This month it was all change in my vegetable patch as I harvested the last of the crops I planted in the spring and planted new things to grow over the winter. On a bit of whim I decided to plant some onion sets (mini bulbs) at one end of the patch, which should be ready to eat in early summer next year.

Onion is one of the oldest known cultivated plants and the earliest archaeological evidence of onions in human settlements dates back to around 5000 BC (Bronze age). It is grown all over the world where it features as a staple vegetable in a variety of dishes. It is not clear where they originated from, but there is some evidence that they may have come from southwestern Asia. Most cultivated onions are varieties of the common onion (Allium cepa L.) but some other onion species are cultivated too.

If you have ever cooked with onions you will know that when the bulbs are wounded they release a chemical that stings our eyes and can make us cry. This chemical – which has the catchy name syn-propanethial-S-oxide – doesn’t tend to put us off eating onions, but it does help them to defend themselves against herbivores and other pests. Charles Darwin hypothesized that tears triggered by cutting onions are the same as tears of sadness (2). However, he was later proved wrong because tears of sadness actually release extra “waste” proteins that aren’t found in onion tears.

Onions may also help to protect other plants from disease. Intercropping is a farming practice in which two or more crop species are grown in alternating rows. It has been used for a long time to increase crop productivity and to help control disease. Intercropping may help to protect plants against diseases by decreasing the number of attacks by the microbes that cause them, or by boosting the resistance of the host plant. Most studies so far have focused on investigating the first possibility, but little is known about whether plants release signals that can boost defense in their intercropping companion.

In Northeast China, a variety of A. cepa L called the potato onion is often the preferred companion plant to tomatoes. Tomatoes (but not onions) are susceptible to infection by a fungus called Verticillium dahlia, which causes a disease called tomato Verticillium wilt. A group of researchers recently investigated whether intercropping tomato with the potato onion is an effective way to control this disease (Fu et al. 2015).

Fu et al. found that when tomato and potato onion plants were grown together, molecules secreted from the tomato plants (but not the onion plants) inhibited the germination of fungus spores and also inhibited the growth of the fungus. This effect is due to the presence of the onion plants because molecules secreted from tomato and onion plants that were grown separately did not limit the growth of the fungus. Further experiments show that the onion plants trigger the expression of defense genes in the tomato plants.

These results indicate that intercropping tomatoes and potato onions may help to reduce the number and severity of outbreaks of Verticillium wilt. However, since these experiments were carried out under controlled conditions and the tomato plants were deliberately exposed to the fungus, large scale field trials would be needed to find out whether this effect is actually relevant in the field.

Onion is the (long-awaited!) Organism of October (apologies for the delay).

References:

Wikipedia: Onion (retrieved 21/10/15) Law, B (2010) Fifty plants that changed the course of history. David and Charles. Fu X, Wu X, Zhou X, Liu S, Shen Y and Wu F (2015) Companion cropping with potato onion enhances the disease resisitance of tomato against Verticillium dahlia. Frontiers in Plant Science 6:726 doi: 10.3389/fpls.2015.00726