ASHS 2025 Conference

Carbon dioxide (CO₂) concentration and light conditions, including both intensity and spectral quality, are key environmental factors influencing plant growth, photosynthetic efficiency, and fruit production in tomatoes. Recent studies on greenhouse tomato varieties have reported that short-term exposure to elevated CO₂ concentration (800 ppm) and higher light intensity enhances plant growth and photosynthetic activities; additionally, long-term exposure to supplementary far-red photons increases dry mass partitioning to fruits, resulting in higher fruit yield. However, limited information is available regarding the interactive effects of CO₂ concentration, light intensity, and far-red photons in dwarf tomatoes for indoor production. Therefore, this study aimed to quantify the interactive effects of CO₂ level, photosynthetic photon flux density (PPFD; 400-700 nm), and supplementary far-red photon on growth, photosynthetic responses, and fruit production in dwarf tomato ‘Red Robin’. Plants were grown under two CO₂ levels (ambient: 400 ppm; elevated: 1500 ppm) × two light intensities [PPFD of 250 µmol m^-2 s^-1 (PPFD250) and 500 µmol m^-2 s^-1 (PPFD500)] × two supplementary far-red levels (0% or 15% of respective PPFD). Plants grown under elevated CO₂ generally exhibited increased plant height and stem biomass than those grown under ambient CO₂ levels. Elevated CO₂ improved fruit yield under low light intensity (PPFD250 with or without far-red supplementation) compared to ambient CO₂; however, this effect was not observed under high light intensity. Light intensity had a stronger impact on total leaf area and fruit mass under ambient CO₂. Specifically, under ambient CO₂, plants grown at PPFD500 showed lower total leaf area but higher fruit biomass than those grown at PPFD250. However, under elevated CO₂, the effects of light intensity became less pronounced, with no significant difference in fruit mass among the light treatments. Supplementation with 15% far-red photons did not cause any significant differences in tomato growth and morphology. Overall, these findings indicate that elevated CO₂ level significantly enhances fruit production under low light intensity treatments, regardless of supplementary far-red photons. However, the potential benefits associated with high light intensity treatments (PPFD500 and PPFD500 + 15% far-red photons) appear to diminish when plants were grown under elevated CO₂ conditions.