Tracking canopy chlorophyll fluorescence with a low-cost light emitting diode platform

Abstract

Chlorophyll fluorescence measured at the leaf scale through pulse-amplitude modulation (PAM) has provided valuable insight into photosynthesis. At the canopy- and satellite-scale, solar-induced fluorescence (SIF) provides a method to estimate the photosynthetic activity of plants across spatiotemporal scales. However, retrieving SIF signal remotely requires instruments with high spectral resolution, making it difficult and often expensive to measure canopy-level steady-state chlorophyll fluorescence under natural sunlight. Considering this, we built a novel low-cost photodiode system that retrieves far-red chlorophyll fluorescence emission induced by a blue light emitting diode (LED) light-source, for two hours at night, above the canopy. Our objective was to determine if an active remote sensing based nighttime photodiode method could track changes in canopy-scale LED-induced chlorophyll fluorescence (LEDIF) during an imposed drought on a broadleaf evergreen shrub, Polygala myrtifolia. Far-red LEDIF (720 - 740 nm) was retrieved using low-cost photodiodes (LEDIFphotodiode) and validated against measurements from a hyperspectral spectroradiometer (LEDIFhyperspectral). To link the LEDIF signal with physiological drought response, we tracked stomatal conductance (gsw) using a porometer, two leaf-level vegetation indices — photochemical reflectance index, PRI; normalized difference vegetation index, NDVI— to represent xanthophyll and chlorophyll pigment dynamics, respectively, and a PAM fluorimeter to measure photochemical and non-photochemical dynamics. Our results demonstrate a similar performance between the photodiode and hyperspectral retrievals of LEDIF (R2=0.77). Furthermore, LEDIFphotodiode closely tracked drought responses associate with a decrease in photochemical quenching (R2=0.69), Fv/Fm (R2=0.59), and leaf-level PRI (R2= 0.59). Therefore, the low-cost LEDIFphotodiode approach has the potential to be a meaningful indicator of photosynthetic activity at spatial scales greater than an individual leaf and over time.

Hyperspectral and Photodiode Retrievals of Nighttime LED‐Induced Chlorophyll Fluorescence (LEDIF) for Tracking Photosynthetic Phenology in a Vineyard

“…This is in contrast to Brissette et al. (2023), who found close correspondence between the photodiode and hyperspectral derived LEDIF values. This is likely due to lower signal‐to‐noise ratio for the photodiode sensors (low intensity ∼0.01 μmol m −2 s −1 signal and ∼2.5 signal‐to‐noise ratio; Figure 2), which they were placed further away from the canopy in a less controlled environment as was done in the experiment by Brissette et al.…”

“…This is in contrast to Brissette et al (2023), who found close correspondence between the photodiode and hyperspectral derived LEDIF values. This is likely due to lower signal-to-noise ratio for the photodiode sensors (low intensity ∼0.01 μmol m −2 s −1 signal and ∼2.5 signal-to-noise ratio; Figure 2), which they were placed further away from the canopy in a less controlled environment as was done in the experiment by Brissette et al (2023) (∼0.05 μmol m −2 s −1 signal). For the hyperspectral sensor, we manually set a 0.6 s integration time to maximize signal-to-noise ratio while avoiding saturation (low intensity ∼2.5 signal-to-noise ratio Figure 2).…”

“…Compared to the hyperspectral-based LEDIF signal, the photodiode-based LEDIF signal demonstrated minimal seasonality and did not correlate well with SIF (Figures 5 and 7). This is in contrast to Brissette et al (2023), who found close correspondence between the photodiode and hyperspectral derived LEDIF values. This is likely due to lower signal-to-noise ratio for the photodiode sensors (low intensity ∼0.01 μmol m −2 s −1 signal and ∼2.5 signal-to-noise ratio; Figure 2), which they were placed further away from the canopy in a less controlled environment as was done in the experiment by Brissette et al (2023) (∼0.05 μmol m −2 s −1 signal).…”

“…Thus, LEDIF offers a technique to monitor changes in chlorophyll content, and likely potential photosynthetic capacity uninhibited by instantaneous conditions such as PAR that would impact SIF dynamics. This work complements a recent study demonstrating the LEDIF system for detecting short‐term drought stress response tracking pigment, PAM fluorescence, and stomatal conductance dynamics (Brissette et al., 2023), as well as a tractor mounted system (Romero et al., 2021). Beyond phenology, chlorophyll determination via R:FR has also been used to assess changes driven by plant growth development, health, stress events and stress tolerance (Buschmann, 2007).…”

“…Brissette et al (2023) explore the use of a Red-Far-Red photodiode sensor for tracking drought response. They find strong agreements between LEDIF retrieved from a hyperspectral sensor and a Red-Far-Red photodiode sensor in addition to strong correlations between LEDIF with PAM fluorescence metrics of chlorophyll fluorescence, photochemistry, and porometer measurements of stomatal conductance (Brissette et al, 2023). This highlights the potential for low-cost nighttime measurements of canopy chlorophyll fluorescence for tracking the dynamics of photosynthetic activity.…”