Day/night Temperature Effects on Kale Photosynthesis and Leaf Morphology

Red Russian Kale (Brassica napus var. pabularia) was grown under 25 day/night temperature combinations to quantify differences in yield, leaf morphology, development rate and impact on the photosynthetic capacity/behavior. Kale was germinated and growth under a 24/13 C day/night temperature with an 18 h photoperiod. Plants were moved to five environmental growth chambers (maintained to achieve a 10, 15, 20, 25 and 30C leaf temperature) with an 8 h photoperiod (300 μmol m^-2s^-1) when four true leaves had unfolded. Plants were moved among chambers at 0800 and 1600HR daily to achieve 25 day/night temperature regimes. Plants were grown in Sunshine LC8 media, were fertilized twice weekly with a 400ppm N solution of Cal-Mag 15-5-15, and were watered as needed otherwise, with care taken not to leach pots. After 56 d, photosynthetic responses to temperature (10-30C; 5 C interval), irradiance (0-1200 μmol m^-2s^-1; 200 μmol m-2 s-1 interval), and carbon dioxide (CO₂) concentration (50-1200 ppm; 200 ppm interval) were collected on the youngest fully expanded leaf using a LI-COR LI-6400XT Portable Photosynthesis System. When one environmental variable was assessed, the other two were held constant:  temperature=20C, irradiance=300 umol m-2 s-1, CO2=400 ppm.  Photosynthesis of plants grown with 10-25C day temperatures saturated at 800 μmol m-2 s-1, while photosynthesis of plants grown with a 30C day temperature saturated at 600 μmol m-2 s-1 (400 ppm CO2, 20C).   Among plants growth at constant temperatures, the maximum and minimum photosynthetic rate occurred on plants grown at 10C (21.3 μmol m-2 s-1 CO2 fixed) and 25C (9.77 μmol m-2 s-1 CO2 fixed) when plants were exposed to high irradiance (1200 μmol m^-2s^-1) and ambient CO2 (400 ppm).   Among plants growth at constant temperatures, the maximum and minimum photosynthetic rate occurred on plants grown at 15C (19.5 μmol m-2 s-1 CO2 fixed) and 25C (14.2 μmol m-2 s-1 CO2 fixed) when plants were exposed to high CO2 (1200 ppm) and ambient irradiance (300 μmol m-2 s-1). Data were also collected on unfolded leaf number, leaf fresh and dry weight, and leaf dimensions (up to 4 expanded leaves). Fresh weight ranged from 8.2 g/leaf in the 20/10C day/night temperature regime to 1.6 g/leaf in the 10/30C day/night temperature regime.  Photosynthetic response curves, leaf unfolding rates, and leaf fresh weight data presented here provide growers with critical information to optimize yield and make temperature, irradiance and CO2 management decisions.