Light Spectrum Differentially Affects the Yield and Phytochemical Content of Microgreen Vegetables in a Plant Factory

Abstract

Light quality exerts considerable effects on crop development and phytochemical content. Moreover, crops grown as microgreens are ideal for plant factories with artificial lighting, since they contain greater amounts of bioactive compounds compared to fully-grown plants. The aim of the present study was to evaluate the effect of broad-spectra light with different red/blue ratios on the yield, morphology, and phytochemical content of seven microgreens. Mustard, radish, green basil, red amaranth, garlic chives, borage, and pea shoots were grown in a vertical farming system under three light sources emitting red/blue ratios of about 2, 5, and 9 units (RB2, RB5, and RB9, respectively). Mustard exhibited the most profound color responses. The yield was enhanced in three microgreens under RB9 and in garlic under RB2. Both the hypocotyl length and the leaf and cotyledon area were significantly enhanced by increasing the red light in three microgreens each. Total soluble solids (Brix) were reduced in 4 microgreens under RB2. The total phenolic content and antioxidant capacity were reduced under RB2 in 6 and 5 microgreens, respectively. The chlorophylls were variably affected but total the carotenoid content was reduced in RB9 in three microgreens. Overall, light wavelength differentially affected the microgreens' quality, while small interplays in spectral bands enhanced their phytochemical content.

Keywords: Brassica; PFAL; antioxidant content; artificial lighting; carotenoids; controlled environment agriculture; phenolics; photomorphogenesis; sprouts; vertical farming.

Figure 1

Colorimetric parameters (A) lightness and (B) hue angle of seven microgreens grown in a plant factory under three light treatments. (C) Mustard microgreens showing the most profound responses to light spectra among the studied species. Within each microgreen species, bars (± SE) followed by different letters are significantly different (p ≤ 0.05). Mean values were computed from n = 24 measurements. RB2: red/blue ratio = 2; RB5: red/blue ratio = 5; RB9: red/blue ratio = 9; a.u.–arbitrary unit.

Figure 1

Colorimetric parameters (A) lightness and (B) hue angle of seven microgreens grown in a plant factory under three light treatments. (C) Mustard microgreens showing the most profound responses to light spectra among the studied species. Within each microgreen species, bars (± SE) followed by different letters are significantly different (p ≤ 0.05). Mean values were computed from n = 24 measurements. RB2: red/blue ratio = 2; RB5: red/blue ratio = 5; RB9: red/blue ratio = 9; a.u.–arbitrary unit.

Figure 2

(A) Hypocotyl length, (B) leaf and cotyledon area, and (C) yield of seven microgreens grown in a plant factory under three light treatments. Within each microgreen species, bars (± SE) followed by different letters are significantly different (p ≤ 0.05). Mean values were computed from n = 24 (hypocotyl length, and leaf and cotyledon area) or n = 6 (yield) measurements. RB2: red/blue ratio = 2; RB5: red/blue ratio = 5; RB9: red/blue ratio = 9.

Figure 3

(A) Total soluble solids, (B) total phenolic content, and (C) antioxidant capacity (FRAP) of seven microgreens grown in a plant factory under three light treatments. Within each microgreen species, bars (±SE) followed by different letters are significantly different (p ≤ 0.05). Mean values were computed from n = 3 measurements. RB2: red/blue ratio = 2; RB5: red/blue ratio = 5; RB9: red/blue ratio = 9.

Figure 3

(A) Total soluble solids, (B) total phenolic content, and (C) antioxidant capacity (FRAP) of seven microgreens grown in a plant factory under three light treatments. Within each microgreen species, bars (±SE) followed by different letters are significantly different (p ≤ 0.05). Mean values were computed from n = 3 measurements. RB2: red/blue ratio = 2; RB5: red/blue ratio = 5; RB9: red/blue ratio = 9.

Figure 4

(A) Chlorophyll a, (B) chlorophyll b, and (C) total carotenoid contents of seven microgreens grown in a plant factory under three light treatments. Within each microgreen species, bars (± SE) followed by different letters are significantly different (p ≤ 0.05). Mean values were computed from n = 3 measurements. RB2: red/blue ratio = 2; RB5: red/blue ratio = 5; RB9: red/blue ratio = 9.

Figure 5

Spectral distribution of (A) RB2, (B) RB5, and (C) RB9 LED light treatments. RB2: red/blue ratio = 2; RB5: red/blue ratio = 5; RB9: red/blue ratio = 9.