Even plants, from towering palm trees to vegetables such as onions and eggplants, can get stressed. Now, a team of engineers has designed a new kind of sensor that attaches directly to plants and peers inside of them to track how they respond to stressors such as drought and fires in real time. Described in a study publishing November 12 in the Cell Press journal Cell Reports Physical Science, the coin-sized device is made up of a sensor and incredibly small needles that poke inside plants without harming plants in the process. The team hopes the device could one day help farmers know when their plants are stressed out.
"For Saudi Arabia and other arid regions, where agricultural production is strongly influenced by high temperatures and limited water resources, technologies that improve our understanding of plant stress responses could be especially valuable," says first author Mufeeda Mundummal of King Abdullah University of Science and Technology (KAUST) in Thuwal, Saudi Arabia.
The research hinges on a small but important molecule: hydrogen peroxide. Plants naturally produce small quantities of hydrogen peroxide throughout the day. The molecule sends signals that help plants activate their defenses against pathogens such as fungi and bacteria.
When plants are exposed to stressful conditions, such as too much heat or light, their cells can produce a surge of hydrogen peroxide. At high doses, the molecule damages plant tissues and may cause other problems.
Monitoring hydrogen peroxide levels in plants has proved tricky for scientists—at least, without causing damage by collecting pieces from plants.
"Plants are constantly responding to their surrounding environment, but many of these responses are invisible to us," says senior author Khaled Salama of KAUST.
Salama and his colleagues wanted to make the invisible visible. To do that, the team used a laser to create a sensor for hydrogen peroxide made out of graphene. The sensor sits in a device that is about the size of a small coin and rests on top of plant leaves.
The device's needles are just 0.4 millimeters wide and can suck up small amounts of fluid from inside leaves. They're also so tiny that they don't cause the plants any serious injuries.
In a series of experiments, the researchers put the sensors to the test. They attached the devices to Aloe vera plants growing in a greenhouse. Then, they shined especially bright lights on some of the plants.
The succulents got a little stressed out. At normal growing temperatures, or around 19 degrees Celsius (66 degrees Fahrenheit), hydrogen peroxide levels in the plants began to spike almost immediately after the team turned on the bright lights. That surge was even more severe at hotter growing temperatures, or 28 degrees Celsius (82 degrees Fahrenheit). In both cases, the hydrogen peroxide levels declined after about an hour.
The findings that plants react so much to changes in temperature and light could spell trouble for many important plant species as the planet's climate changes, and heat waves grow more common, say the authors.
The team still has work to do before real farmers can use the devices to monitor their plants, including testing their designs on a range of different plant species, but their device could open the door to real-time insights on the inner workings of plants with an unprecedented level of detail.
"Our research moves plant-health monitoring toward direct measurement of physiological stress signals, rather than relying only on visible symptoms or external environmental conditions," Mundummal says.
