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Our interpretation of photosynthetically active radiation in plants has evolved since the 1970s with new data explaining the underlying mechanisms. To update McCree's founding work, this study explored the spectral response of photosynthesis in young tomato (Solanum lycopersicum cv. Beefsteak) and lettuce (Lactuca sativa cv. Breen) plants using a narrow-spectrum light unit and a portable photosynthesis system equipped with a whole plant chamber. Highly resolved spectral photosynthesis curves using 1-nm increments at 10 nm full width at half maximum (FWHM) were generated. Results show that the lowest quantum yields were observed at 450 nm and 660 nm, two wavelengths commonly used to improve photosynthesis in research. Different trends and amplified peaks were observed among the spectral quantum yield curves of tomato and lettuce plants and those of earlier studies with red and blue light. An opposing phenomenon was observed, where blue light is more efficient than red light. This is based on the narrower wavelength data acquired in both experimental plant species. Findings represent the most detailed and highly resolved spectral photosynthesis and quantum yield curves to date using experimental model plants (tomato and lettuce)., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Optimizing photosynthetic lighting is essential for maximizing crop production and minimizing electricity costs in controlled environment agriculture (CEA). Traditional lighting methods often neglect the impact of environmental factors, crop type, and light acclimation on photosynthetic efficiency. To address this, a chlorophyll fluorescence-based biofeedback system was developed to adjust light-emitting diode (LED) intensity based on real-time plant responses, rather than using a fixed photosynthetic photon flux density (PPFD). This study used the biofeedback system to maintain a range of target quantum yield of photosystem II (Φ PSII ) and electron transport rate (ETR) values and to examine if the adjustment logic (Φ PSII or ETR-based) and crop type influence LED light intensity. The system was tested in a growth chamber with lettuce (Lactuca sativa) 'Green Towers' and cucumber (Cucumis sativus) 'Diva' to maintain six ETR levels (30, 50, 70, 90, 110, 130 μmol·m -2 ·s -1 ) and five Φ PSII levels (0.65, 0.675, 0.7, 0.725, 0.75) during a 16-hour photoperiod. The ETR-based biofeedback quickly stabilized the target ETR within 30-45 minutes, whereas the Φ PSII -based system needed more time. The system adjusted light intensities according to target values, acclimation status, and crop-specific responses. For example, to maintain a target ETR of 130 μmol·m -2 ·s -1 , the gradual increase in Φ PSII over time due to light acclimation allowed the required PPFD to decrease by 35 μmol·m -2 ·s -1 . Lettuce showed higher photosynthetic efficiency and lower heat dissipation than cucumber, leading to higher PPFD adjustments for lettuce. This biofeedback system effectively controls LED light, optimizing photosynthetic efficiency and potentially reducing lighting costs., (© 2025 The Author(s). Physiologia Plantarum published by John Wiley & Sons Ltd on behalf of Scandinavian Plant Physiology Society.)