Review of the Buddleja x weyeriana hybrids and future prospects for the introgression of yellow flower colour into Buddleja davidii

Updated March 2014.

Introduction

Horticulturalists have always relished the prospect of creating new plants by the hybridisation of geographically separated species. One of the earliest Buddleja hybrids was what became known as B. x weyeriana. During WW1 van de Weyer of Smedmore House, at Corfe Castle, pollinated the South American species B.globosa with pollen from B.davidii var.magnifica, a native of China (van de Weyer 1920). His F1 plants were mostly greyish-white, intermediate between the two species and somewhat of a disappointment but the F2 generation (assumed to be a cross of two F1 plants) was much more successful. Two of these, B.x weyeriana 'Golden Glow' and 'Moonlight', are still popular today and most other B.x weyeriana cultivars are probably sports from these original plants, as is the case with B. x weyeriana 'Sungold' (Fig.1) (De Vogel 1967).

Although the current B.x weyeriana yellow hybrids are attractive and popular garden shrubs all have ball-shaped heads in an interrupted panicle. The race is on to produce a new hybrid that more closely resembles B. davidii with long uninterupted panicle inflorescences and a clear yellow colour. Such a plant would have a high commercial value in the horticultural trade.



Fig. 1: B.x weyeriana 'Sunglold' shows the typical inflorescence of this hybrid.

Buddleja globosa and Buddleja davidii

Fig 2a: A 1906 illustration of B. variabilis var. Magnifica (B. davidii), the cultivar or variety used by Weyer in his hybridisation.

What about the parent species? Both are hardy in the UK and very popular in gardens. B. davidii is hermaphrodite, tetraploid (chromosome number of 76) and from China. The plant used by van de Weyer is an early cultivar (or natural variety) he describes as B. variabilis var. Magnifica, a violet-coloured B. davidii. Normally it would flower from July in the UK and the inflorescences form long conical panicles with several hundred individual perfect flowers (Fig.2a). Modern cultivars often have larger flower spikes but this variety would have been the best available to van de Weyer, as his hybridisations pre-date virtually all the known cultivars.



Fig 2b: A modern B. davidii cultivar with a very long panicle.

B.globosa is dioecious, diploid (chromosome number of 38), and a native of the Chilean Andes. It is a common belief that it is hermaphrodite like B. davdii but in common with most New World Buddlejas individuals are either male or female. The exception is the Stachyoides series, e.g. B.tubiflora, which is hermaphrodite and there are a also few trioecious species (a population which can be male, female and hermaphrodite). The confusion arises with B. globosa because the flowers appear to have both anthers and a pistil but only one or the other is functional, either the anthers or ovary are empty. This is called cryptic dioecy or occasionally micro-dioecy. It is a difficult to correctly identify the genders and often requires dissection, staining and microscopic examination of the flowers. For the amateur horticulturalist simple observation may suffice to identify the gender of a B. globosa plant providing there is a suitable pollinator in the vicinity: the females will produce seed pods and the males never will. Dioecy has evolved as a means to facilitate out-crossing and is a relatively simple mechanism compared to the evolution of self-incompatibility in monoecious and hermaphrodite species (Norman 2000).

Fig. 3: B. globosa inflorescence and, inset, seed pod.

B. globosa normally flowers in May/June and rarely overlaps the flowering period of B. davidii so van de Weyer was fortunate indeed to manage this crossing. Fortunate too, in that he happened to own a female plant. The inflorescences are one terminal and several pairs of pedunculate globose heads, each about 1.0 - 3.0 cm in diameter and with 30 - 50 flowers. Although the gross structure of the inflorescence is very different the individual flowers are quite similar, except the corolla tube is much reduced in B. globosa.

Flower Colour in Buddleja

The purple/lilac flower pigments in the corolla of Asian Buddlejas are presumably anthocyanins as in other members of Scropulariaceae. Anthocyanins are just one type of flavonoid, other flavonoids include flavones and aurones. All are derived from the same precursors called chalcones (fig 4A), which are themselves yellow if they accumulate in the flower's cells. Flavones can be yellow or colourless (but reflecting near-UV wavelengths so visible to insects) and are often co-pigments. Floral flavones rarely give a bright yellow colour but are more likely to appear cream or pale yellow. Aurones are a less common class of flavonoid restricted to a few plant families including Antirrhinum (formerly Scrophulariaceae, now assigned to Plantaginaceae) and are responsible for a bright yellow colour (Asen et al, 1972). In opened flowers the level of the flavones can be five to ten times higher than that of the anthocyanins or aurones (Ono et al 2006).

However, it is common for yellows and oranges to be the result of chemically different pigments: carotenoids. The yellow pigment in the centre (throat) of Asian Buddleja flowers is non-cyclic crocetin-digentobiose ester (a carotenoid) (Tallent-Halsell and Watt, 2009), much the same as the pigment as in saffron. Flavones are also known to be present in the flowers of Asian Buddleja (Matsuda et al, 1995).

The orange pigmentation of B.globosa and other New World Buddleja flowers has not been determined but it could be both a carotenoid-type compound (e.g.: beta-carotene; crocetin) and/or a flavonoid (an aurone). Flavones and carotenoids are virtually unbiquitous in higher plants and certainly present in other tissues of B. globosa (Backhouse et al, 2007). It is predictable that B.x weyeriana 'Golden Glow' has a yellow corolla but with a lilac flush, the former a feature of B. globosa, the latter derived from B. davidii. In the yellow B. x weyeriana hybrids there is a division between the golden yellow of the corolla and the orange of the throat (see fig.1), a similar division seen in most B. davidii cultivars. This makes it more likely that the pigment of the corolla and the throat are chemically different and could suggest that the yellow of the corolla is an aurone.

Fig. 4A: Simplified biosynthetic pathway for flavonoid pigments: Flavones, anthocyanins and aurones share common precursors (chalcones, in this example tetrahydroxy-chalcone (THC)). Arrows can represent multiple enzyme steps and the simplest example (i.e.: the fewest hydroxyl substitutions) for each category is shown. (Adapted from Ono et al 2006)

Fig. 4B: A. Anthocyanin (general struture) showing the positions of the side groups (R); B. Apigenin, a flavone found in Buddleja;
C. Beta-carotene; and D. Crocetin digentobiose ester.

This lilac colour can be suppressed in B. davidii to produce white-flowered plants. Rather than there being an absence of or fault in the genes that produce anthocyanins, their production is blocked by a gene called Alb2. This is the case in at least one white B.davidii cultivar, Nanho Alba (Tobutt 1993) and probably the same applies to other white or very pale cultivars. I have seen a near white cultivar (Les Kneale) suddenly produce a dark lilac flower spike where the anthocyanin suppression has failed locally (Fig. 5A). Sungold and Honeycomb also completely suppress anthocyanin production but this can be restored (by loss of effective suppression) in rare instances and the lilac colour then completely masks the yellow in the corolla (Fig. 5B). This is strong evidence for the suppression hypothesis of white flower colour.

Fig.5: Les Kneale (A) and Sungold (B) demonstratng the loss of control over anthocyanin production.

Van de Weyer's Original Plants


Fig. 6: Golden Glow was described in Weyer's original article (1920)

The original report of van de Weyer (1920) makes fascinating reading. His B. globosa x B. davidii F1 generation were virtually all greyish white tinted with lilac but for one yellowish-white near-sterile hybrid, and had 'ball-shaped' flowers. He selected one grey-white plant crossed presumably to a sibling or self-pollinated, the latter assuming the self-incompatibility of B. davidii wasn't inherited (Chen et al, 2011), to collect seeds from to form his F2. The F2 population was a mixed bag: grey, violet flowers and yellowish flowers in ball-like clusters borne on spikes, as well as a larger number of plants with violet panicles much like B. davidii var. Magnifica. 'Golden Glow'(Fig. 6) was his chosen best plant to exhibit from the former group. He also sowed a F3 generation but there are no details about these, but it is intriguing to speculate that 'Moonlight' (Fig. 7), first exhibited in 1923 (Anon., 1923), may in fact be from this F3 population although no details are given. Other than these two named cultivars his plants are now lost.

Fig. 7: Moonlight, another one of Weyer's original plants.

Genetic make-up of the Hybrids

It could be assumed that B.x weyeriana would be triploid as the progeny of diploid and tetraploid parent species, however it has been known for some time that it is tetraploid like B. davidii with 76 chromosomes (Moore 1960). It also has a somewhat larger genome that either parent as determined by the DNA content of the cell, being some 26% greater than B.davidii (Van Laere et al, 2009). Genomic in situ hybridisation (GISH) analysis of the chromosomes from B.x weyeriana Sungold revealed that of the 76 chromosomes 36 were from B. davidii, 28 from B. globosa and 12 recombinant chromosomes (a mix of B. globosa and B. davidii) (van Laere et al, 2010). This nearly equal contribution from each parent suggests that van de Weyer's original plants were the result of so-called unreduced gametes in his B.globosa flowers and the F1 was also tetraploid. Unreduced gametes (B. globosa ovules in this case) have not undergone meiotic reduction and still have the parental complement of chromosomes (2n = 38). These 2n gametes are then more inclined to successful fertilisation by the normal 1n pollen of B.davidii with an equal number of 38 chromosomes, as a mismatch can be a barrier to successful seed formation and germination (van Laere 2008).

Unreduced gametes are well documented in the diploid Asian Buddleja B. lindleyana. These have been demonstrated as pivotal in the breeding of a B. lindleyana x B. davidii hybrid (Elliot et al, 2004) and reciprocal B. davidii x B. lindleyana hybrids (van Laere et al 2008), the latter giving rise to the Buddleja 'Argus' cultivars. However this is not the case with all Buddleja hybrids between species of differing ploidys. A cross of B. davidii and B. asiatica (a tender Asian diploid species) resulted in a triploid, fully sterile hybrid released commercially as Buddleja 'Asian Moon' (Renfro et al 2007).

Recent Efforts to Breed a Yellow B. davidii Hybrid

In order to cross B. globosa and B. davidii successfully it seems likely that one must rely on unreduced gametes being produced in B. globosa, which isn't necessarily a common occurrence. So two separate projects have come to the same conclusion that it would be a good first step to artificially produce a tetraploid B. globosa. In the work of Rose et al (2000) a cultivar CSS18 was chosen for manipulation as it reliably set seed i.e.: it was female. Colchicine (a mitotic inhibitor) was used to induce polyploidy in nodal section culture and plants micro-propagated. When mature the tetraploid plants obtained had smaller leaves, smaller flowers and shorter internodes than the diploid. Although several crosses were made and later generations were also grown at the East Malling Research Station, Kent (K. Tobutt, personal communication) no further reports have been published so we don't know how successful the progeny were.

In the work of van Leare (2011) several methods were employed to induce tetraploidy, both in seedlings and shoots, using oryzalin and trifluralin (more potent mitotic inhibitors). Many tetraploid plants were successfully propagated and one was chosen (GLO34) to pollinate B.davidii 'Nanho Alba'. GL034 must have been male as the reciprocal-cross failed and in any case the induction of tetraploidy is unlikely to also induce hermaphrodism. It should be noted here that neither report mentions that B. globosa is a dioecious species. The F1 progeny of the cross (Fig. 8) were rather different to van de Weyer's F1 plants, possibly as a result of the different B. davidii cultivar used in the cross. Of seven plants five were yellow and two purple but with evidence of yellow pigmentation. The numbers are too few to work out the genetics but certainly support the assertion that Nanho Alba is dominant heterozygous at one locus for anthocyanin suppression (Tobutt 1993). The flowers are less globose that B.x weyeriana 'Sungold', and in interrupted panicles intermediate between the species (Fig. 8). Breeding is ongoing (van Laere, personal communication) but the results have yet to be reported.


Fig. 8: F1 plants B. davidii x B. globosa (reproduced from Van Laere et al, 2011).

These are not the only reports of artificial chromosome doubling in Buddleja. B. madgascarensis is an African species (section: Nicodemia) with yellow panicles but is not at all cold hardy (Leeuwenberg 1979). It also has potential to be used for the introgression of yellow colour into B. davidii, selecting hybrids with good cold hardiness. A cross of B. madgascarensis x B. crispa, both diploid species, has been made and gave rise to a diploid, sterile hybrid. Sterility precludes further breeding and diploidy limits its application in crosses with B. davidii. Oryzalin was used to induce tetraploidy in the hybrid and fertility was reinstated; these tetraploid plants were successfully crossed with B.davidii 'Nanho Alba' (Dunn and Lindstrom 2007). None of the surviving seedlings of this cross are yellow but they fade to orange/yellow as the flower ages. They are not sterile and the plants are also large which limits their appeal. They are extremely drought tolerant and winter-hardiness has not been an issue (Dunn and Lindstrom, personal communication).

B.x weyeriana X B. davidii Hybrids

Van de Weyer (1920) reported that Golden Glow was not very fertile but all the B.x weyeriana cultivars do in fact produce seed sporadically. Pollen sterility is suspected for many of the cultivars. They are readily pollinated by B. davidii. A number of B.x weyeriana x B. davidii hybrids are becoming popular garden varieties: Pink Pagoda and Blue Boy from Peter Moore (Longstock Nursery); Bicolor from Mike Dirr (University of Georgia); Flutterby Grande Peach Cobbler and Vanilla from Peter Podaras (Cornell University). Each one gains its colour from the pollen (B. davidii) parent, but with a difference that seems due to a small residual persistence of yellow pigment in the corolla. Bicolor's purple colour changes to orange-bronze as the flower fades, due to the partial breakdown of anthocyanins allowing the yellow-orange pigment to show through. Flutterby Vanilla is off-white retaining almost nothing of the yellow outside of the coralla tube.

Van Laere (2008) carried out reciprocal crosses of B. davidii 'Nanho Alba' with B.x weyeriana 'Sungold', managing to extract a small amount of poorly viable Sungold pollen. The seedlings were a mix of pale purple and white, as would be expected from what is known about the genetics of white flower colour (Tobutt, 1993). Sadly, no residual yellow pigment seems to be present in the corolla in the white seedlings from this cross. I have grown a number of B.x weyeriana x B. davidii seedlings out of Honeycomb open-pollinated in a densely Buddleja-planted garden - they are mixed colours as a result and very variable in many ways. A couple of plants with pale blue flowers show a phenomenon similar to Bicolor in that the blue in the flowers would fade leaving a primrose yellow background (Fig.9), a phenomenon noted by van de Weyer in his F1 plants (Weyer, 1920). The pigments responsibles for the yellow colour of the B.x weyeriana corolla are unknown but it is probably necessary to have the gene(s) that segregates the yellow into the corolla in a B. davidii type plant such as in the seedling below (Fig. 9). The potential is there to retain yellow in the corolla and not just the corolla tube, whilst breeding out the lilac/purple anthocyanins (or perhaps more correctly breeding in their suppression) leading to yellow flowers with the inflorescence morphology of B. davidii.




Fig. 9: B.x weyeriana Honeycomb x B. davidii seedling showing the residual yellow colour in the outer corolla.


The fertility status of the B.x weyeriana x B. davidii hybrids is intriguing. The two Flutterbys mentioned above are fully pollen and seed sterile, whereas Bicolor produces copious viable pollen but never sets seed (Peter Podaras, personal communication). Blue Boy and Pink Pagoda fail to set seed but whether they produce pollen is unknown. Out of some twenty plants of my own that have reached maturity only one is able to set poorly viable seed, and a couple of others appear to produce seed but these fail to germinate. Most fail to produce seed capsules at all. This raises the possibility that the sex determinants of B. globosa are reasserting themselves in this generation, causing sterility in some and gender-specification in others. This presents a challenge for breeding further generations in that at least one parent must be capable of setting seed (either by being female or hermaphrodite) and the most promising plants could well not have this attribute.

Conclusions

Sophisticated and traditional plant breeding efforts are underway to produce the first yellow flowered ostensible B. davidii although such a plant hasn't been released to commerce yet. The tendency to sterility in hybrids may prove a real hindrance in many cases. The full range of orange flowered New World Buddlejas has yet to be utilised in hybridisations with Asiatic plants, as most of them are not common or even known to be in cultivation. Some of these may prove excellent partners for hybridisation with B. davidii: the series Stachyoides are hermaphrodite and there are tetraploid species in other series. Several are high altitude species that may imbue cold hardiness (Norman 2000).

The alternative path to success may be traditional plant breeding with the B.x weyeriana x B. davidii hybrids but the fertility status of many of these plants is a disadvantage. The potential does appear to be there to breed at the very least a primrose yellow B. davidii-type plant. Whichever method is ultimately successful we eagerly await such a cultivar.

Acknowledgments

My thanks for Katrijn van Laere (Instituut voor Landbouw- en Visserijonderzoek / Institute for Agricultural and Fisheries Research, Belgium) for checking the text for accuracy and to Ann Croft for the photo of the Sungold reversion.

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