LED Illumination Spectrum Manipulation for Increasing the Yield of Sweet Basil (Ocimum basilicum L.)

Abstract

Manipulation of the LED illumination spectrum can enhance plant growth rate and development in grow tents. We report on the identification of the illumination spectrum required to significantly enhance the growth rate of sweet basil (Ocimum basilicum L.) plants in grow tent environments by controlling the LED wavebands illuminating the plants. Since the optimal illumination spectrum depends on the plant type, this work focuses on identifying the illumination spectrum that achieves significant basil biomass improvement compared to improvements reported in prior studies. To be able to optimize the illumination spectrum, several steps must be achieved, namely, understanding plant biology, conducting several trial-and-error experiments, iteratively refining experimental conditions, and undertaking accurate statistical analyses. In this study, basil plants are grown in three grow tents with three LED illumination treatments, namely, only white LED illumination (denoted W*), the combination of red (R) and blue (B) LED illumination (denoted BR*) (relative red (R) and blue (B) intensities are 84% and 16%, respectively) and a combination of red (R), blue (B) and far-red (F) LED illumination (denoted BRF*) (relative red (R), blue (B) and far-red (F) intensities are 79%, 11%, and 10%, respectively). The photosynthetic photon flux density (PPFD) was set at 155 µmol m^-2 s^-1 for all illumination treatments, and the photoperiod was 20 h per day. Experimental results show that a combination of blue (B), red (R), and far-red (F) LED illumination leads to a one-fold increase in the yield of a sweet basil plant in comparison with only white LED illumination (W*). On the other hand, the use of blue (B) and red (R) LED illumination results in a half-fold increase in plant yield. Understanding the effects of LED illumination spectrum on the growth of plant sweet basil plants through basic horticulture research enables farmers to significantly improve their production yield, thus food security and profitability.

Keywords: artificial lighting; energy use efficiency; far-red illumination; growth analysis; light exposure; medicinal plants; protected horticulture; water use efficiency.

Figure 1

Effective wavelength bands that affect the growth of sweet basil plants.

Figure 2

The photosynthetic photon flux densities (PPFD) of the Heliospectra^TM grow lights measured using a LaserCheck optical power meter, at different distances from the light-emitting diode (LED) source, for all tents.

Figure 3

(A) Photos of typical sweet basil pots from the BRF*, BR*, and W* grow tents after 28 days of LED illumination. (B) BRF* illumination spectrum; (C) BR* illumination spectrum; and (D) W* illumination spectrum. W*—only white LED illumination; BR*— combination of red (R) and blue (B) LED illumination with relative R and B intensities of 84% and 16%, respectively); BRF*— combination of R, B and far-red (F) LED illumination, with relative R, B and far-red (F) intensities of 79%, 11%, and 10%, respectively.

Figure 4

Effect of different LED illumination spectra on the growth, and development parameters, such as (A) fresh mass (g); and (B) dry mass of sweet basil (Ocimum basilicum L.) plants grown in 2 m² tents. Different letters indicate a significant difference among the treatments, p ≤ 0.05.

Figure 5

Effect of the different LED illumination spectra on the mean values of (A) water use efficiency (WUE) and (B) energy use efficiency (EUE) of sweet basil (Ocimum basilicum L.) plant grown in 2 m² tents. Different letters indicate a significant difference among the treatments, p ≤ 0.05.

Figure 6

Biomasses of basil plants after 6 weeks in the W* illuminated tent and after 4 weeks in the BRF*-illuminated tents. Different letters indicate a significant difference among the treatments, p ≤ 0.05.

Figure 7

The normalized values obtained in the first experiment in September 2020 compared to values obtained in the second experiment in March 2020.