Photosynthetic Responses of Swiss Chard, Kale, and Spinach Cultivars to Irradiance and Carbon Dioxide Concentration

The impact of irradiance (0-1200 mu mol.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 mu mol 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 mu mol.m(-2) .s(-1), respectively)] across varieties. The light saturation points (LSPs) for chard and kale were similar at 884-978 mu mol CO2.m(-2).s(-1) but for spinach, the LSP was higher at 1238 mu mol CO2.m(-2).s(-1). Dark respiration was lowest on kale and highest on spinach (-0.83 and - 5.00 mu mol 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 mu mol CO2.m(-2) .s(-1)). Spinach Pn 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.