Increased Plant Quality, Greenhouse Productivity and Energy Efficiency with Broad-Spectrum LED Systems: A Case Study for Thyme (Thymus vulgaris L.)

Abstract

A light-emitting diode (LED) system covering plant-receptive wavebands from ultraviolet to far-red radiation (360 to 760 nm, "white" light spectrum) was investigated for greenhouse productions of Thymus vulgaris L. Biomass yields and amounts of terpenoids were examined, and the lights' productivity and electrical efficiency were determined. All results were compared to two conventionally used light fixture types (high-pressure sodium lamps (HPS) and fluorescent lights (FL)) under naturally low irradiation conditions during fall and winter in Berlin, Germany. Under LED, development of Thymus vulgaris L. was highly accelerated resulting in distinct fresh yield increases per square meter by 43% and 82.4% compared to HPS and FL, respectively. Dry yields per square meter also increased by 43.1% and 88.6% under LED compared to the HPS and FL lighting systems. While composition of terpenoids remained unaffected, their quantity per gram of leaf dry matter significantly increased under LED and HPS as compared to FL. Further, the power consumption calculations revealed energy savings of 31.3% and 20.1% for LED and FL, respectively, compared to HPS. In conclusion, the implementation of a broad-spectrum LED system has tremendous potential for increasing quantity and quality of Thymus vulgaris L. during naturally insufficient light conditions while significantly reducing energy consumption.

Keywords: biomass efficacy; daily light integral; energy consumption; light-emitting diode; plant morphology; volatile organic compounds.

Figure 1

Biomass yields and partitioning of Thymus vulgaris L. cultivated under different supplemental lighting systems during fall and winter in Berlin, Germany. LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent light. (A) Fresh matter yields in gram per plant *, (B) Dry matter yields in gram per plant *, (C) Leaf and shoot dry matter partitioning in gram per plant **. * Presented are mean plant yields of four independent spatial replications per light treatment (n = 4) ± standard deviation (SD) of 32 harvested plants per spatial replication and light treatment (N = 384, n = 128 plants per supplemental light treatment, n = 32 plants per spatial replication). Significant differences (p ≤ 0.01) were determined according to Dunnett’s T3 multiple comparisons test after Brown-Forsythe and Welch ANOVA test (p ≤ 0.001). Different letters indicate significant differences. ** Presented are mean dry leaf and shoot matter yields of four independent spatial replications per light treatment (n = 4) ± SD (standard deviation) of 16 harvested plants per spatial replication and light treatment (N = 192, n = 64 plants per supplemental light treatment, n = 16 plants per spatial replication). Significant differences (p ≤ 0.05) were determined according to Dunnett’s T3 multiple comparisons test after Brown-Forsythe and Welch ANOVA test (p ≤ 0.001). Different letters indicate significant differences.

Figure 2

Visual appearance of Thymus vulgaris L. at harvest cultivated under different supplemental lighting systems during fall and winter of Berlin, Germany. LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent light.

Figure 3

Irradiance profiles (W m⁻² nm⁻¹) of the experimental plots (1 m²) underneath each supplemental lighting system. (LED = light-emitting diode, HPS = high-pressure sodium lamp, FL = fluorescent light).

Figure 4

Light spectra of the three artificial light sources (light-emitting diode (LED) = solid line, high-pressure sodium lamp (HPS) = dashed line, fluorescent light (FL) = dotted line) used during the greenhouse experiment.