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).

Conclusion By measuring the photosynthetic light-response curves of Gynura formosana under 24 monochromatic light wavelengths within the 400-750nm spectral band,the results demonstrated that the net photosynthetic rate peaked under blue light at 450 nm and red light at 650nm, identifying these two wavelengths as the core effective light qualities driving photosynthesis in this species. The yellow-green spectral region readily induced photoinhibition in G.formosana, and its energy proportion should therefore be restricted.Far-red light (730nm and 750nm)resulted in a lower light saturation point and a higher light compensation point, which can be attributed to the "red drop" effect and enhanced dark respiration, rendering it unsuitable as a primary light source. On this basis, a composite spectral strategy is proposed, with red and blue light as the dominant components, supplemented by low-intensity green and far-red light, to synergistically enhance photosynthetic performance while modulating plant morphogenesis. This study provides a theoretical reference for the design of precise supplemental lighting systems for G.formosana in protected cultivation.