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

The magnitude of chlorophyll fluorescence emission represents both chlorophyll content and energy quenching processes enabling its application to serve as a proxy of photosynthetic activity. Thus, there is interest in advancing methods for canopy‐scale monitoring of chlorophyll fluorescence. Remotely sensed solar‐induced fluorescence (SIF) retrievals offer daytime monitoring of chlorophyll fluorescence, which can serve as an indicator of photosynthesis. However, it represents an instantaneous measurement during the day, which is strongly influenced by incoming radiation, solar angle, and sun/shade fraction—making it difficult to tease out baseline information on plant health and potential photosynthetic capacity—which could be tracked by changes in fluorescence yield (independent of sunlight). Recent advances have demonstrated the potential for inducing nighttime chlorophyll fluorescence via LED light sources at the canopy‐scale, which can be retrieved as LED‐induced chlorophyll fluorescence (LEDIF), potentially serving as a baseline indicator of plant health and photochemical capacity, independent of daytime conditions. In this study, we explored two methods of LEDIF retrievals: (a) hyperspectral sensor (1.33 nm full‐width half max) and (b) low‐cost Red‐Far‐Red photodiode sensor. LEDIF retrieved by the hyperspectral sensor demonstrated strong correlations with daytime SIF and gross primary productivity during mid to end of season phenology (R2 > 0.70). In contrast, phenological dynamics of LEDIF retrieved by the photodiode sensor was more subtle, likely due to weaker signal‐to‐noise ratio, but still demonstrated some potential. Overall, LEDIF offers a technique to monitor nighttime chlorophyll fluorescence emissions (and changes in its spectral shape with a hyperspectral sensor) to assess canopy‐scale phenology of photosynthetic potential. Plain Language Summary: Monitoring the baseline health status of plants is difficult because of their dynamic responses to the environment over the course of a day to sunlight, temperature, and water availability. Measuring plants at night offers an alternative method to assess a plant's baseline health status. Here we use blue LED lights at night to induce the reemissions of light from leaves via a red‐far‐red chlorophyll fluorescence signal detected by a low‐cost system. We found that this signal closely tracks the seasonal variation of photosynthesis. Because this signal was measured at night, unconfounded by daytime variation of sunlight, temperature, and water availability, our low‐cost system offers an approach to assess a plant's photosynthetic capacity and baseline health. Key Points: Low‐cost hyperspectral and red‐far‐red photodiode sensors with blue LED lights offer nighttime retrieval of chlorophyll fluorescence (LEDIF)LEDIF can track photosynthetic phenology similar to tower‐based solar‐induced fluorescence and gross primary productivityLEDIF may serve as a more direct assessment of baseline plant status unaffected by incoming radiation [ABSTRACT FROM AUTHOR]