Tracking the temporal dynamics of canopy-scale chlorophyll fluorescence with a low-cost nighttime LED platform

Solar induced fluorescence (SIF) has shown promise to indirectly estimate the photosynthetic capacity of plants across spatiotemporal scales, allowing us to see beyond canopy structure. However, the signal from chlorophyll fluorescence is incredibly small, making it difficult, labor-intensive, and expensive to measure canopy-level SIF during the daytime. In light of this, we built a low-cost sensor system consisting of blue LED lights and photodiode radiometers centered in the PAR, red and far-red spectral regions as a means to provide continuous timeseries of steady-state canopy fluorescence. We established a controlled drought and recovery experiment using the plant Polygala myrtifolia, Sweet Pea Shrub, to test out our method of capturing nighttime chlorophyll fluorescence. The blue LEDs and photodiode radiometers were placed ~20cm above the canopy and turned on nightly for 2 hours. The experiment lasted for three-weeks during where we induced a drought by stopping irrigation so we could observe how well our method tracked stress events and recovery. To validate the sensitivity of this approach to subtle changes in canopy fluorescence, we measured leaf and canopy level reflectance and radiance, fluorescence parameters using a monitoring PAM device (MONI-PAM), and stomatal conductance using a LICOR Li600 porometer. With our nighttime LED platform, we were able to track the steady-state fluorescence dynamics of P. myrtifolia during both drought and recovery. Following a baseline of non-stressed conditions, we observed a prominent decline of fluorescence and photosynthetic activity during drought, and then noted a slight recovery during the return to well-watered conditions. This pattern was corroborated by the MONI-PAM, spectrometer, and porometer measurements. The development of this method can help inform timeseries of canopy scale steady-state fluorescence for less than $500 USD. This nighttime fluorescence methodology also has the immediate potential to be used to validate both satellite and tower-scale temporal SIF dynamics. Further, because of its cost-effective nature, this may allow for more equitable, repeatable experiments to be performed across a variety of ecosystems worldwide.