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Plants 2019, 8, 93 3 of 12 treatment LED light chamber. Plants were provided with a half strength Hoagland nutrient solution. The pH and electrical conductivity in nutrient solution were measured weekly and adjusted to pH 5.8 and electrical conductivity to 1.85 mS cm−1. All leafy vegetables, including lettuce, spinach, basil, and kale, were harvested after five weeks of treatment and sweet pepper was harvested after seven weeks. The experiment was repeated twice. 2.2. LED Light Treatments Plants were illuminated by light emitting diodes (LEDs) with different percentages of red (R, 661 nm) and blue (B, 449 nm) light. Four spectral treatments were used in this study, namely 83% R + 17% B, 91% R + 9% B, 95% R + 5% B, and 100% R (control). The photoperiod was 16/8 h (day/night), photosynthetic photon flux density (PPFD) was 200 ± 5 μmol m−2 s−1, day/night temperature was 21 ◦C ± 2.5 ◦C, and relative humidity was 65 ± 5% in the growth chamber. The LED lights were prototypes from General Electric Lighting Solutions (Lachine, QC, Canada). These consisted of 1.78 m × 0.08 m × 0.02 m linear fixtures, on which were placed an array of 16 LEDs. Irradiance was measured routinely using a quantum sensor (MQ-200; Apogee Instruments, Logan, UT, USA). 2.3. Plant Growth and Biomass Measurement Plant height and the number of leaves from the plants were recorded after five weeks. Plant height was measured to the tip of the youngest leaf. The fresh mass (FM) of the plant parts were determined immediately after harvesting. The drying temperature was 80 ◦C and duration of drying was no less than 72 h until a stable mass was attained. Plant tissues were weighed again after drying to record dry biomass. Leaf tissue samples and sweet pepper fruits were frozen (−22 ◦C) before lyophilization. Phytochemicals and antioxidant properties were analysed from freeze dried leaf tissues (leafy vegetables) and fruit tissues (pepper). 2.4. Chlorophyll and Carotenoid Analysis For chlorophyll and carotenoid analysis, the method described by Lichtenthaler [26] was used. Aliquots of 100 mg of dried and ground leaf tissue and sweet pepper fruit tissue were added to 10 mL of 80% acetone, mixed well, and incubated overnight at 4 ◦C in darkness. The supernatant was withdrawn after centrifugation at 5000 rpm for 10 min at 25 ◦C and absorbance were recorded at 646.8, 663.2, and 470 nm against 80% acetone blank using a spectrophotometer (PowerWaveTM XS Microplate Reader, BioTek Instruments, Inc. Vermont, USA). The amount of chlorophyll a, chlorophyllb, total chlorophyll, and carotenoids were determined according to Lichtenthaler (1987). 2.5. Antioxidant Capacity Measurement Antioxidant activity was measured by the scavenging capacity of the stable 2, 2-diphenyl-1 picryl hydrazyl (DPPH) free radical assays [27,28]. Briefly, 25 μL of appropriately diluted samples were added to 200 μL of DPPH solution (dissolved in methanol) in a well of a 96-well plate. The mixture was allowed to react at room temperature in the dark for 4 h before the absorbance was measured at 517 nm against a methanol blank using the spectrophotometer (PowerWaveTM XS Microplate Reader, BioTek Instruments, Inc. Vermont, USA). The radical scavenging activity (RSA) percentage was determined from Equation (1), considering the absorbance of methanol as the control (A0) and the sample (Ax): RSA (%) = [(A0 − Ax)/A0] × 100 (1) 2.6. Statistical Analysis The statistical analysis was performed using analysis of variance (ANOVA). Data analysis was processed by one-way analysis of variance (ANOVA), combined with the Tukey’s (Tukey Simultaneous 95% CIs) least significant difference (LSD) test at the confidence levels of p < 0.05 by using MinitabPDF Image | Blue Light added with Red LEDs Enhance Growth Characteristics
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