When searching for papers for a review on plant sensors, I came across many experiments that used sensors on plant materials, such as cell suspensions or plant cuttings, and even a few where the sensor was placed onto the leaf or stem of a plant, but one paper I found that really excited me involved the printing of a drought sensor directly onto guard cells of plant stomata. Water is essential for plants, making up 80-90% of their mass, and the effects of drought can therefore be devastating, leading to reduced growth, oxidative damage and eventually death. Stomata, typically located on the underside of leaves, are tiny openings that allow gaseous water and CO2 to flow in and out of the plant, opening and closing depending on the amount of sunlight, temperature, humidity and, importantly, the amount of water available to a plant through its roots. Observing the opening and closing of the stomata can therefore be used to monitor plant hydraulics and potentially predict a plant's response to environmental change.
Koman et al. have developed a sensor design that was printed directly onto the stomata of the peace-lily, chosen due to its large stomata, that could monitor the opening and closing of stomata successfully. The sensor fabrication was relatively simple (see Figure 1): after locating a closed stoma on the underside of a leaf, a microfluidic chip made of PDSM (silicone) was placed over the stoma, conductive ink injected into the chip, dried, rinsed with water, and the chip removed. The conductive ink, made of carbon nanotubes and SDS surfactant, was biocompatible, water-based, and most importantly, not found to interfere with normal stomatal behaviour.
Figure 1. Silicone microfluidic chip injected with conductive ink to form sensor on plant stoma.
The sensor worked on the basic principle of the circuit being complete upon stomatal closure and breaking upon stomatal opening. Stomatal opening and closure is closely related to illumination, where they open with more light to allow in CO2 for photosynthesis and close in darkness to prevent water loss, and so the sensors were tested by turning illumination of white light on and off and measuring the resistance of the sensor.
Figure 2a shows the aperture of the stoma, as measured by optical microscopy. You can clearly see how the aperture increased to ~4-5 um when the stoma was illuminated (yellow section) and decreased back to 0 um when the light was turned off (grey section), expected behaviour for a stoma (note that the red line shows the behaviour of a bare stoma and the black shows that of a stoma with the printed sensor, where little difference was observed between the two).
Figure 2b shows the resistance of the printed sensor on the stoma, where resistance increased to over 1000 kOhm when the stoma was illuminated and decreased upon removal of light, corresponding well with the behaviour of the stoma, suggesting that the sensor can be used to determine whether a stoma is open or closed. You'll notice that the "light off" resistance increases gradually over time: the researches suggest that the sensors eventually failed after 5-7 open-close cycles, but I think the fact they are reusable at all is very exciting at this stage of research.
a) Stomatal aperture, as measured by optical microscopy, upon light-on (yellow) and light_off (grey), where red line is bare stoma and black line is stoma with sensor.
b) Resistance of sensor upon light-on (yellow) and light_off (grey).
You probably have also noticed the slight lag between the light being turned on and off and the change in stoma aperture. This "latency" in stomatal opening and closing has also been investigated by the researchers here using their new sensor design. Under normal watering conditions, they found a latency of ~7 mins after light-on and ~53 mins after light-off. After measuring the aperture for 3 days, they then simulated drought by no longer watering the plant, and found that the opening latency increased to ~25 mins and closing latency decreased to ~45 mins. This suggests the plant was trying to decrease water loss by slowing down stomatal opening and speeding up stomatal closure.
In another experiment, they put a plant under drought conditions and watered the plant on the second day, where they found opening latency decreased from ~20 mins down to ~8 mins, and closing latency increased from ~45 back up to ~53 minutes, demonstrating the attempted recovery of the plant after severe conditions.
Is this the direction plant sensors could be going in? ...I'm not so sure. Although the results are amazing, the precision and time required required to set up the experiment would make it unfeasible for wide use. Having said that, I would definitely love to see more direct sensors like this, shortening the gap between technology and plants.
Koman, V. B., Lew, T. T. S., Wong, M. H., Kwak, S. Y., Giraldo, J. P., & Strano, M. S. (2017). Persistent drought monitoring using a microfluidic-printed electro-mechanical sensor of stomata: In planta. Lab on a Chip, 17(23), 4015–4024. https://doi.org/10.1039/c7lc00930e
All images © The Royal Society of Chemistry 2017