Which Crops Can Survive Drought? Nanosensors May Offer Clues
Keith Gillogly
For crops, climate change is literally a growing problem.
The warming of our planet has increased the odds of drought worsening throughout the world. In the US, it’s threatening soybean, corn, and wheat production, and the future isn’t looking much wetter. So with water in short supply, we might try to plant crops that manage it more efficiently. “A big focus today is breeding for a changing climate,” says Abraham Stroock, a professor of chemical and biomolecular engineering at Cornell University. “We want to discover new traits and their genetic origins for resilience in a hot and dry climate of our future—of our today in many parts of the world.”
But first, researchers have to better understand how existing plants manage their water flow. Stroock and his colleagues have developed a nanoscale sensor called AquaDust that uses tiny fluorescent dyes to illuminate how water moves through plant tissue—a minimally invasive way for breeders and biologists to assess crops’ health at the microscopic level. Their work is described in a new paper published in the Proceedings of the National Academy of Sciences in June.
Water is vital for plants. It keeps cells hydrated and is critical for photosynthesis. But it’s also under tension. Plants work to draw water out of the soil and up into their leaves. Meanwhile, the surrounding atmosphere is pulling that water out through evaporation, especially when the air is hot and dry. The measure of this tension is called “water potential.” Knowing a plant’s water potential matters because it correlates with its growth, yield, and the delicate balance between water loss and carbon dioxide uptake. During a drought, a plant with very negative water potential might dry up and die.
Currently, the gold standard for measuring this tension is a tool called the Scholander pressure chamber. It’s about the size and shape of an open lunch box, and it contains a pressure gauge and sample chamber, plus an external tank of pressurized gas. After sealing a leaf in the chamber, the researcher turns up the pressure, which forces the liquid out of the plant to get a reading. But this technology is decades old, it's heavy, and if you want to get a reading across an entire leaf, you have to cut it off and destroy it, says Stroock, a coauthor on the PNAS study and the associate director of the Cornell Institute for Digital Agriculture.
So Stroock’s team is taking a different approach, one that keeps the leaves alive.
They started by developing nanoparticles they dubbed AquaDust, microscopic sensors made from a hydrogel that expands or contracts in response to changes in water availability. The particles can be mixed into a solution, creating a pinkish liquid.
© Photograph: Piyush Jain/Cornell University Pictured is a confocal micrograph of maize leaf highlighting array of stomatal cavities separated by vascular bundles where epidermal cell wall autofluorescence is false-colored as blue, chloroplasts as green, and hydrogel nanoreporter (AquaDust) of water potential in leaves as red
For the current study, the researchers injected the solution into maize leaves, which they chose, in part, because the crop is critical to worldwide food supply. The nanosensors coated the outside of the leaf’s cells, swelling or shrinking based on how much water was available.
The dye molecules in AquaDust fluoresce at different wavelengths, depending on their proximity to each other, and these wavelengths can be measured with an instrument called a spectrometer. When water is readily available, the nanoparticles swell, pushing the dyes apart and creating a peak in the green wavelength the dyes emit. When there’s not much water, the nanoparticles shrink, and the dyes move closer together, resulting in a peak in the yellow wavelength. Then the researchers can convert the emission spectrum readings into water potential measurements, all without harming the plant.
The technique can be applied to different locations along the leaf to track water flow, says Piyush Jain, a study coauthor and mechanical engineering PhD candidate at Cornell. “What that allows us to do is basically model the water flow through different tissues, starting from the stem to different parts of the leaf,” he says.
The researchers focused their AquaDust measurements on the area just beneath the leaf’s surface, where plants carry out important functions like taking in CO2, releasing water vapor into the atmosphere, and packaging sugars created by photosynthesis. To breed crops that manage water better, having a better grasp of the biology and behavior of water at such critical points will be very helpful, the researchers say.
Ultimately, the technology might be used in real-world situations, like for workers in fields or greenhouses. It might even be possible to someday spray AquaDust over a field and then use a multispectral camera to quickly measure water potential across hundreds of plants
.
© Photograph: Siyu Zhu/Cornell University A researcher using AquaDust in a corn field.
And while that’s still a far-off development, AquaDust sounds like useful technology, says Irwin Goldman, professor of horticulture at the University of Wisconsin, Madison, who wasn’t involved in the study. “Using any sort of remote sensing technology—in this case they’re using nanosensors—is an enormous leap forward,” he says. “My sense of this technology is that it is the future, really.”
Breeders have focused on developing drought-resistant crops for some time, says Goldman. “For at least the last 15 years, there’s been a sense in the plant-breeding community that we need to be incorporating selection for greater resilience in our crops as part of our breeding programs, that it’s not enough to just breed higher-yielding or better quality, or for disease resistance,” he says. But, he points out, it will be a long process to identify which plants best defy water loss and which genes are linked to that resiliency, before then pairing them with other desirable traits like good nutrition and flavor. “Once we identify the genes, that’s very helpful, but it doesn’t necessarily get us all the way to the end of the project,” he says. “We still have to find useful combinations.”
For now, AquaDust is primarily a research tool, not something that’s ready to be rolled out at scale that farmers or breeders could use to, say, assess 1,000 plants in an hour. For one thing, the injected solution itself contains water, which must evaporate before anyone can take a measurement. “We wait for about a day to get the leaf to come back into its natural state,” says Jain.
AquaDust’s application and readout methods would need to be refined before it could be ready for such high-throughput measurements or commercial products. But in the meantime, being able to precisely target the flow of water within plants might help researchers solve some mysteries. One of them, says Stroock, is whether plants ever allow the innermost layers of their leaves, called mesophyll, to dry out. For years, the conventional wisdom was that they avoid it, but indirect measurements by other labs now suggest that it’s a possibility. Being able to test this directly with AquaDust could fundamentally alter our understanding of how plants manage their water and how they handle the stress caused by dry inner tissue, he says.
“We believe there are very exciting questions to answer in the lab that take precedence over commercialization,” Stroock says. “Right now, Iowa farmers are not calling us to say, ‘Can we cover our field with AquaDust?’”
Those farmers are probably just hoping for rain. But, someday, technology like nanosensors might help them out when those hopes run dry.
