For the first time a plant's magnetic field has been measured. Although the plant in question, the carnivorous Venus flytrap (Dionaea muscipula), is a candidate for the vegetable kingdom's most unusual member, the discovery could lead to better ways to probe the health of more common flora.
In low-nitrogen soils insects and spiders offer such a rich and tempting source of nutrients plants have independently evolved carnivory many times, although sadly a quarter of these are now endangered. However, the rapid leaf closures the Venus flytrap uses to secure its prey are exceptionally rare – most carnivorous plants prefer more passive methods such as hairs that point inwards or slippery slopes.
When movement triggers a flytrap's hairs an electrical signal is transmitted within the plant, stimulating the leaves to close within a second, an astonishingly fast movement by plant standards. The electrical signals animal nerves use to transmit messages produce magnetic fields that can be detected to trace what is occurring, and Johannes Gutenberg University Mainz PhD candidate Anne Fabricant thought the same should apply with Venus flytraps.
"You could say the investigation is a little like performing an MRI scan in humans," Fabricant said in a statement. "The problem is that the magnetic signals in plants are very weak, which explains why it was extremely difficult to measure them with the help of older technologies."
Fabricant is part of a team that used atomic magnetometers to address this. In Scientific Reports they have announced the detection of temporary magnetic fields produced when the flytrap's cells transmit a shut message.
"We have been able to demonstrate that action potentials in a multicellular plant system produce measurable magnetic fields, something that had never been confirmed before," Fabricant said. The sensors use an atomic vapor in a glass cell to detect highly localized magnetic fields with a strength of just 0.5 picoTesla without the supercooling required of MRI machines. "The signal magnitude recorded is similar to what is observed during surface measurements of nerve impulses in animals," she added. For comparison, a fridge magnet is around 20 billion times stronger.
Along with the brushing of hairs that alerts the plant to the presence of prey, Venus flytraps have been found to respond to heat and salty water. The team chose to stimulate their plants by raising the thermometer 4ºC/s (7.2ºF/s) from room temperature, judging warmth less likely to produce background noise than mechanical stimulation, which the ultra-sensitive monitors would confuse with magnetic fields from the plant. The possibility of a field-inducing electrical potential rose with the temperature, passing 50 percent at 33.8ºC (92.8ºF) and reaching 100 percent at 40ºC (104ºF).
Venus flytraps are such popular houseplants that poaching is one of the biggest threats in the small area to which they are native. Nevertheless, they and the waterwheel plant (the only other snap-trap user) are of minor global significance.
Fabricant and colleagues think other plants also produce magnetic fields, but these are likely to be even weaker, and therefore more challenging to detect. However, they believe such fields may indicate disease or other stressors and hope portable detection devices could someday allow farmers to non-invasively track their crops' health.