High Throughput Phenotyping Technology | UAV-based High Throughput Phenotyping in Plant Breeding

One way to classify HTP methods is to separate them into four categories: Field-based phenotyping platforms (FBPP), cable-suspended, satellite imaging technology, and aerial vehicles. FBPP and cable-suspended HTP devices are limited by their inability to conduct large-scale phenotyping and inflexible maneuverability while satellite imaging technology is limited by their spatial and temporal resolution (Bai et al., 2016; Bai et al., 2019). Aerial vehicles includes both manned and unmanned aerial vehicles (UAV, aka drones). Of the two, UAVs are becoming a more enticing option for phenotypic data collection due to their high spatial and temporal resolution and flexibility in spectral resolution and maneuverability (Mick, 2023). For additional examples of some of these platforms you can find information in the Song et al. (2021) article “High-throughput phenotyping: Breaking through the bottleneck in future crop breeding”. These platforms are used to carry different sensors that can measure phenotypes. Sensors can be mechanical, chemical, or optical. Mechanical and chemical sensors measure plant morphological and chemical traits using a physical sample of the plants (Burrell et al., 2017; Forristal & Keppel, 2001). Optical sensors can measure these characteristics in a non-destructive manner making them a more ideal option for plant phenotyping.

Like our eyes, optical sensors are instruments designed to detect specific wavelengths of light in the electromagnetic spectrum. The electromagnetic spectrum is the entire range of electromagnetic waves organized by wavelength or frequency (Fig. 4).

Figure 4. *The electromagnetic spectrum.* Humans visually sense wavelengths in the visible spectrum while sensors can capture values of light reflectance in the infrared and ultraviolet spectrum.

Reprinted from EM spectrum by P. Ronan, 2007, *Wikimedia Commons*. CC BY-SA 3.0.

Our eyes and brain are biologically designed to detect and recognize the wavelengths in what is called the visible light region of the electromagnetic spectrum. HTP sensors are engineered to measure the level of reflectance and absorption of light in the entire electromagnetic spectrum. Different sensors are sensitive to a different range of wavelengths of light. Table 1 provides a general range of wavelengths measured by each sensor type, but the specific wavelengths captured vary depending on the sensor brand used.

Table 1. Five sensors used for high throughput phenotyping and the wavelengths they detect in nanometers (nm) including ultraviolet light (UV), red-green-blue (RGB), red edge, near infrared (NIR), and short-wave infrared (SWIR).
Sensor	Wavelengths
RGB	400-700 nm
Multispectral	UV (100 nm – 400 nm) + RGB (400-700 nm) + Red Edge (670-780 nm) + NIR (700-1400 nm)
Hyperspectral	UV (100- 400 nm), + RGB (400-700 nm) + Red Edge (670-780 nm) + NIR (700-1400 nm) + SWIR (1100- 3000 nm)
LiDAR	NIR (700-1400 nm)
Thermal	8,000- 14,000 nm

This range of wavelength detection sensitivities provide plant biologists and plant breeders combinations of sensors to test which can measure differences in how plants interact with (absorb / reflect) light. The question is ‘do these light reaction differences relate to the traits the plant breeder needs to evaluate?’.