"Farmers have a very good eye. They can look at their crop if they've been farming for a long time and say, 'My crop needs water, I can tell,'" says Richard Stoner, founder of the biotech and agriculture company, AgriHouse.

So who might know more about what a plant needs than a farmer or a greenhouse owner? How about the plant itself? What if plants could tell us when they are thirsty?

With funding from the National Science Foundation (NSF), AgriHouse has developed a leaf sensor that is enabling plants to do just that.

The clear plastic sensor measures about an inch by a quarter of an inch; light enough to fit on almost any leaf.

Here's how it works: When a plant starts drying out, its leaves contract. That change can be measured in electrical voltage. The sensor transmits those voltage changes to a cell phone, or computer, so a farmer can use the data to turn irrigation equipment on or off.

So before even the keenest eye of a farmer or a florist might notice a plant starting to wilt, the sensor can transmit a message to "send water."

First developed as a space solution

Measuring the water deficit stress in a plant did not begin as an idea for earthly plants. The technology was first developed by aerospace engineers and the doctoral research work of Hans-Dieter Seelig of the University of Colorado at Boulder's BioServe Space Technologies Center.

"The original inventors were looking at [the sensor] as a device to go to Mars, where the astronauts would have to spend over 70 percent to 80 percent of their waking hours farming," explains Stoner. "They’ll need devices that would be in the greenhouses on Mars that would free up their time."

Stoner and his colleagues have developed other products to help plants grow in space. One, a nutrient called Organic Disease Control (ODC), was flown and tested on the Mir space station. The experiments were conducted with adzuki beans.

"Where the ODC was placed in the solution, the plants grew taller than those that were controls with just water," says Jim Linden, professor emeritus of the Chemical and Biological Engineering Department at Colorado State University (CSU). "The product is being tested on all kinds of commodity crops, corn, soybeans, tomatoes, peppers, around the world. Farmers are very interested in it to increase their yield."

"NASA couldn't fly into space things that are harmful to humans, or harmful to the environment. So what we were able to do is create this liquid product to replace pesticides. AgriHouse has since received EPA approval on this as an eco-friendly bio pesticide," says Stoner.

Stoner has also worked with Linden and Ken Knutson, CSU professor emeritus of horticulture and landscape architecture, on aeroponics projects.

"Aeroponics is the process of growing plants in a complete air environment, no soil, no water except when the water is applied through a hydro-atomized spray," explains Stoner.

That's likely the way future astronauts will be growing food on long-term Mars missions.

How the leaf sensor works

The leaf sensor works the same way on aeroponic crops as it does on traditionally grown crops in soil. And one of the long-term goals for the sensors is to enable them to not just alert a farmer that a plant is thirsty, but, once they are provided with the proper equipment, to turn on the water themselves.

Stoner showed off an aeroponic setup in the company's Berthoud, Colo., lab, which has geraniums, ivy and coleus plants, plus a few young pine trees. A sensor is hooked to a geranium leaf.

"We're taking the input from the leaf sensor, taking that voltage reading, converting it to a number that we can understand, and then when the algorithm senses a change in this number, it activates the entire aeroponic system to provide just the right amount of moisture to the root systems," says Stoner.

As if on cue, as soon as Stoner finishes explaining the setup, a sprayer turns on to wet the roots of this array of plants.

"You didn't see any changes; none of us can; but the plant is sensitive enough to pick up a change," continues Stoner.

At a U.S. Department of Agriculture research farm in Greeley, Colo., beans attached to leaf sensors used 25 percent less water with the sensors than the plants did when watered using standard irrigation practices. Stoner estimates that one sensor could be placed every 20 or 30 acres to provide enough information to irrigate an entire crop.

Another objective of the NSF-supported research is to create a wireless version of the sensor.

At the AgriHouse lab, electrical engineering student Matt Aldrich from the University of Colorado at Colorado Springs says he had never really thought about designing computer systems for plants until this project came along.

"I'm not really big into greenhouses. But this definitely seems like something that is viable. It seems to be something that is vital for our future, and it can definitely help us with our overuse of water," says Aldrich.

There are other devices on the market to help farmers decide when to irrigate; products that measure moisture in the soil and the temperature of leaves. But Stoner believes the precision of the AgriHouse leaf sensor can mean big savings in water and energy.

"It's another tool in the toolbox," adds Stoner. "We believe that this device can be a low cost approach for information that was never available before."

This research was funded by NSF through grant number 0712605.

This story was originally written for Science Nation and was republished with permission here. Video: Science Nation, Miles O'Brien/Science Nation Correspondent, Marsha Walton/Science Nation Producer.