LED-induced chlorophyll fluorescence during heat and drought stress as assessed in a microcosm experiment on sunflower

Like sun-induced fluorescence (SIF), LED-induced fluorescence (LEDIF) became frequently used to establish and analyze leaf- and canopy-level stress responses. Different plant phenotypes (trees, understory shrubs, crops, vineyards, etc.) were subjected to, in most of the studies, blue LED illumination during the night or in darkened boxes for assessing either the entire broad-band (650-850 nm) spectrum of LEDIF or one of the wavelength bands of the red (~ 690 nm) and far-red (~ 740 nm) peak emissions. It seems to be however less common to apply close to “white” LED lighting, mixed from different wavelength ranges all below 650 nm (to overcome spectral overlap of red excitation and emission) as a light source. Moreover, stress manipulation in microcosm experiments is also scarce within studies while detecting LEDIF signal changes.

In our study, we established a microcosm experiment with four treatments on sunflowers: well-watered – no heat stressed, well-watered – heat stressed, water-stressed – no heat stressed, and water-stressed - heat stressed. The plants were gradually exposed to the treatments during the two months of the experiment between October and December 2024. We captured reflectance and the broad-band fluorescence spectra above the canopy with a VIS-NIR spectrometer facing downwards toward the canopy between the LED panels. We followed the response of the plants to the imposed stress by weekly/bi-weekly measurements and analyzed the changes in the shapes of the curves. We also captured the canopy architecture with side-view photos and leaf area growth with top-view photos. There was a clear increase in the LEDIF signal during the canopy development, and then a heterogeneous response depending on the treatment.