Unraveling the Role of Red:Blue LED Lights on Resource Use Efficiency and Nutritional Properties of Indoor Grown Sweet Basil

Abstract

Indoor plant cultivation can result in significantly improved resource use efficiency (surface, water, and nutrients) as compared to traditional growing systems, but illumination costs are still high. LEDs (light emitting diodes) are gaining attention for indoor cultivation because of their ability to provide light of different spectra. In the light spectrum, red and blue regions are often considered the major plants' energy sources for photosynthetic CO₂ assimilation. This study aims at identifying the role played by red:blue (R:B) ratio on the resource use efficiency of indoor basil cultivation, linking the physiological response to light to changes in yield and nutritional properties. Basil plants were cultivated in growth chambers under five LED light regimens characterized by different R:B ratios ranging from 0.5 to 4 (respectively, RB_0.5, RB₁, RB₂, RB₃, and RB₄), using fluorescent lamps as control (CK₁). A photosynthetic photon flux density of 215 μmol m^-2 s^-1 was provided for 16 h per day. The greatest biomass production was associated with LED lighting as compared with fluorescent lamp. Despite a reduction in both stomatal conductance and PSII quantum efficiency, adoption of RB₃ resulted in higher yield and chlorophyll content, leading to improved use efficiency for water and energy. Antioxidant activity followed a spectral-response function, with optimum associated with RB₃. A low RB ratio (0.5) reduced the relative content of several volatiles, as compared to CK₁ and RB ≥ 2. Moreover, mineral leaf concentration (g g^-1 DW) and total content in plant (g plant^-1) were influences by light quality, resulting in greater N, P, K, Ca, Mg, and Fe accumulation in plants cultivated with RB₃. Contrarily, nutrient use efficiency was increased in RB ≤ 1. From this study it can be concluded that a RB ratio of 3 provides optimal growing conditions for indoor cultivation of basil, fostering improved performances in terms of growth, physiological and metabolic functions, and resources use efficiency.

Keywords: Ocimum basilicum L.; energy use efficiency (EUE); land surface use efficiency (SUE); nutrient use efficiency (NUE); plant factories with artificial lighting (PFALs); water use efficiency (WUE).

FIGURE 1

Growth and resources use efficiency in basil plants grown under LED lights with different R:B ratio (closed symbols) or under fluorescent lights (open symbols). (A) Fresh weight, (B) chlorophyll content, (C) water use efficiency (WUE), (D) Energy Use Efficiency (EUE), (E), land Surface Use Efficiency (SUE) of a single layer (circles), or a vertical farming structure of five-layers (triangles) and ten-layers (square), and (F) overall Nutrient Use Efficiency (NUE) (based on total content of N, P, K, Ca, Mg, and Fe and expressed as g FW g⁻¹ nutrients). Data referring to experiments 1, 2, 3, and 4 (charts A–E, 36 replicate plants) and experiments 2 and 3 (chart F, 18 replicate plants) are indicated as mean ± SE. Different letters indicate significant differences at P ≤ 0.05.

FIGURE 2

(A) Fresh weight, (B) chlorophyll index, and (C) PSII quantum efficiency of basil plants grown under LED lights with different R:B ratio in the spectrum (RB_0.5, RB₁, RB₂, RB₃, and RB₄). The data, referring to experiment 5, are presented as mean values ± SE (15 replicate plants). Different letters indicate significant differences at P ≤ 0.05. In (D), whole plant images created using PlantExplorer^TM are included. The greater presence of red pixels indicates lower values of F_q′/F_m′, whereas higher yellow/green areas indicate higher F_q′/F_m′ values, as defined in the integrated legend.

FIGURE 3

(A) Stomatal conductance in basil plants grown under LED lights with different R:B ratio in the spectrum or under fluorescent light (open symbol). Mean values (experiments 3 and 4) ± SE (18 replicate plants). Different letters indicate significant differences at P ≤ 0.05. (B) Linear relationship in experiment 3 (red) and experiment 4 (green) between mean values of stomatal conductance and Water Use Efficiency (WUE). Open symbols represent CK₁.

FIGURE 4

(A) Antioxidant capacity (FRAP) and (B) total flavonoids concentration in basil plants grown under LED lights with different R:B ratio in the spectrum or under fluorescent light (open symbol). Data referring to experiments 2 and 3 are indicated as mean ± SE (18 replicate plants). Different letters indicate significant differences at P ≤ 0.05.

FIGURE 5

Fold change from control treatment (fluorescent light, CK₁) in the amount of selected volatiles in basil leaves from plants grown under LED lights with different R:B ratio in the spectrum (RB_0.5, RB₁, RB₂, RB₃, and RB₄). Mean values of 3 replicate plants per treatment (experiment 3) ± SE (3 replicate plants).