White-flowered Watsonia

Watsonias are grown as garden ornamentals for their spectacular display of flowers in spring. The wild species introduced from South Africa in the 19th century all had flowers in shades of pink, red, orange or purple. H.H. Arderne’s discovery of a pure white-flowered, acyanic, clone of Watsonia borbonica was big news in 1888, and soon it was a feature display at Kew Gardens. Its popularity and wide dissemination as a garden plant are documented by the numerous articles in gardening magazines of that time. Early 20th century breeders – notably John Cronin in Australia and Luther Burbank in the USA – used Arderne’s White with the species then available in their hybridisation and selection programs.

Arderne’s White

Acyanic Watsonia plants have white flowers with cream anthers. They also have completely green leaves, and stems, because they lack the anthocyanin pigments. On the other hand, wild-type plants have coloured flowers with purple anthers; their leaves may have brown or red pigmentation on the margins and bases.

Some results from my watsonia breeding program from 1997 to 2010 strongly suggested that Arderne’s White and five other acyanic cultivars derived from Watsonia borbonica are due to a single recessive allele, a, with the wild-type allele A allowing the formation of anthocyanins. The same allele was present in some coloured cultivars, as shown by the appearance of roughly 25% acyanic progeny when these were crossed. It also produced recessive acyanic progeny in experimental crosses with Watsonia aletroides and W. meriana. It seems most likely that the a allele was introduced into cultivation via Arderne’s White. But since it is a recessive it must have been present in the gene pool before a rare homozygous plant happened to appear and survive long enough for Mr Arderne to notice it.

All the acyanic Watsonia accessions that I examined have something else in common. Their production of anthocyanins could have been blocked at several different points in the chemical pathway, but in fact they all lack the enzyme chalcone synthase, which is needed to produce all the flavonoid pigments, the yellow chalcones as well as the anthocyanins. This fits the explanation that they all have a mutation at the same A locus that inhibits the production of this particular enzyme. To complicate matters, some temporarily show a pink tinge when the white flowers open in cold weather. The possibility that these have a complementary mutation at a different locus was ruled out by a test cross, which produced only acyanic progeny. If two loci had been involved, only cyanic progeny would have been expected from this cross.

A classic, but all too brief, paper by Horn (1963) mentioned the interesting observation of a 3:5 segregation ratio when he crossed Arderne’s White with three different Watsonia hybrids that had coloured flowers, implying that the acyanic gene was dominant. The lack of further detail makes it difficult to compare his experiment with mine. It may be that one or two of the three hybrids were Aa heterozygotes, giving some acyanic progeny when crossed with the homozygous aa Arderne’s White.

the a allele introduced into another Watsonia line with quite different morphology

Many Watsonia cultivars have been characterised in horticultural literature and retail catalogues simply as ‘white-flowered’. However, a named cultivar of an ornamental perennial is normally a vegetatively propagated clone that has constant horticultural properties due to its constant genotype. Descriptions need to be based on several characters if we want to define unambiguously cultivars with unique genotypes. Descriptions would ideally be supported by a type specimen and molecular data as well. A decade ago there was a lot of talk about cultonomy – the taxonomy of cultivated plants – as a separate subject, and there was still an International Code of Nomenclature for Cultivated Plants separate from the more general International Code of Nomenclature used for plants, fungi and algae. We seem to have gone backwards since then, and I fear that the Procrustean attempt to fit plants created by humans into a system of nomenclature intended for wild taxa will cause a gradual loss of knowledge of cultivars.

Ornamental flower cultivars are selected for the shape and colours of their flowers, and for growth traits that make them ‘gardenable’, i.e. convenient to use in gardens. But other morphological characters that have not been directly selected may be useful indicators to distinguish related cultivars. Among Watsonia cultivars, these characters include the relative length of the style, shape of floral bracts, orientation of stamens and shape of the seed capsule. For example, two genotypes of white-flowered Watsonia that John Cronin produced in the 1920s had become lumped together in the old nursery that preserved them: both are tall slender plants with white funnel-shaped flowers. But one has a style equal equal in length to its stamens, while the other has a much longer style that protrudes from the flower. When the two are grown side by side for a few years, other small differences in flowering time, mean height and maximum leaf width can be noticed.

References

Cooke, D.A. (2010) Genetics of white-flowered cultivars derived from Watsonia borbonica (Iridaceae). J.Adelaide Bot. Gard. 24: 33-38. Download the PDF here

Horn, W. (1963) Flower colour inheritance in some Iridaceae. Naturwissenschaften 50(15): 527-528.

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