Rocket salad (Eruca sativa (Mill.) Thell.) is a very popular fresh vegetable belonging to the Brassicaceae family. Nitrate (NO3−) considers the most important form of nitrogen easily taken up in large amounts by most horticultural crops. Light-emitting diodes (LED) are considered a promising technology for greenhouses and can be used either as a main or as a supplementary light source. Some studies indicated that LED light influenced the quality of horticultural products. Accordingly, the objective of this study was to evaluate the effects of various LED with different spectral composition when provided as supplemental lighting on activities of enzymes involved in NO3− assimilation (i.e. nitrate reductase (NR), nitrite reductase (NiR), and glutamine synthetase (GS)) of rocket plants in greenhouse conditions. The analysis of variance (ANOVA) results demonstrated that the activities of enzymes were significantly affected by light treatments. Between LED treatments, 50% red + 50% blue and 25% red + 75% blue exhibited the highest NR activities. Also, LED lights caused a remarkable increase in NiR activity compared to control conditions. Blue and 25% red + 75% blue treatments represented the highest enhancement in NiR activity relative to control (74.1% and 61.2%, respectively). In addition, 75% red + 25% blue and 50% red + 50% blue had the highest GS activities. Our experiments indicated that LED as supplemental lighting could be used to stimulate the activity of nitrogen metabolism enzymes and thus decrease NO3− and NH4+ accumulation in rocket plants.
