Quantifying the impact of high night temperature on NPQ dynamics using canopy reflectance

High night temperatures (HNT) can depress the photosynthetic performance of the plants, with consequential reductions in crop yield. To quantify the impact of HNT on plant photosynthesis, understanding non-photochemical quenching (NPQ) behavior is crucial given its mechanistic link to the downregulation of photosynthesis. Yet, how HNT alters the dynamics of NPQ remains poorly understood. In this study, we used a controlled walk-in growth chamber with phenotyping equipment for whole plants to obtain NPQ and the quantum yield of photosynthesis (Φ_PSII) images combined with imaging spectroscopy to investigate NPQ under controlled temperature conditions (pre-HNT and HNT). Tomato plants were consecutively exposed to 3-day phases with day/night temperatures of 35/20°C (pre-HNT, day 1-3), 35/28°C (HNT, day 4-6) and 35/20°C (recovery, day 7-9). HNT induced nocturnal NPQ elevation that persisted in the following day, resulting in consistently higher NPQ throughout the diurnal cycle compared to the pre-HNT (t-test, p<0.05). This carryover effect suggests a prolonged photoprotective state triggered by nighttime heat stress. Meanwhile, Φ_PSII showed no nighttime difference among phases, but exhibited decreases near peak daytime temperatures. HNT further shifted the Φ_PSII-fluorescence yield curve downward, resulting in lower fluorescence yield at similar Φ_PSII values. A further objective was to monitor the change in NPQ through non-destructive image spectroscopy wherefore we employed partial least squares regression (PLSR) to estimate NPQ with using canopy reflectance (450-780 nm). Our PLSR results confirmed that NPQ can be estimated with an R² of 0.93 based on canopy reflectance, and the predicted NPQ captured the HNT-induced increase during both night and day. From the variable importance in projection (VIP) analysis, we found that nighttime and daytime NPQ shared similar VIP peaks in green (500-600 nm) and red-edge (680-750 nm) region, indicating that consistent spectral features underlie NPQ dynamics regardless of light conditions. Our findings extend the understanding of how increased temperature activates NPQ dynamics and highlight that spectral reflectance contains informative signals for capturing temperature-driven photoprotective responses.