The advance from engineers at MIT in the US reported in this week's "The Engineer" is claimed to be the earliest indicator of drought for agricultural applications.

“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,” said Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT and senior author of the new study.

The MIT sensor takes advantage of plants’ stomata, which are small pores in the surface of a leaf that allow water to evaporate. As water evaporates from the leaf, water pressure in the plant falls, allowing it to draw water up from the soil.

According to MIT, plant biologists know that stomata open when exposed to light and close in darkness, but the dynamics of this opening and closing have been little studied because there hasn’t been a good way to directly measure them in real time.

“People already knew that stomata respond to light, to carbon dioxide concentration, to drought, but now we have been able to monitor it continuously,” said Volodymyr Koman, MIT postdoc and lead author of the paper, which appears in Lab on a Chip. “Previous methods were unable to produce this kind of information.”

To create their sensor, the MIT researchers used an ink made of carbon nanotubes dissolved in sodium dodecyl sulphate, an organic compound that does not damage the stomata. This ink can be printed across a pore to create an electronic circuit. When the pore is closed, the circuit is intact and the current can be measured by connecting the circuit to a multimeter. When the pore opens, the circuit is broken and the current stops flowing, allowing the researchers to precisely measure when a single pore is open or closed.

By measuring this opening and closing over a few days, under normal and dry conditions, the researchers found that they can detect, within two days, when a plant is experiencing water stress. They found that it takes stomata about seven minutes to open after light exposure and 53 minutes to close when darkness falls, but these responses change during dry conditions. When the plants are deprived of water, the researchers found that stomata take an average of 25 minutes to open, while the amount of time for the stomata to close falls to 45 minutes.

For their study, the researchers tested the sensors on a peace lily, which has large stomata. To apply the ink to the leaves, they created a printing mould with a microfluidic channel. When the mould is placed on a leaf, ink flowing through the channel is deposited onto the leaf surface.

The MIT team is now working on a new way to apply the electronic circuits by placing a sticker on the leaf surface.