Objective Photosynthesis of Gynura formosana under varied monochromatic lights was analyzed to optimize the condition for greenhouse cultivation of the vegetable.

Method A CIRAS-3 photosynthesis system with built-in monochromatic lights was constructed for the experiment. Photosynthetic responses of G. formosana plants exposed to 24 monochromatic wavelengths varying from 400nm to 750nm were measured. Quantified leaf gas exchange was obtained from a non-rectangular hyperbola model with an adjusted R²＞0.96.

Results A maximum net photosynthetic rate (NPR) of 8.8 μmol·m⁻²·s⁻¹ of the plants was reached under blue light at 450nm. The stomatal conductance peaked under 650 nm red light at 73 mmol·m⁻²·s⁻¹, which corresponded to an NPR of 7.0 μmol·m⁻²·s⁻¹. Photoinhibition occurred under yellow-green light at a photosynthetic photon flux density（PPFD）of 400 μmol·m⁻²·s⁻¹. The far-red lights at 730 nm and 750 nm produced significantly lower light saturation point (LSP) but significantly higher light compensation point (LCP). It was postulated to relate to the quantum efficiency decline on light utilization of the plants caused by the "red drop" phenomenon and the upregulated dark respiration rate after activation of the phytochrome signaling pathway.

Conclusion The optimal lighting to enhance the light utilization and promote morphological regulation and growth of G. formosana might include the use of red and blue lights as the primary source and supplemented with low-intensity green and far-red light.