The same plant

By Jeff Burbrink
Ag & Natural Resources Extension Education, Purdue Extension LaGrange County

LAGRANGE — Many of our most popular vegetable plants come from the mustard family, or Brassicaease, which contains 338 known genus and more than 3,700 species worldwide. One of the subgroups in that family is the genus Brassica.

While the genus Brassica is a large group by itself, I will focus on three particular species of this genus that provide much of our food:

B. oleracea (e.g., kale, collard greens, cabbage, Brussels sprouts, kohlrabi, broccoli, cauliflower),
B. napus (e.g. rutabaga, rape)
B. rapa (e.g., turnip, Chinese cabbage, bok choi)

All of these plants can potentially cross-pollinate within their species. For example, kale, kolhrabi and Brussel sprouts can cross pollinate with each other because they are all oleracea species. Turnips and Chinese cabbage can cross pollinate with each other, too.

However, kale cannot cross pollinate with rutabagas or turnips, because they are a different species. The fact some plants can cross pollinate does not affect the taste or the look of this year’s plant. If the seeds were saved, the offspring would be some odd combination of the two parents, and may or may not be very desirable.

The development of these plants is an incredible story. Just look at Brassica oleracae, a single plant which humans have been modifying through artificial selection for over 1,000 years. In the original form, it is a weedy little plant growing between rocks in the Mediterranean. By selecting and breeding the plant for bigger leaves, or larger buds, or better taste, at least seven different looking plants, all sharing the same scientific name, Brassica oleracae, are fed to people all over the world.

Kale and collard greens were the first to be domesticated, probably before 300 B.C. In the 1200’s, red and green cabbage was selected from kale plants for its large terminal bud. Brussel sprouts appeared at the same time, bred for their tiny cabbage-like buds that grow along the stem.

Kohlrabi, the odd-looking plant that looks amazingly like the little green men in the Toy Story movie, first appeared in the 1400’s, selectively bred from kale for its thick round stalk. Broccoli came along in the 1500’s, again from kale, and was selected for its larger flower stalk, which are harvested before the plant flowers. A hundred years later, cauliflower was developed from some broccoli varieties, focusing on the flower buds once again.

This fascinating story, spanning more than 2000 years, shows humans have been tinkering with the genetics of our food for a long time. While some people are very concerned about genetically modified organisms, I will contend these new lab techniques like CRISPR are just a way for us to do the same thing our ancestors did, but in a more precise, direct, and faster way.

In fact, if you really think about it, even plants that are labeled “non-GMO” really are GMOs, even so-called heirloom varieties. The popular tomato, Brandywine, is the product of years of selected breeding, not natural selection. No plant we cultivate in 2023 has avoided human tinkering. The term GMO is not really a very accurate label.

The other issue that can keep plants of the same species from crossing with each other is simply the timing of flowering and pollen drop. Broccoli and cauliflower may be able to cross pollenate, but if the flowers emerge at different times, the physical transfer of the pollen is not going to happen. Plant breeders need to take into account planting dates if they wish to force pollination between two similar but off-timing species.