LED Light Quality Impacts Plant Content of Nutritionally Important Pigments in Sprouting Broccoli

Light-emitting diodes (LED) are an alternative choice for lighting in controlled environments with the added benefit of light quality management.  Many studies now show changes in light quality can impact secondary metabolite accumulation in specialty leafy crops.  The objective of this study was to compare impacts of white LEDs with different ratios of blue/red LEDs on the content of nutritionally important pigments in Sprouting broccoli (Brassica oleracea var italica).  Broccoli were seeded into soil-less media and germinated under greenhouse conditions before transfer to controlled environment chambers 7 days after seeding (DAS).  Experimental sole source LED light quality treatments were: 1) white; 2) 5% blue (447 nm) / 95% red (627 nm); 3) 10% blue / 90% red; 4) 20% blue / 80% red; 5) 40% blue / 60% red; and 6) 60% blue / 40% red, with a 14-hour photoperiod and a light intensity of 250 µmol·m^-2·sec^-1 for all treatments.  The experiment was repeated three times.  All plants were harvested 30 DAS.  Broccoli plant shoot fresh mass (P=0.033) varied in response to light quality treatments.  Because of the impacts of light quality treatments on shoot fresh mass, pigments were analyzed on a per plant fresh mass content.  The content of antheraxanthin (P=0.002), β-carotene (P=0.032), chlorophyll a (P=0.014), chlorophyll b (P=0.009), lutein (P=0.035), zeaxanthin (P=0.016), and the pool of xanthophyll cycle pigments (zeaxanthin + antheraxanthin + violaxanthin; P=0.054) within the Sprouting broccoli plants varied with changes in LED light quality.  The 5% blue/95% red treatment resulted in higher content for most pigments compared to all other light treatments.  Interestingly, the content of zeaxanthin in the broccoli was higher under broad-spectrum white LEDs as compared to all other narrow-band light treatments.  The xanthophyll cycle pigments are vital for energy dissipation of excess absorbed light, and zeaxanthin is directly involved in preventing photo-oxidative stress.  Application of LEDs and proper management of light quality may be beneficial for improving the nutritional content of baby leafy greens in controlled environments.