For farmers, the earlier they know that their crops need water, the better – they can't just wait until the plants are visibly wilting. With this in mind, MIT scientists led by Prof. Michael Strano have developed a new type of sensor that's printed directly onto a plant's leaf.
Plants have small pores in their leaves known as stomata, which open during the day and close up at night. While they're open, they allow water to evaporate from within the plant. This causes the water pressure within the plant to drop, which in turn causes the plant to draw more water up from the soil, through its roots.
In the case of the peace lilies that were tested, the stomata typically take about seven minutes to open after exposure to light, and 53 minutes to close once darkness falls – that's if the plant has enough water. If the plant is deprived of water, however, the opening time increases to an average of 25 minutes, while the closing time is reduced to 45 minutes.
That's where the sensor comes in.
It's made from an electrically-conductive ink containing carbon nanotubes, and is printed across a single stomata to form an electrical circuit. While the stomata is closed, the circuit is intact, and the current is able to flow unimpeded. Once it opens, however, the circuit is broken and the current can no longer flow.
The scientists found that after monitoring the sensor for two days – using a current-measuring device known as a multimeter – they could determine whether or not a plant was experiencing water stress, based on how long it took the stomata to open and close.
"This appears to be the earliest indicator of drought that we have for agricultural applications," says Strano. "It's hard to get this information any other way. You can put sensors into the soil, or you can do satellite imaging and mapping, but you never really know what a particular plant is detecting as the water potential."
MIT is now looking at commercializing the technology, which may ultimately take the form of a sticker that can be applied to leaves.
A paper on the research was recently published in the journal Lab on a Chip.
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