There was a time when superconducting quantum interference devices (SQUIDS) were the bees knees when it came to measuring magnetic fields. Although SQUIDS require a dizzying array of paraphernalia to keep them operating at supercool temperatures, nothing could better their sensitivity or performance. “
All that has changed with the dramatic improvements that have been made in recent years with spin-exchange relaxation-free (SERF) magnetometers. These work by sending a beam of circularly polarised light through a small cloud of rubidium atoms. Any small magnetic field tends to align the electrons in the atoms causing them to absorb polarised light passing through the cloud. So the amount of transmitted light is a measure of the strength of the field.
SERF’s are at least as sensitive as SQUIDS, capable of detecting fields in the nanoG regime (the Earth’s magnetic field on the surface is about 500 milliG). They are small, just a few cubic millimetres in size, and best of all, can operate at room temperature without complicated cryogenic stuff.
Consequently, SERF’s are opening up entirely new types of magnetic field measurements.
Corsini and buddies have used their SERFs to attempt to measure the magnetic field generated by “Trudy”, a rare-flowering plant currently residing at the University of California Botanical Garden in Berkeley.
Trudy is an amorphophallus, a genus which takes its name from the Greek for “misshapen penis”. She flowers only once every six years or so in an event that has become as famous as it is rare. When Trudy blooms she produces a putrid stench reminiscent of decaying corpses that attracts insects and tourists alike for miles around.
During this process, her spadix (the misshapen penis part of the flower) heats up by as much as 30 degrees C in cycles of 30 minutes or so.
Corsini and co reasoned that if the temperature rise is the result of ionic currents in the plant then they must also be associated with a biomagnetic field of about 10 microG. And the speed of the process might just make the measurements possible over a reasonable period of time.
So he and the others set up shop in the University of California Botanical Garden armed with a pair of SERFs capable of making such sensitive measurements.
It turns out, of course, that these measurements are trickier than they sound. Trudy’s greenhouse has heaters which regularly switch on and off to maintain a tropical temperature, it is filled with numerous botanists and tourists who are all curious to see and smell Trudy in her finest hour and it is uncomfortably close to the BART commuter train line into San Francisco.
Corsini and co take up the tale: “We visually observed the anthesis (beginning of the blooming phase) at approximately 9 PM on the night of June 22.”
At that point they were already recording but what did they see? “The BART-free time periods (1 - 5 AM) are clearly visible as relatively magnetically quiet periods on each of the two magnetometer channels,” say they say, adding that “large magnetic field fluctuations are also visible during the Garden open hours (9 AM - 5 PM)” when visitors are milling around.
They also spot some strange jumps in the measurements. “Discontinuities in the data were caused by inadvertent moving of the pot and/or the sensors,” presumably due to visitors stumbling into the plant and the surrounding equipment.
But what of the biomagnetic field? Corsini and co say their measurements place an upper bound of 0.6 microG on the amplitude of biomagnetism generated by the plant. In other words, the biomagnetic fields were too small to measure.
That’s a shame but what went wrong? It’s likely that Corsini and co over-estimated the size of the field that ionic currents in the plant could generate. They assume that the heat is generated electromagnetically, by the flow of ionic current through a resistor.
That may not be the case. Trudy’s hot flushes could also be caused by some kind of exothermic chemical process. If that’s the case, the fields may be too small even for a SERF to pick up.
But more sensitive measurements should be possible. Corsini and friends now plan to carry out their measurements in more isolated environments using more manageable plants. A venus fly trap, for example, ought to fit neatly into a standard physics lab and can be triggered at will.
Trudy was certainly worth a try, given her size and metabolic rate, but it looks as if we’ll have to wait a little longer for the first unambiguous detection of a biomagnetic field generated by a plant.
At least next time, Corsini and co will be able to carry out their work without the aroma of decaying corpses in the air.
Ref: arxiv.org/abs/1006.3578: Search For Plant Biomagnetism With A Sensitive Atomic Magnetometer