Casting Nasturtiums

Nasturtiums are a source of a little confusion in the botanical and horticultural worlds.

The trouble is, there's more than one plant that goes under this name.  The name nasturtium is derived from the Latin for "twisted nose", a reference to the strong smells and tastes of these plants.  These characteristic smells and tastes come from glucosinolates, or mustard oils.  Glucosinolates are a family of compounds that contain sulphur and nitrogen; their main role is the deterrence of grazers.  However, some insects (cabbage aphid, cabbage white caterpillar) are able to either break down or sequester the glucosinolates and have the cabbage resource all to themselves.

Watercress growing in a stream in Wellington

First, a plant that has Nasturtium as its scientific name.  This is the watercress, a member of the mustard family, Brassicaceae.  For a long time there was thought to be just one species, Nasturtium officinale.  The advent of chromosome counting turned up a surprise: some plants had four sets of chromosomes instead of the usual two, and they also had small but consistent differences in flower size, seed ornamentation, and arrangement of seeds in the pods.  A new species of watercress, N. microphyllum, was named.  Hybrids between them are mostly sterile.

Later, it was decided that Nasturtium as a genus wasn't very different from Rorippa, and the two genera were merged under the latter name.  Our two watercresses are now called Rorippa nasturtium-aquaticum and R. microphylla.  The glucosinolates of watercress are quite mild and make them a nice addition to salads and sandwiches; some of the native New Zealand bitter-cresses (Cardamine) have similar flavours.

Garden nasturtium, Tropaeolum majus.

What about the garden nasturtium?  This one has nasturtium as its common name, hence the confusion. Its scientific name is Tropaeolum majus, and it's in a different family (Tropaeolaceae) from watercress.  Its leaves are a great addition to salads, and their flavour is a bit like capers (Capparis, family Capparaceae). In fact, glucosinolate producing plants are found in about 8 or 10 families.  These families look so different from each other that it was assumed glucosinolates must have evolved independently many times in unrelated groups.  For instance glucosinolates are found in the Cleome family (Cleomaceae), the mignonette family (Resedaceae), and the papaya family (Caricaceae).  It seems morphology was telling us these families weren't related, while chemistry was telling us they were.

Spider flower, Cleome hassleriana.

How can botanists sort out a conflict like this?  Doesn't it threaten our notions of evolution?  Well, no.

One approach, but not a rigorous one, is simply to privilege (to borrow a verbed noun used in the humanities) one set of data over another.  That's how we mostly dealt with the glucosinolate problem; we assumed that morphology trumped chemistry, and that glucosinolates had arisen many times independently in mostly unrelated families.  Lazy, and wrong, as it turns out.

Mignonette, Reseda lutea.

First, when data seem to conflict like this, we need to look at the differences and similarities and see if there are developmental differences among some of the similarities, or underlying structural similarity in some of the differences.

Another approach is to find a third and independent set of data.  That came along in the 1990s with the huge DNA sequencing effort in plant taxonomy.  And here was a genuine surprise (Rodman et al. 1993): nearly all the glucosinolate families are closely related, because the DNA data agree with the chemical data.   We need to re-assess the morphological differences.

It seems glucosinolates have arisen just twice: once in a large group of families (now grouped together as Order Brassicales, e.g., Brassicaceae, Capparaceae, Tropeolaceae, Caricaceae, Resedaceae) and another time in the unrelated small genus Drypetes (Euphorbiaceae).  Future study will focus on the huge range of different growth forms, flowers, fruits, and seeds in the Brassicales.  It's going to be interesting as the genes that control this development are discovered and their function becomes understood.

Reference
Rodman, JE; Price, RA; Karol, K; Conti, E; Sytsma, KJ; Palmer, JD 1993:  Nucleotide sequences of the rbcL gene indicate monophyly of mustard oil plants.  Annals of the Missouri Botanical Garden 80: 686–699.

Sweet alyssum, Lobularia maritima (Brassicaceae)