anthocyanin appears purplish (Plate XII, fig. 13).

That it is the flavone which changes and not the anthocyanin alone in the cases just mentioned is made still clearer by bleaching the anthocyanin in au aqueous solution of SO2, which does not affect the flavones. In every ease the magenta or purple sectors have ivory grounds—not yellow.

Two such plants have been used in breeding and they have proved to be heterozygous for yellow, and have given magenta or purple seedlings in F1.

The position with regard to the loss of ivory flavone from a flower carrying yellow flavone and anthocyanin is not clear. Although it is difficult to perceive ivory in the presence of yellow flavone, yet, as will be shown, presence or absence of the ivory pigment does make a proportionate difference to flavone colour. It is to be expected therefore that, should ivory be lost when yellow flavone and anthocyanin are present, a change will occur in the colour of the flower. Sectorial variations have been observed which have every appearance of being due to such a change.

In the mutations so far described apparently only the colour of the flower changes, and not the intensity of pigmentation. Thus an orange flower sports pale magenta sectors; deep orange, deeper magenta; scarlet, purple—and so on.

Intermediate Forms. The facts so far presented have been from the two extreme groups: (1) ivory-magenta-purple, (2) yellow-orange-scarlet. Mention was made of intermediate colour forms, which are more difficult to understand. Careful examination of many hundreds of these has led to the establishment of the following facts:

(1) The yellow and ivory flavones may be present together in varying amounts.

(2) Yellow flavone partially obscures ivory.

(3) The proportion of ivory to yellow gives rise to several shades of colour ranging from ivory, through cream and primrose, to yellow.

(4) When anthocyanin is present the flower colour is determined by the proportion of ivory and yellow flavones, e.g. On an ivory ground it purple; on cream, more crimson; on pale yellow, nearer scarlet; on a good yellow ground, scarlet. The precise limits of the range of the flower colour as determined by the association of anthocyanin and flavone are not clear at present, but so far as my observations have gone the above scheme seems to be approximately correct, though doubtless many moderating influences of a minor nature frequently come into play.

(b) Distribution of anthocyanins and flavones.

(i) Anthocyanins. As previously stated anthocyanin is developed only when flavone is present. It is obvious therefore that where anthooyanin is present the flower colour is due to the combined effect of the anthocyanin and flavone, and that only chemical analysis can finally establish the true colour of the particular anthocyanin(s) involved. Any reference here to an anthocyanin colour by name is made with this reservation.

Apart from evidence to be quoted later there is some reason to believe that at least two distinct anthocyanins occur in the garden dahlia, namely (1) a geranium red, (2) purple. Each is found in several intensities (the above names merely indicating certain degrees of pigmentation), but their distribution and qualitative variation and their reactions with ammonia and sulphurous acid differ appreciably.

Since the geranium red anthocyanin is never present alone, but associated with flavone, the colour of the flower as seen may generally be described as "scarlet.''

The "scarlet" is the less variable of the two anthocyanin colours. The tinged, pale orange, deep orange, scarlet and deep scarlet colours may be different concentrations of the same pigment, for it is possible to make an almost perfect gradation from the most faintly tinged to the deepest of scarlets. Especially is this true if flowers of different ages are used, since there is a considerable facing with age of both anthocyanins and flavones, distinct colour varieties overlapping each other. Thus the intensity of colour of a fully expanded flower of variety "A" will be identical with a newly opened flower of variety "B," and so on.

This fading of flower colours begins with the unfolding of the petals and suggests that the production of anthocyanin is complete (or practically so) at the opening of the capitulum, and that increase in size of the, petals brings about a corresponding dilution of the pigment, the colour differences between some varieties being merely quantitative.

One exception has been observed. A certain pale magenta flowered variety is not fully coloured until the flower is expanded—the buds and the young flowers being almost pure ivory.

The distribution of anthocyanin may vary, the concentration being least at the tip of a petal; least at, or even absent from, both tip and base; deeper on the backs of the petals or vice versa; concentrated on the edges or in the middle of the petals, etc.

Different individuals with apparently the same yellow or ivory grounds may have different intensities of anthocyanin, from the faintest tingeing to very intense coloration—as if varying doses of the same pigment occurred on the same ground.

The quantitative variations described for "scarlet" anthocyanin were observed in flowers whose yellow grounds were apparently identical.

In the scarlet series the presence of small amounts of ivory flavone may alter qualitatively the observed colour of the flowers, but in the purple series no such interference is possible. Hence any qualitative variation in this series is likely to be (1) the direct expression of the modification of anthocyanin or (2) due to the occurrence of different anthocyanins   .

Variations may be observed in the magenta-purple series (a) inclining to rose, (b) inclining to violet, and there are aces iii which they seem to occur together in the same flower.

It is interesting to note that apparently a similar variation occurs in D. Maxoni.

Pigmentation of the stems of the dahlia, though distinct from flower colour, is nevertheless not entirely unrelated. White and ivory varieties never have colour in the stems or in the sub-epidermis of the tuberous roots—yellows rarely have. The yellows which have faintly tinged stems are not at present understood, and seem to be exceptional. All other coloured varieties observed have coloured stems.

There are several well-known varieties which have deeply coloured leaves and stems, and in such plants this pigmentation extends also to the disc—florets, bracteoles, etc. all being coloured. Various intergrades have been seen between the deepest and the palest of these deeply pigmented plants. Presence of abundant stem pigment confers an added intensity upon the flower colour.

Normally, all variations of flower colour are only expressed in the ray petals—the disc florets remaining the usual yellow colour—i.e. flavones are present and give the characteristic reaction with ammonia in every part of the capitulum except the pollen. Thus the disc florets of a normal white rayed variety all carry flavones—the petals alone having no pigment.

There is another kind of white, however, in which pigments are absent from the whole of the capitulum—bracts, bracteoles, disc florets and rays. This white is the extreme expression of a condition which most frequently shows itself in coloured edges to otherwise white petals. The well-known variety "Union Jack" is typical of this condition. I have attempted to analyse the behaviour and inheritance of this abnormal pigment distribution, but it appears to be extraneous to the general distribution and inheritance of pigments in Dahlia. The results of this part of the investigation will appear in a later paper, when the work has been carried further.

(ii) Flavones. Different intensities of yellow flavone may be present in a petal, all the petals of a given plant showing the same distribution. A common distribution is one in which the basal quarter, third, half or two-thirds of the petal is cream, primrose or yellow, passing to ivory, cream or primrose at the tip. Anthocyanin on such a ground appears mostly scarlet at the base and bluish toward, the tip (Plate XII, figs. 7 and 8).

(c) Reactions of anthocyanins and flavones with ammonia and SO2

As indicated on p. 128 the change of colour occurring when white, ivory or yellow petals are fumed with ammonia is as follows:

Petal colour Changes to
White No change
Ivory Lemon yellow
Yellow Intense orange

The ammonia test is a well-known method used for detecting the presence of anthocyanin in plant tissues. If anthocyanin is present a green or bluish colour usually develops upon fuming, though possibly other substances in the cell sap may modify this green or bluish appearance. In addition to the many fumings made with ammonia in the course of these experiments, petals have been bleached with SO2, and the reactions of different flower colours with these two reagents noted.

The observations may be briefly summarised as follows:

SO2.— SO2 does not affect the flavones. It half bleaches the deeper coloured petals and almost entirely bleaches tinged varieties. Penetration is increased if the petal is first fumed in ammonia and then bleached—complete removal of the anthocyanin pigments resulting, thus revealing the flavone ground.

Ammonia.— (a) Magenta and purple flowers give green and bluish-green reactions. (b) Orange and scarlet give an intense reddish-brown coloration. (c) Intermediate forms give intermediate reactions.

The intense reddish-brown which develops when orange or scarlet petals are fumed is of some interest, since this is not typical reaction of anthocyanin with ammonia.

All the other species I have grown conform, after their kind, to the above results.