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

Abstract

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 (Pn; per area basis) was determined to facilitate maximizing yield in controlled environment production. Spinach, chard, and kale maximum Pn 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 (−0.83 and −5.00 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 Pn 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 Pn was more responsive to increasing irradiance than to CO2. Kale Pn was more responsive to increasing CO2 than to irradiance, and chard Pn was equally responsive to increasing CO2 or irradiance. Implications and limitations of this work when “upscaling” to whole-plant responses are discussed.