Effect of LED Spectrum on the Quality and Nitrogen Metabolism of Lettuce Under Recycled Hydroponics

Abstract

Light quality optimization is an efficient method for improving the growth and quality of lettuce in plant factories. In this study, lettuce seedlings were illuminated under different light-emitting diode (LED) lights, namely, red-blue (RB), red-blue-green (RBG), red-blue-purple (RBP), and red-blue-far-red (RBF) LED lights, to investigate the effect of light quality on growth, quality, and nitrogen metabolism. The combination of 75% red and 25% blue light was set as the basic light source, and 20% of green, purple and far-red light were added to basic light source, respectively. All the treatments were set to 200 μmol m–2 s–1. Results showed that the fresh weight and dry weight of aboveground lettuce under RBG, RBP, and RBF treatments were significantly lower than those under the RB treatment because of the decrease in the effective photon flux density for chlorophyll absorption. The vitamin C content of the lettuce leaves was increased by about 23% with the addition of purple light. For nitrate reduction, the addition of green light significantly increased the nitrite content of the lettuce leaves. It also promoted the reduction from nitrite to ammonium through the activation of the nitrite reductase (NiR) expression and enzyme activity. The nitrate and ammonium content decreased with the addition of purple light because of the inhibited NR and NiR expression and enzyme activity. For nitrogen assimilation, individual (e.g., Asp, Glu, and Leu) and total amino acids were induced to increase by adding green, purple, and far-red light. The addition of light was hypothesized to have inhibited protein biosynthesis, thereby causing the accumulation of amino acids. Correlation analysis showed that the relative expression levels between HY5 and NR/NiR presented a significantly negative correlation. Transcription factor HY5 might mediate the regulation of light quality on nitrogen metabolism by inhibiting NR and NiR expressions. It might also exert a negative effect on nitrate reduction. Further studies via genome editing techniques on the identification of HY5 functions for nitrate assimilation will be valuable. Nevertheless, the results of this work enrich the understanding of the effect of light quality on nitrate metabolism at the level of gene expression and enzyme activity.

Differential Metabolic Responses of Lettuce Grown in Soil, Substrate and Hydroponic Cultivation Systems under NH4+/NO3− Application

“…A few studies on the composition of lettuce metabolites in hydroponic systems have been reported [ 37 , 38 , 39 ]. It has not been fully explored how different NH 4 + /NO 3 − ratios affect the sugar compounds, amino acid contents and polyphenolic compounds in lettuce and how these metabolic changes influence nutritional values in lettuce.…”

Cold Stress Resistance of Tomato (Solanum lycopersicum) Seedlings Is Enhanced by Light Supplementation From Underneath the Canopy

“…The gas exchange, chlorophyll fluorescence, and P700 redox state were measured in vivo by using the LI-6400XT Photosynthesis System (Li-Cor Incorporation, United States) and the Dual PAM-100F (Heinz Walz, Effeltrich, Germany) as described in previous reports ( Grieco et al, 2012 ; Pietrzykowska et al, 2014 ). The light-adapted curves were recorded after 2 min of exposure to various PPFDs ( Lu et al, 2019 ; Li et al, 2021 ).…”

“…Phy A is the predominant photoreceptor of far-red (FR) light and phy B is the primary photoreceptor of red (R) light ( Chen and Chory, 2011 ). In addition, the transcription factor ELONGATED HYPOCOTYL5 (HY5) can be activated by photoreceptors to promote downstream photomorphogenesis ( Li et al, 2021 ). Many studies have shown that the above molecules play key roles in cold tolerance ( Chen and Chory, 2011 ; Li et al, 2021 ; Wang et al, 2021 ).…”

“…In addition, the transcription factor ELONGATED HYPOCOTYL5 (HY5) can be activated by photoreceptors to promote downstream photomorphogenesis ( Li et al, 2021 ). Many studies have shown that the above molecules play key roles in cold tolerance ( Chen and Chory, 2011 ; Li et al, 2021 ; Wang et al, 2021 ). It has been shown that light signals regulate chloroplast avoidance movement through phy to reduce photodamage in plants ( Kasahara et al, 2002 ; Jaedicke et al, 2012 ; Suetsugu et al, 2017 ).…”

“…To maintain constant light intensity at the top of the canopy, the height of the lamp needs to be adjusted constantly; however, light at the bottom of the canopy is inevitably reduced ( Rowse et al, 2016 ). These light sources also produce heat that is conducive to crop growth, but as thermal light sources, they cannot be placed very close to the plant surface or they will easily burn young tissues and cause leaf photoinhibition ( Niinemets and Keenan, 2012 ; Li et al, 2021 ). In comparison, light-emitting diodes (LEDs) are considered to be a suitable light source for interlighting because of their low heat production (less likely to burn leaves), non-residual and non-toxic effects, and long operating lifetimes.…”