Updates to McCree's photosynthetically active radiation curve - 55 years later.

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).