Crop fluorescence measurements from UAVs

In field measurement of photosynthesis can be made in an indirect way by determining the sun-induced chlorophyll fluorescence (SIF), which is produced in natural daylight and can be detected with remote sensing techniques. Earlier studies have shown that SIF can be measured at global level from satellites. To further explore the potential of SIF in agriculture, especially in precision agriculture, a fine spatial resolution is playing a key role. Unmanned Aerial Vehicles (UAVs) are versatile platforms that have a potential to acquire SIF at a low flying altitude (from 10 to 120 m). UAVs can also acquire data at high temporal resolution on specific areas, e.g. diurnal measurements, and thus support airborne and satellite-based measurements through validation and interpretation.

As part of her PhD research, Na Wang and her colleagues from the lab of Geo-information Science and Remote Sensing and the Finnish Geospatial Research Institute are investigating the potential of a UAV-based spectroscopy approach for understanding crop SIF and photosynthesis in precision agriculture at the field level. She uses the Fluorspec system for this purpose. The system is based on a non-imaging hyperspectral spectrometer (Ocean Insight QE-Pro) and capable of measuring SIF both from the ground and in a UAV based set-up. The results of her initial experiments have shown that the FluorSpec can collect high spectral resolution data, repeatedly over time, providing strong support for novel scientific approaches in monitoring crop photosynthetic activities and growth conditions (e.g., heat stress and drought stress). SIF measurements from a UAV platform over a potato and sugar beet field exhibited obvious diurnal patterns, and their differences demonstrated that SIF varied depending on the crop types. The variations shown in the same flight mainly due to the crop species, growth status and spatial heterogeneity were consistent with previous studies, which shows that the UAV-based FluorSpec system is a reliable system to measure actual SIF values. Follow up studies will focus on the use of the Fluorspec system to investigate SIF as stress indicator in water limited sugar beet cropping fields.

The results of her study are published in the International Journal of Applied Earth Observation and Geoinformation in a paper entitled ‘Diurnal variation of sun-induced chlorophyll fluorescence of agricultural crops observed from a point-based spectrometer on a UAV’. The paper is open-source and can be accessed through the following link: https://doi.org/10.1016/j.jag.2020.102276

Abstract:

Unmanned Aerial Vehicle (UAV)-based measurements allow studying sun-induced chlorophyll fluorescence (SIF) at the field scale and can potentially upscale results from ground to airborne/satellite level. The objective of this paper is to present the FluorSpec system providing SIF measurements at the field level onboard a UAV, and to evaluate the potential of this system for understanding diurnal SIF patterns for different arable crops. The core components of FluorSpec are a point spectrometer configured to measure in the O2 absorption bands at sub-nanometer resolution, bifurcated fibre optics to switch between the downwelling irradiance and upwelling radiance measurements, and a laser range finder allowing accurate atmospheric correction. The processing chain is explained and the capability of the novel Spectral Shape Assumption Fraunhofer Line Discrimination (SSA-FLD) method to retrieve SIF was tested. To test the reliability of FluorSpec diurnal SIF measurements, near-canopy diurnal SIF was monitored during the growing season over potato and sugar beet plants with a ground-based setup. The two crops exhibited a clear diurnal SIF pattern, which positively correlated with the photosynthetically active radiation (PAR). The divergence in diurnal patterns between SIF and PAR indicated that the crops might be suffering from heat stress. A significant correlation between SIF and the Photosystem II Quantum Yield was obtained. By mounting the FluorSpec on a UAV, SIF measurements were obtained over the same crops during a clear day. UAV-based SIF also exhibited a pronounced diurnal pattern similar to the ground-based measurements and it showed clear spatial variation within different crop fields. The obtained results demonstrate the ability of the FluorSpec system to reliably measure plant fluorescence at ground and field level, and the possibility of the UAV-based FluorSpec to bridge the scale gap between different levels of SIF observations.