Cress Kung-Fu

It might look like the weediest part of salad, but like a cunning martial artist, cress uses its opponent’s strength to defeat it.

When you plant some plants next to cress, something odd happens – they grow well. Too well in fact, so they’re far too tall for their roots, and are weakened. Scientists have suspected that cress is releasing something into the soil to mess with its neighbors, but haven’t known what until now. Stephen C Fry, in his Open Access paper Potassium, not lepidimoide, is the principal “allelochemical” of cress-seed exudate that promotes amaranth hypocotyl elongation, shows that cress seeds release potassium, a well-known nutrient, into the soil, but that alone can’t explain everything that’s going on.

In a recently published study, Fry examines low-molecular weight cress-seed exudate. This is something cress adds to the soil when it germinates. The goal of the study was to identify what exactly this mysterious exudate is. To do this, Fry used high-voltage electrophoresis, a way of pulling apart the component chemicals under an electrical charge, effectively laying out all the ingredients of the chemical mixture.

One surprise was that cress was releasing potassium. This is unusual, as potassium is a valuable nutrient for a plant, at least it is normally. Fry tested confirmed this by seeing what happens to amaranth seeds when they are exposed to potassium. He found the hypercotyl – the first shoot out of the seed – grows much better than usual. That sounds like good news, but cress is sneaky. At the same time as it’s helping the shoots of its neighbors to grow, it’s working on the roots too.

Whatever the cress is exuding, it reduces the root growth of its neighbors. Tests show that it’s not the potassium working on the roots, so the cress must be exuding something else. It’s this combination of actions that makes cress a formidable opponent. Its mystery mix of chemical exudate makes amaranth grow tall, but with poor stability due to its stunted roots. When there’s some kind of stress, like a strong wind, the Amaranth would topple, its growth exceeding its ability to stand.

It’s not entirely clear if leaching the potassium is selected for by evolution, or if it’s something cress can’t help doing while it’s a seed. It may be that potassium loss is undesirable and unavoidable, and that cress exudes more chemicals to take advantage of the lost potassium – meaning there’s less competition around when it’s grown, and able to take it back up.

While cress isn’t the biggest plant in your salad, it’s still a fighter, using other plants’ greed for nutrients against them.