Effect of LED Lights on Secondary Metabolites and Antioxidant Activities in Red Pakchoi Baby Leaves

Abstract

Pakchoi (Brassica rapa subsp. chinensis) is one of the most widely consumed vegetables in Asian countries, and it is high in secondary metabolites. The availability, quantity, and quality of light play a critical role in the growth and development of plants. In this study, we investigated the effect of LEDs (light-emitting diodes; white, blue, red, and red + blue) on anthocyanin, glucosinolates, and phenolic levels in red pakchoi baby leaves. On the 24th day after sowing (DAS), red baby pakchoi leaves were harvested, and shoot length, root length, and fresh weight were measured. Among the different LED treatments, there was no significant difference in shoot length, whereas the highest root length was achieved in the red + blue LED treatment (23.8 cm). The fresh weight also showed a significant difference among the different LED treatments. In total, 12 phenolic and 7 glucosinolate individual compounds were identified using high-performance liquid chromatography (HPLC) analysis. The highest total glucosinolate (2937 μg/g dry wt) and phenolic (1589 μg/g dry wt) contents were achieved in baby leaves exposed to red + blue light. Similarly, the highest contents of total anthocyanins (1726 μg/g dry wt), flavonoids (4920 μg/g dry wt), and phenolics (5900 μg/g dry wt) were achieved in the red + blue treatment. Plants exposed to red + blue LED light showed the highest accumulation of anthocyanin, glucosinolates, and phenolic compounds. For antioxidant activity, DPPH (2,2-diphenyl-1-picrylhydrazylradical) free radical scavenging, ABTS (2,2-azinobis (3-ethylbenzothiazoline)-6-sulfonic acid) radical scavenging, and reducing power assays were performed, and the antioxidant activity of red pakchoi baby leaves grown under red + blue LED light was found to be the best. The metabolic profiling of the identified metabolites revealed distinct separation based on the secondary metabolites. This research will be helpful for farmers to choose the best LED light combination to increase the secondary metabolic content in pakchoi plants.

Figure 1

(A, B) Effect of different LED lights on shoot length and root length in pakchoi baby leaves was determined 24 DAS in growth chambers. Different letters denote a significant difference in means (p < 0.05) using the Tukey test.

Figure 2

Effect of different LED lights on fresh weight in pakchoi baby leaves was determined 24 DAS in growth chambers. Different letters denote a significant difference in means (p < 0.05) using the Tukey test.

Figure 3

Heatmap with the Euclidean distance measure of relative contents of metabolites in pakchoi baby leaves grown under different LED lights. The relative metabolite concentrations are represented by the color scale (2 to −2) on the right, with high and low concentrations denoted by red and blue, respectively.

Figure 4

PCA (A) and PLS-DA (B) scores and loading plots of the metabolites found in pakchoi leaves grown under different LED lights.

Figure 5

Main components separating pakchoi leaves grown under different LED lights are based on the VIP scores attained via the PLS-DA model. The VIP scores are shown by the black dots, reflecting the degree of importance of the metabolites.

Figure 6

(A,B) DPPH and ABTS free radical scavenging activity in red pakchoi baby leaves grown under different LED lights. Different letters in the values denote statistically significant differences among the means, using the Tukey test (p < 0.05), while values are shown as mean ± SD.