Optimizing Carbon Dioxide Enrichment to Balance Yield, Functional Food Quality, and Economic Feasibility in Plant-Factory-Cultivated Kale

Kale is widely recognized as a nutritional superfood. This study investigated the impact of carbon dioxide (CO2) concentrations (400, 800, and 1200 µmol mol−1) on the growth, yield, physiological responses, and nutritional contents of two kale cultivars (‘Curly Kale’ and ‘Red Ursa’) grown in a plant factory. A completely randomized design was used to evaluate these parameters. Based on the results, increasing the CO2 concentration to 1200 µmol mol−1 significantly enhanced stem height, shoot, and root fresh weight and dry weight in ‘Curly Kale’ and ‘Red Ursa’, compared to the other CO2 concentrations. Increasing CO2 concentration to 1200 µmol mol−1 significantly enhanced net photosynthesis rate, stomatal conductance, transpiration rate, and water use efficiency in ‘Curly Kale’. In addition, compared to ambient CO2, the increase in the CO2 concentration to 800 µmol mol−1 significantly increased the vitamin C, soluble protein, and total phenolic contents, while reducing the nitrate accumulation in both cultivars. However, further elevation to 1200 µmol mol−1 CO2 markedly decreased the vitamin C content and total amino acids, including both the essential and non-essential amino acids. Among the tested concentration gradients, 800 µmol mol−1 CO2 was identified as the most cost-effective level for maintaining nutrient density, whereas 1200 µmol mol−1 CO2 increased unit production costs for ‘Red Ursa’ due to a lack of significant yield returns. In conclusion, enriching the CO2 concentration to 800 µmol mol−1 provided a balance between improved growth, photosynthetic performance, and optimal nutritional quality, while ensuring economic feasibility and preserving the superfood identity of kale.