Red and yellow Chasmanthe floribunda

The South African Chasmanthe floribunda (Salisb.)N.E.Br., is a common garden plant in southern Australia. It could be called a low-care relative of Gladiolus, in the family Iridaceae. The wild-type has an orange-red perianth with some yellow on the perianth tube, a purple inflorescence axis and purple anthers. But some of the plants in cultivation have a completely yellow perianth and yellow anthers, with a green inflorescence axis.

Wild-type

I have examined perianths of both flower types under a light microscope at 200X. The red anthocyanin pigment was dispersed through the cell sap in the red flowers, but could not be found in the yellow flowers; the yellow pigment (presumably a carotenoid) was concentrated in chromoplasts in both red and yellow flowers.

Yellow variant

No plants with intermediate flower colour were observed, although the species regenerates freely from self-sown seed (to the extent of becoming a weed of native vegetation). Because of this clear distinction, the yellow variant had been formally named as C. floribunda var. duckittii by Louisa Bolus (1933).

Apart from its lack of red pigment, the yellow variant is indistinguishable from the wild-type in its morphology. De Vos (1985) had suggested that the portion of the perianth tube below the insertion of the stamens (the hypanthium) was shorter in the yellow flowers but did not cite any measurements. However, flowers that I have measured in suburban Adelaide gardens and roadside populations in the Adelaide hills have hypanthium lengths of 9-11 mm, irrespective of their flower colour.

Experimental crosses
The red pigment is also absent from the plumule of the seedling and etiolated shoots emerging from the corms; they are cream in this variant but reddish-tinted in wild-type plants. This suggested the simple experiment of crossing red- and yellow-flowered plants and scoring their progeny at germination for pigment.

Cross pollination was carried out between a ‘red’ and a ‘yellow’ plant; reciprocal crosses were made to check for any apomixis or accidental self-pollination.

• Flowers of a ‘red’ were pollinated with pollen from a ‘yellow’
  Result: 30 ‘red’ seedlings, 32 ‘yellow’
• Flowers of a ‘yellow’ were pollinated with pollen from a ‘red’
  Result: 31 ‘red’ seedlings, 35 ‘yellow’

These results are in accord with a 1:1 ratio (Chi-square 0.281, yielding a probability of 0.595). Four randomly selected plants from each of the ‘red’ and ‘yellow’ seedlings were grown through to flowering; all four scored as red produced wild-type red flowers, all those scored as yellow produced pure yellow. No intermediates between red and yellow were produced in the experiment.

The simplest hypothesis is that a single gene is involved, with red fully dominant over yellow; the red plant used in the cross must therefore have been a heterozygote to give the observed 1:1 ratio in the F1. Selfing of the two plants used in the experiment was not succesful due to self-incompatiblility (which is very common in Iridaceae).

From this I concluded that yellow-flowered C. floribunda plants do not warrant formal taxonomic recognition. They are acyanic (i.e. anthocyanin lacking) variants, analogous to the white-flowered forms of many garden ornamentals that are no longer formally recognised except at the level of cultivars. This experiment was first published in December 1998 on my old website, which is archived here by the Wayback Machine, and the conclusion was also mentioned in the Horticultural Flora of South-Eastern Australia. My only rethink since then has been that there is a bit of variation in the intensity of the red flowers; possibly the more intense ones are homozygous for the wild-type allele and the paler ones with more yellow visible on the tube are the heterozygotes.

References
Bolus, L. (1933) Plants – new or noteworthy. South African Gardener 23: 46-47.
Cooke, D.A. (2005) Iridaceae. In Spencer, R.D., Horticultural Flora of South-Eastern Australia 5: 196-262.

de Vos, M.P. (1985) Revision of the South African genus Chasmanthe (Iridaceae). South African J. Botany 51: 253-261.

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