Tuesday, January 28, 2014
The Physics arXiv Blog
The study of the electrical properties of plants is sleepy backwater of botany that has never generated widespread interest or finding. For example, in 1995 one group carefully evaluated the physiological state of a cucumber by measuring it electrical impedance. Others have done the same for olive trees. That's surely interesting work but hardly likely to set the world on fire.
Today, Andrew Adamatzky at the University of the West of England in Bristol argues that the electrical properties of plants have been overlooked. And he takes one step to putting this right by measuring the electrical properties of lettuce seedlings, just three or four days after they've sprouted.
Adamatzky's interest is more than mere curiosity. His idea is that plants can be thought of as organic wires that could become the infrastructure behind an entirely new generation of biological circuits, sensors and even information processors that grow for scratch with little more than a sprinkle of water and a little soil.
Before any of that can happen, however, the electrical properties of plant wires must be fully characterized. Hence this work.
Adamatzky's work is a set of experiments to measure these wires' resistance, electrical potential transfer function and any other relevant characteristics.
These experiments are extremely simple. They consist of placing a lettuce seedling across a 10 mm gap in a circuit, passing a one microAmp current through it and simply measuring its resistance and electrical potential over a period of 10 minutes. Adamatzky repeated this for 25 different seedlings.
The results are straightforward. The average resistance of the seedlings was 2.76 megaOhms. "This is much higher than resistance of conventional conductors yet relatively low comparing to resistance of other living creatures," says Adamatzky. For example, the resistance of the slime mold, which grows protoplasmic tubes that also act as wires of about the same length and lettuce seedlings, is some three megaOhms.
Interestingly, the resistance and other electrical properties of lettuce seedlings oscillate at frequencies that have no clear periodicity although there are dominating frequencies. "The three most dominating frequencies observed are 0.043 Hz, 0.084 Hz, and 0.009 Hz," says Adamatzky.
Exactly what causes this oscillation isn't clear but Adamatzky thinks they are probably the result of changes in the cytoplasmic flow in the root, stem and leaves which change the diameter and volume of the tubes, and hence its electrical properties.
The next challenge for Adamatzky is to incorporate these kinds of organic wires into self-growing circuits that connect biosystems with silicon devices.
One hurdle is to find ways of controlling the direction of growth of the seedlings to create circuits. Adamatazky's early attempts at this have not yet been successful.
Another challenge will be to find useful applications for plant wires. One idea is to use them as temperature sensors since their resistance is thought to be sensitive to heat and cold. That could allow horticulturists to better monitor growing conditions. Other ideas in the comments section please.
But it is in the field of unconventional computing that Adamatzky's real interests lie. Over the last few years, he has studied the extraordinary information processing power of the single-celled creature, slime mold.
The result is numerous papers describing their computational feats. For example, slime mold can find its way out of maze with little difficulty.
But they are temperamental creatures that are hard to grow and maintain. Adamatzky's main motivation in studying lettuce seedlings is to find hardier organisms that will be easier to exploit.
Just how far he can go with lettuce isn't clear. But the possibility of being able to grow circuits that extract information from the environment and process it in useful ways, provides plenty of reason to experiment further.
Ref: arxiv.org/abs/1401.4396: Towards Plant Wires