Morphological and Photosynthetic Parameters of Green and Red Kale Microgreens Cultivated under Different Light Spectra

Abstract

Microgreens are plants eaten at a very early stage of development, having a very high nutritional value. Among a large group of species, those from the Brassicaceae family, including kale, are very popularly grown as microgreens. Typically, microgreens are grown under controlled conditions under light-emitting diodes (LEDs). However, the effect of light on the quality of grown microgreens varies. The present study aimed to determine the effect of artificial white light with varying proportions of red (R) and blue (B) light on the morphological and photosynthetic parameters of kale microgreens with green and red leaves. The R:B ratios were for white light (W) 0.63, for red-enhanced white light (W + R) 0.75, and for white and blue light (W + B) 0.38 at 230 µmol m^-2 s^-1 PPFD. The addition of both blue and red light had a positive effect on the content of active compounds in the plants, including flavonoids and carotenoids. Red light had a stronger effect on the seedling area and the dry mass and relative chlorophyll content of red-leaved kale microgreens. Blue light, in turn, had a stronger effect on green kale, including dry mass. The W + B light combination negatively affected the chlorophyll content of both cultivars although the leaves were significantly thicker compared to cultivation under W + R light. In general, the cultivar with red leaves had less sensitivity to the photosynthetic apparatus to the spectrum used. The changes in PSII were much smaller in red kale compared to green kale. Too much red light caused a deterioration in the PSII vitality index in green kale. Red and green kale require an individual spectrum with different proportions of blue and red light at different growth stages to achieve plants with a large leaf area and high nutritional value.

Keywords: Brassica oleracea var. acephala L.; LEDs; blue light; red light.

Figure 1

Spider plot of selected fluorescence parameters characterising the changes in PSII of green (A) and red (B) kale microgreens cultivated under different spectral compositions of light. All values are shown as per cent of the control (W light) (W light = 100). The parameters are divided into the following groups: energy fluxes, quantum yields and efficiencies, specific energy fluxes (calculated per single RC), and generalised parameters of PSII functional activity, i.e., performance indexes. (KGW—green kale under white light, KGB—green kale under white + blue light, KGR—green kale under white + red light, the same for KR—red kale). The presented data are mean values based on 20 replicates.

Figure 2

Changes in the non-photochemical fluorescence quenching (NPQ): (A) photochemical quenching in the relative reduction state of QA reflecting the fraction of open PSII reaction centres (qP) (B) and vitality index of PSII indicative of interactions between the light-stage reactions activated by PAR absorption and the dark reactions of photosynthesis (Rfd) (C) in green and red kale microgreens cultivated under different spectral compositions of light. Columns with different letters are statistically different for different lights and types of kale (p < 0.05). Mean values from two cycles represent the average of six plants per replication (experimental unit) and four replications per treatment in each cycle.

Figure 3

Raman spectra for microgreens grown in different spectral compositions of light together with the marked positions of the peaks characteristic of the relevant functional groups (carotenoids, polysaccharides, chlorophyll, lipids, carbohydrates, flavonoids, and proteins). (KGW—green kale under white light, KGB—green kale under white + blue light, KGR—green kale under white + red light, the same for KR—red kale). The spectra are means from 5 independent measurements.

Figure 4

The hierarchical clustering shows the relation to the spectral composition of light. (KGW—green kale under white light, KGB—green kale under white + blue light, KGR—green kale under white + red light, the same for KR—red kale). HCA was performed based on the whole FT-Raman spectra range of 800–1850 cm⁻¹ which gave more than 270 variables.

Figure 5

The spectral composition of light-emitting diodes (LED) used during microgreens growth with areas of blue (430–480 nm), red (620–700 nm), and far-red (700–780 nm) light highlighted. (A) One hundred per cent of white light, (B) white light enhanced in the blue light range, and (C) white light enhanced in the red light range.