Low Red to Far-red ratio increases resistance to CO₂ diffusion and reduces photosynthetic efficiency in low light grown tomato plants

Application of light-emitting diode technology has opened opportunities to optimize light spectrum for crop production greenhouses and vertical farms. In addition to photosynthetically active radiation, far-red (FR) light has shown potential for enhancing leaf photosynthesis. However, additional FR also alters the red to far-red ratio (R:FR) and induces a shade-avoidance response (SAR) that changes leaf nitrogen, thickness and mass. These acclimations can potentially also alter the resistance to CO₂ diffusion, which can limit photosynthesis. Tomato plants were grown with and without additional FR at two light levels. Changes in photosynthetic responses to light and CO₂ diffusion resistance, as well as leaf mass, thickness and nitrogen content were assessed. At low light additional FR strongly reduced leaf maximum photosynthesis, leaf mass, thickness and nitrogen, and increased the resistance to CO₂ diffusion. These effects were to a much lesser extent present in plants grown at high light intensity. Tomato leaves grown under low light intensity supplemented with FR, show much stronger SAR and a larger increase in CO₂ diffusion resistance than plants grown under high light, which negatively influences their photosynthesis at increasing light intensity. Only if the negative effects of sensitivity to FR and SAR response can be avoided, supplemental FR light may be beneficial to enhance photosynthesis in greenhouse and vertical farm systems.