Photosynthetic responses of swiss chard, kale, and spinach cultivars to irradiance and carbon dioxide concentration

The impact of irradiance (0-1200 µmol·m^-2·s^-1) and carbon dioxide concentration (CO2; 50-1200 ppm) on kale (Brassica oleracea and B. napus pabularia; three cultivars), Swiss chard (chard, Beta vulgaris; four cultivars), and spinach (Spinacea oleracea; three cultivars) photosynthetic rate (P_n; per area basis) was determined to facilitate maximizing yield in controlled environment production. Spinach, chard, and kale maximum P_n were 23.8, 20.3, and 18.2 µmol CO₂·m^-2·s^-1 fixed, respectively, across varieties (400 ppm CO₂). Spinach and kale had the highest and lowest light compensation points [LCPs (73 and 13 µmol·m^-2·s^-1, respectively)] across varieties. The light saturation points (LSPs) for chard and kale were similar at 884-978 µmol·m^-2·s^-1, but for spinach, the LSP was higher at 1238 µmol·m^-2·s^-1. Dark respiration was lowest on kale and highest on spinach (L0.83 andL5.00 µmol CO₂·m^-2·s^-1, respectively). The spinach CO₂ compensation point (CCP) was lower (56 ppm) than the chard or kale CCP (64-65 ppm). Among varieties, ‘Red Russian’ kale P_n saturated at the lowest CO₂ concentration (858 ppm), and ‘Bright Lights’ chard saturated at the highest (1266 ppm; 300 µmol·m^-2·s^-1). Spinach P_n was more responsive to increasing irradiance than to CO₂. Kale P_n was more responsive to increasing CO₂ than to irradiance, and chard P_n was equally responsive to increasing CO₂ or irradiance. Implications and limitations of this work when “upscaling” to whole-plant responses are discussed.