ACID rain damages crops. In particular, it damages rice, because many rice-growing countries, which are predominantly in Asia, do not have in place the pollution-control mechanisms that are now routine in the wheat-growing continents of Europe and North America. A rice crop soaked by acid rain can be saved if it is rinsed with clean water. But it is not always obvious when that needs doing, for rainfall varies in its acidity and is not always acidic enough to cause harm.

What is needed is a cheap and reliable way of finding out whether a particular set of plants have actually been stressed by a fall of acid rain. And Wang Xin of Nankai University, in Tianjin, China, thinks he has one. It relies on the reaction of soil bacteria to molecules secreted by plant roots.

Dr Wang knew from the botanical literature that most plants secrete from their roots a mixture of carbohydrates, amino acids and fatty acids that are food for microbes. He also knew, from previous experiments, that some of these microbes generate a weak electric current while they are feeding, and that monitoring this current reveals their level of activity. With all of this in mind, he theorised that acid rain would disrupt metabolic activity in leaves, decrease the flow of organic compounds from the leaves to the roots, and thereby alter the behaviour of soil microbes enough to generate a signal that could warn farmers that their crops were in need of a rinse.

Tests proved him right. Spraying experimental rice plants with artificial acid rain immediately cut their release into the soil of three relevant bacterial foodstuffs—fumaric acid, galactose and glucose. The bacteria responded as expected. The electric current they generated dropped by an average of 300 microamps within two minutes of the plants being sprayed.

That, as Dr Wang describes in ACS Sensors, is a change easily measurable using cheap electrodes. And by wiring those electrodes to a transmitter, the plants’ cries of distress can be texted instantly to the farmer tending them, who can then arrange for a second shower, this time of clean water, to restore them to health before any permanent damage occurs.



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