Imagine walking around the Newport Beach Back Bay in sunny southern California in late December. The cool breeze from the ocean is pleasant, the bright sun is warm on your sunblocked face, and the rocky and sandy landscape is covered with small shrubs, although the soil is still visibly dry. You check back in after the “winter” rain passes through and the same rocky and sandy soil is now covered with little green rosettes that have quickly sprouted after the recent dose of precipitation. A few weeks later, you return, and those same plants have bolted and flowered. They’re growing like weeds, can be as tall as or taller than you, and produce hundreds of seeds per plant! The predominant species you’re seeing is Brassica rapa, commonly known as the field mustard. It will grow almost anywhere, under a large variety of conditions, and are cultivated for agriculture (cultivars include turnips and canola), but the ones you see are wild and invasive. It is unclear how B. rapa was introduced from the old world, but it can now be found in all of the United States.
Agricultural and scientific importance
An annual plant, B. rapa has famous relatives such as the plant “lab rat” Arabidopsis thaliana and Brassica oleracea which is the cultivated species that gives rise to broccoli, cauliflower, brussel sprouts, cabbage, bok choi, and kohlrabi among others. The large number of Brassica species is indicative of the great genetic diversity, and such genetic diversity enables organisms to adapt to novel environments and evolve in response to changing environments (both very handy for invaders!). For example, a previous study utilized the resurrection paradigm to demonstrate that in the face of changing precipitation patterns, populations of B. rapa were able to genetically respond in a few years and evolved a significantly different shorter flowering time in response to drought.
Identifying Brassica rapa
Brassica rapa is characterized by pinnate and hairy rosette leaves close to the ground, a stalk with clasping leaves, and cluster of flowers at the terminal branches. The individual flowers have four yellow petals and characteristic of the Brassica family, has four stamen and two staminodes. Fertilized flowers lose their leaves and the carpel elongates to form pods or siliques filled with seeds. Though B. rapa is cultivated for its thick taproot, wild varieties can be harvested for their rosette leaves or flower buds.
Eating the mustard family
The mustard family has evolved to produce a class of organic compounds called glucosinolates. It is a natural pesticide which makes insect herbivory low, but to humans who consume glucosinolates at low levels (relative to body size), it can be a desirable flavor! Check back soon to see how I harnessed the flavor of field mustard.