LED-induced chlorophyll fluorescence during heat and drought stress in microcosm experiments on sunflower and wheat

Chlorophyll fluorescence, emitted mainly in the red and far-red spectral ranges (about 650–850 nm), provides direct information on photosynthetic functioning and plant stress responses. Solar-induced chlorophyll fluorescence (SIF) offers information on photosynthetic activity at the canopy scale under natural light conditions, but its interpretation is strongly influenced by variable illumination and canopy structure. Actively induced fluorescence using LED light sources offers a controlled alternative for fluorescence spectra measurements. LED-induced chlorophyll fluorescence (LEDIF) enables observations under standardized conditions, independent of ambient light variability, and allows more direct access to baseline fluorescence properties linked to plant physiological status. LEDIF is therefore well-suited for studying stress responses in controlled experiments.

In this study, controlled microcosm experiments were conducted on sunflower and wheat following the same experimental protocol to investigate plant responses to drought and heat stress using LEDIF. Plants were subjected to four treatments: well-watered – no heat stressed, well-watered – heat stressed, water-stressed – no heat stressed, and water-stressed - heat stressed. All experiments lasted approximately two months in 2024-2025, with stress applied gradually. Chlorophyll fluorescence was induced using an actively controlled 11-channel multispectral LED illumination system. Broadband fluorescence (650–850 nm) and reflectance spectra (350–850 nm) were recorded above the canopy using a downward-facing VIS–NIR spectrometer positioned between the LED panels, while canopy architecture and leaf area development were monitored using side- and top-view RGB images. LEDIF increased during canopy development of the sunflower plants, after which clear treatment-dependent responses emerged. Sudden heat stress applied to well-watered plants caused a decline in fluorescence comparable to that in gradually drought-stressed sunflower plants. While plants exhibited similar growth during the initial phase, drought induced strong divergence in canopy development, with well-watered plants maintaining healthy canopies and drought-stressed plants showing severe canopy loss. Wheat plants consistently exhibited lower fluorescence intensity than sunflower plants and a stronger temporal decline in LEDIF, reflecting greater loss of green leaf area. Leaf angle changes supported these responses, with water-stressed plants displaying shifts toward flaccid, senescing leaves.