Increases in Shoot Tissue Pigments, Glucosinolates, and Mineral Elements in Sprouting Broccoli after Exposure to Short-duration Blue Light from Light Emitting Diodes

Abstract

Microgreens are specialty leafy crops harvested just above the roots after the first true leaves have emerged and are consumed fresh. Broccoli (Brassica oleacea var. italica) microgreens can accumulate significant concentrations of cancer-fighting glucosinolates as well as being a rich source of other antioxidant phytochemicals. Light-emitting diodes (LEDs) now provide the ability to measure impacts of narrow-band wavelengths of light on seedling physiology. The carotenoid zeaxanthin has been hypothesized to be a blue light receptor in plant physiology. The objective of this study was to measure the impact of short-duration blue light on phytochemical compounds, which impart the nutritional quality of sprouting broccoli microgreens. Broccoli microgreens were grown in a controlled environment under LEDs using growing pads. Seeds were cultured on the pads submerged in deionized water and grown under a 24-hour photoperiod using red (627 nm)/blue (470 nm) LEDs (350 μmol·m−2·s−1) at an air temperature of 23 °C. On emergence of the first true leaf, a complete nutrient solution with 42 mg·L−1 of nitrogen (N) was used to submerge the growing pads. At 13 days after sowing, broccoli plantlets were grown under either: 1) red and blue LED light (350 μmol·m−2·s−1); or 2) blue LED light (41 μmol·m−2·s−1) treatments for 5 days before harvest. The experiment was repeated three times. Frozen shoot tissues were freeze-dried and measured for carotenoids, chlorophylls, glucosinolates, and mineral elements. Comparing the two LED light treatments revealed the short-duration blue LED treatment before harvest significantly increased shoot tissue β-carotene (P ≤ 0.05), violaxanthin (P ≤ 0.01), total xanthophyll cycle pigments (P ≤ 0.05), glucoraphanin (P ≤ 0.05), epiprogoitrin (P ≤ 0.05), aliphatic glucosinolates (P ≤ 0.05), essential micronutrients of copper (Cu) (P = 0.02), iron (Fe) (P ≤ 0.01), boron (B), manganese (Mn), molybdenum (Mo), sodium (Na), zinc (Zn) (P ≤ 0.001), and the essential macronutrients of calcium (Ca), phosphorus (P), potassium (K), magnesium (Mg), and sulfur (S) (P ≤ 0.001). Results demonstrate management of LED lighting technology through preharvest, short-duration blue light acted to increase important phytochemical compounds influencing the nutritional value of broccoli microgreens.