Milk-vetches, Liquorice and Locoweeds

Flowers of the weeping broom Carmichaelia stevensonii, from here.

Far in the distant past, I commented on a review of the New Zealand brooms of the genus Carmichaelia. The New Zealand brooms were placed by Wagstaff et al. (1999) in the tribe Galegeae, which includes more than 3000 species around the world. The vast majority of these are placed in the genus Astragalus, milk-vetches, which may itself have well over 2500 species and be the largest recognised genus of flowering plant. Other significant members of the Galegeae include the locoweeds (Oxytropis) and Glycyrrhiza, a small genus of less than twenty species that nevertheless deserves praise for being the source of liquorice*.

*I am reliably informed that there are people in this world who do not appreciate the flavour of liquorice. There is simply no accounting for taste.


Flowers of Astragalus monspessulanus, from here.

However, despite its wide recognition, phylogenetic studies of recent years have been unanimous in declaring the Galegeae polyphyletic. It is true that the vast majority of galegeans remain in a clade, with only a few relatively minor genera placed elsewhere (Wojciechowski et al. 2000). The problem is that one of those 'minor genera' happens to be Galega itself, the type genus and therefore sine qua non of the tribe, which is more closely related to the chickpeas of the genus Cicer. Wojciechowski et al. (2000) circumvented this issue by recognising a clade called Hologalegina, uniting the 'Galegeae' with other tribes such as Hedysareae, Trifolieae, Fabeae, Loteae and Robinieae. Within the Hologalegina, the taxa previously assigned to the Galegeae all belong to what is called the IRLC, the 'Inverted Repeat-Lacking Clade'. The name of this clade refers to the loss of a copy of the 25 kb inverted repeat in the chloroplast genome that is otherwise found in almost all other land plants. Members of the IRLC are largely herbaceous, and many of the most commercially significant legumes (such as peas, beans, lentils, clover and alfalfa) belong to this clade.


Liquorice, Glycyrrhiza glabra, photographed by J. C. Schou. The flavour comes from the roots.

With the exception of the aforementioned Glycyrrhiza (which is another phylogenetically distant genus from other galegeans), most of the 'Galegeae' are not commercially grown. They may be commercially eradicated: species of Oxytropis get their name of 'locoweed' from their toxic effects on grazing livestock, and various species of Astragalus and Swainsona are also known for their toxicity. Some species are cultivated as garden plants, such as the weeping broom Carmichaelia stevensonii shown in the top photo (though many places still list it under the older name of Chordospartium stevensonii) and the kaka beak Clianthus puniceus. One particular galegean is widely regarded as the stuff that gardeners' dreams are made of, despite being a notoriously difficult plant to grow. My partner tells me that when he lived in Kalgoorlie as a child, tourist buses would regularly appear outside his house just so that their passengers could see this plant flowering in the front yard. I speak, of course, of Swainsona formosa, Sturt's desert pea:


Photographed by Marj Kibby.

REFERENCES

Wagstaff, S. J., P. B. Heenan & M. J. Sanderson. 1999. Classification, origins, and patterns of diversification in New Zealand Carmichaelinae (Fabaceae). American Journal of Botany 86 (9): 1346-1356.

Wojciechowski, M. F., M. J. Sanderson, K. P. Steele & A. Liston. 2000. Molecular phylogeny of the “temperate herbaceous tribes” of papilionoid legumes: a supertree approach. In: Herendeen, P. S., & A. Bruneau (eds). Advances in Legume Systematics vol. 9, pp. 277–298. Royal Botanic Gardens, Kew.