Blue Light-emitting Diode Treatments Act to Decrease Kale Leaf Tissue Zeaxanthin Concentrations and Non-photochemical Quenching, Thereby Demonstrating a Less Stressful Light Environment

Previous research in our group demonstrated that narrow-band wavelengths from light-emitting diodes (LEDs) improved concentrations of mineral nutrients and health promoting glucosinolates and carotenoids in broccoli (Brassica oleracea) microgreens. The objectives of this study were to: 1) measure the impact of light quality on the concentrations of nutritionally important pigments in baby 'Dwarf Siberian' kale (Brassica oleracea cv. Acephala group); 2) measure the impacts on non-photochemical quenching (NPQ); and 3) correlate NPQ with kale xanthophyll cycle pigments.  Non-photochemical quenching is a mechanism employed by plants to protect light-harvesting complexes from the adverse effects of high light intensity, and indicates plant stress.  Therefore, we compared the LEDs with incandescent/fluorescent light to understand how the treatments differ from regular growth chamber light conditions.  Seeds were sown in 10 cm pots in soilless media and upon emergence of the first true leaf were fertilized with 80 mg N/L solution of 20–20–20 soluble fertilizer (100 mL per pot).  Furthermore, light treatments of: 1) fluorescent/incandescent light; 2) 5% blue (455–470 nm)/95% red (627–630 nm); and 3) 20% blue/80% red were applied in controlled growth chamber environments.  The light intensity was set at 250 µmol·m^-2·s^-1 for all light treatments with a 16-h photoperiod and air temperature of 24 °C.  After 30 days under the light treatments, whole kale plants were assessed for NPQ with a pulse-amplitude modulated fluorimeter.  Consequently, kale plants were harvested and shoot tissues were freeze dried and measured for chlorophyll and carotenoid pigments.  Exposure to blue light LED treatments significantly increased shoot tissue violaxanthin, neoxanthin, zeaxanthin, lutein, total xanthophylls, chlorophyll a, chlorophyll b, and total chlorophyll when compared to the incandescent/fluorescent light treatment.  Results also revealed a significant positive correlation among NPQ and leaf tissue zeaxanthin concentrations, which indicated LEDs provided a less stressful light environment.