Abstract
Ultraviolet A (UV-A) light-emitting diodes (LEDs) could serve as an effective tool for improving the content of health-promoting bioactive compounds in plants in controlled-environment agriculture (CEA) systems. The goal of this study was to investigate the effects of UV-A LEDs at different wavelengths (366, 390, and 402 nm) and durations (10 and 16 h) on the growth and phytochemical contents of mustard microgreens (Brassica juncea L. cv. “Red Lion”), when used as supplemental light to the main LED lighting system (with peak wavelengths of 447, 638, 665, and 731 nm). Plants were grown for 10 days under a total photon flux density (TPFD) of 300 µmol m−2 s−1 and 16-h light/8-h dark period. Different UV-A wavelengths and irradiance durations had varied effects on mustard microgreens. Supplemental UV-A radiation did not affect biomass accumulation; however, the longest UV-A wavelength (402 nm) increased the leaf area of mustard microgreens, regardless of the duration of irradiance. The concentration of the total phenolic content and α-tocopherol mostly increased under 402-nm UV-A, while that of nitrates increased under 366- and 390-nm UV-A at both radiance durations. The contents of lutein/zeaxanthin and β-carotene increased in response to the shortest UV-A wavelength (366 nm) at 10-h irradiance as well as longer UV-A wavelength (390 nm) at 16 h irradiance. The most positive effect on the accumulation of mineral elements, except iron, was observed under longer UV-A wavelengths at 16-h irradiance. Overall, these results suggest that properly composed UV-A LED parameters in LED lighting systems could improve the nutritional quality of mustard microgreens, without causing any adverse effects on plant growth.
Highlights
Because of the rising awareness of health, people are changing their dietary habits and consuming more vegetables and herbs that are low in calories and an important source of minerals, vitamins, and other beneficial phytochemicals
DPPH free-radical scavenging activity was higher at the 10-h duration of Ultraviolet A (UV-A) irradiance (EXP1), whereas Total Anthocyanin (TA) and Ascorbic Acid (AA) contents decreased under these lighting conditions
The content of AA increased significantly after 16-h exposure to 402 nm UV-A, while the α-tocopherol content was significantly higher at 402 nm, regardless of the duration, and at 390 nm and 16-h irradiance compared with other treatments
Summary
Because of the rising awareness of health, people are changing their dietary habits and consuming more vegetables and herbs that are low in calories and an important source of minerals, vitamins, and other beneficial phytochemicals. This has led to higher requirements for the nutritional quality of vegetables, which can be enhanced by the regulation of environmental and agronomic factors. Traditional light sources such as fluorescent, high-pressure sodium (HPS), metal halide, and incandescent lamps have been used in CEA systems These artificial light sources are not spectrally optimal or energy efficient. It is well known that red and blue lights are absorbed by leaves better than other regions of the visible spectrum, and LEDs of such wavelengths are categorized as having the highest photon efficiency; the effect of red to blue light ratio on plants has been detailed in many studies (Massa et al, 2008; Olle and Viršile, 2013; Nelson and Bugbee, 2014; Mitchell et al, 2015; Bugbee, 2016)
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