Despite rapid advancements in technology, the development of wearable plant sensors (WPS) often overlooks the essential aspects of plant biology and agricultural environments. Researchers from Michigan State University, Sejong University, Seoul National University, and Wageningen University have highlighted this issue in a recent review paper.

They emphasized that while engineering advancements have led to a boom in sensor technologies, this growth comes with a significant gap in testing these sensors adequately within real-world agricultural settings. The disconnect between WPS developers and plant biology researchers has impeded the adoption of this promising technology in sectors most in need, such as open fields, greenhouses, and vertical farms.

Wearable plant sensors are designed to attach directly to crops, measuring vital parameters related to growth, water status, nutrients, stress, and microclimate. Advocates believe these devices represent the future of “internet of plants,” which could substantially improve resource efficiency, minimize crop losses, and facilitate precision agriculture.

Lab Testing: A Poor Substitute

The review criticized current experimental methodologies employed in WPS studies. The researchers analyzed 93 experimental studies and uncovered a concerning trend: inadequate field testing.

Only 15 out of the 93 studies—approximately 16%—tested their sensors in authentic field conditions. Moreover, even the few field tests were typically short-lived, averaging just 48 hours. The analysis revealed deeper issues: most studies failed to adhere to basic plant science standards, suffering from environmental blind spots, insufficient replication, and a lack of cross-validation.

A staggering 88% of studies did not adequately outline the environmental conditions under which plants were grown or tested, suggesting a lack of understanding that plants react dynamically to their surroundings. Furthermore, around 73% restricted tests to a single plant or inadequately reported the number of biological replicates, neglecting essential biological variance.

Compounding the issue, 68% of the studies measured data without verifying it against independent metrics, raising doubts about the physiological significance of the recorded signals. The authors stressed the necessity of rigorous testing, underscoring that the calibration between sensor signals and plant responses can differ when a sensor is affixed to a living plant.

Essential Signals and Common Pitfalls

The authors identified crucial signals that WPS can measure, vital for various aspects of crop management. These include environmental factors like light, vapor pressure deficit (VPD), and pesticide levels, as well as physiological metrics such as water status, nutrients, phytohormones, and volatile organic compounds (VOCs).

Many studies mistakenly measured light intensity using lux, a unit designed for human sensitivity, rather than the appropriate plant-focused unit of photosynthetic photon flux density (PPFD). Accurate measurements are critical, as light intensity and spectrum directly impact plant growth.

Furthermore, VPD serves as a valuable indicator of plant water stress, influencing transpiration. Administrating it alongside leaf temperature and relative humidity could provide early drought warnings. Although many sensors can detect residues on surfaces, physiologically relevant performance testing against plant surface components and humidity is necessary.

Designing Sensors for Specific Environments

The authors noted that developers must consider the specific agricultural system for which a sensor is intended since each environment presents unique obstacles. For instance, in open fields, sensors need to withstand various conditions, including sunlight, UV radiation, rain, and wind. Thus, robust encapsulation, flexible mounting, and low-power electronics are essential for long-term monitoring.

In greenhouses, despite more controlled conditions, sensors must be designed to handle high humidity and potential condensation. Integrated WPS data can enhance greenhouse management systems, allowing for climate adjustments based on real-time plant feedback.

Conversely, vertical farming systems require precise control, introducing complexities linked to specialized LED lighting that can interfere with sensor measurements. Synchronizing sensors with lighting cycles may be necessary to avoid signal noise.

The Path to Successful Adoption

To bridge the disconnect between laboratory testing and field applicability, the review proposed key protocols for developers. They must utilize a sufficient number of biological replicates and ensure sensor readings are confirmed in vivo against independent measurements. Stability testing and assessments of the sensor’s impact on plant growth are also crucial.

In conclusion, while there are promising advancements in WPS technology, successful integration into agricultural practices hinges on high-quality research and stringent testing protocols. The authors believe that integrating other sensing methodologies, such as hyperspectral imaging and infrared sensing, could further enhance plant monitoring when combined with WPSs.

Source: ACS Sensors Let’s Get Real: Are Wearable Plant Sensors Ready for Crop Monitoring? https://doi.org/10.1021/acssensors.5c02510 Authors: Hoh Donghee, et al.