Photoperiod and Irradiance Affect Red Russian Kale Leaf Nutrient Content

Red Russian Kale plants (Brassica napus var. pabularia) were grown under short days (8 hour; daylight from 0800–1600 HR) or long days (18 hour; daylight plus supplemental lighting from 0600–2200 HR) in a greenhouse with a 24/13 °C day/night temperature regime in St. Paul, MN.  Under each photoperiod, plants were grown under either of four irradiance levels. Irradiance levels were ambient daylight, +25, +50 or +100 µmol·m^-2 ·s^-1 supplemental high pressure sodium lighting, resulting in eight photoperiod/irradiance lighting regimes from December to February. Plants were fertilized with 15-5-15 Cal-Mag fertilizer (200 ppm N) in the irrigation water as needed. Samples (fifth leaf from bottom of stem) were collected from five plants (1 sample per plant) from each photoperiod/irradiance lighting regime. Total leaf number was also counted, leaf fresh and dry weight were determined, and leaf nutrient content (P, K, Ca, Mg, Fe, Mn, Zn, Cu, and B) was determined using an inductively-coupled plasma spectrophotometer. Total dietary fiber and vitamin C levels were measured from tissue frozen at –80 °C. Leaf total dietary fiber was highest (4.29 g/100 g tissue) and lowest (3.32 g/100g tissue) on plants grown under the 8-h photoperiod with 100 µmol·m^-2·s^-1 supplemental irradiance, and 8-h photoperiod under ambient light, respectively. Vitamin C content was highest (40.8 mg/100g) in plants grown under an  8-h photoperiod plus 100 µmol·m^-2·s^-1 supplemental irradiance. Results indicate vitamin C and total dietary fiber content of Red Russian Kale can be optimized for plants grown at 24/13 °C day/night temperature under an 8-h photoperiod plus 100  µmol·m^-2·s^-1 supplemental high pressure sodium lighting.  Photoperiod and irradiance effects on leaf mineral content will also be discussed.