For the current study, the researchers injected the solution into corn leaves, which they selected in part because the crop is crucial to the world’s food supply. Nanosensors coated the outside of the leaf cells, swelling or contracting based on the amount of water available.
The color 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 colors and creating the tip of the green wavelength that the colors emit. When there is not much water, the nanoparticles shrink and the colors converge, resulting in a peak in the yellow wavelength. Researchers can then convert emission spectrum readings into water potential measurements, all without harming the plant.
The technique can be applied in different places along the leaf to monitor water flow, says Piyush Jain, co-author of the study and a doctoral candidate for mechanical engineering at Cornell. “What allows us to basically model the flow of water 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 below the leaf surface, where plants perform important functions such as CO2, the release of water vapor into the atmosphere and the packaging of sugars created by photosynthesis. Researchers say it will be useful to grow crops that manage water better if we better understand the biology and behavior of water in such critical places.
Ultimately, the technology could be used in real-world situations, such as field workers or in greenhouses. It might even be possible to spray AquaDust across the field one day and then use a multispectral camera to quickly measure water potential on hundreds of plants.
While it’s still a distant development, AquaDust sounds like a useful technology, says Irwin Goldman, a professor of horticulture at the University of Wisconsin-Madison, who was not involved in the study. “Using any kind of remote reading technology – in this case using nanosensors – is a huge leap forward,” he says. “My point of this technology is that it’s really the future.”
Growers have focused for some time on developing drought-resistant crops, Goldman says. “At least for the last 15 years, there has been a feeling in the plant community that we need to include selection for greater resilience in our crops as part of our breeding programs, that it is not enough just to cultivate multigenerational or better quality or disease resistance,” he says. But, he points out, there will be a long process of determining which plants best defy water loss and which genes are associated with that resistance, before pairing them with other desirable traits like good nutrition and taste. “Once we identify the genes, it’s very useful, but it doesn’t have to lead 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 ready to spread, which farmers or growers could use to, say, estimate 1,000 plants per hour. First, the injected solution itself contains water that must evaporate before anyone can take a measurement. “We are waiting for about a day for the leaf to return to its natural state,” says Jain.
The methods of application and reading of AquaDust should be refined before it is ready for such high-throughput measurements or commercial products. But in the meantime, the ability to precisely target the flow of water within plants could help researchers solve some mysteries. One of them, says Stroock, is whether plants ever allow the deepest layers of leaves, called mesophiles, to dry out. For years it has been common wisdom to avoid this, but indirect measurements by other laboratories now suggest that this is a possibility. The ability to test directly with AquaDust could fundamentally change our understanding of how plants manage water and how they cope with the stress caused by dry internal tissue, he says.
“We believe the lab needs to answer very exciting questions that take precedence over commercialization,” Stroock says. “Currently, Iowa farmers are not calling us to say, ‘Can we cover our field with AquaDust?'”
These farmers are probably just hoping for rain. But one day, technology like nanosensors could help them when those hopes dry up.
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