Filtering Light-Emitting Diodes to Investigate Amber and Red Spectral Effects on Lettuce Growth

Abstract

Red and blue light are the principal wavelengths responsible for driving photosynthetic activity, yet amber light (595 nm) has the highest quantum efficiency and amber-rich high pressure sodium lamps result in superior or comparable plant performance. On this basis, we investigated how lettuce plant growth and photosynthetic activity were influenced by broad and narrow light spectra in the 590-630 nm range, by creating amber and red light-emitting diode (LED) spectra that are not commercially available. Four different light spectra were outfitted from existing LEDs using shortpass and notch filters: a double peak spectrum (595 and 655 nm; referred to as 595 + 655-nm light) that excluded 630-nm light, 595-nm, 613-nm, and 633-nm light emitting at an irradiance level of 50 W·m^-2 (243-267 µmol·m^-2·s^-1). Shifting LED wavelengths from 595 nm to 633 nm and from 595 nm to 613 nm resulted in a biomass yield decrease of ~50% and ~80%, respectively. When 630-nm light is blocked, lettuce displayed expanded plant structures and the absence of purple pigmentation. This report presents a new and feasible approach to plant photobiology studies, by removing certain wavelengths to assess and investigate wavelength effect on plant growth and photosynthesis. Findings indicate that amber light is superior to red light for promoting photosynthetic activity and plant productivity, and this could set precedence for future work aimed at maximizing plant productivity in controlled environment agriculture.

Keywords: LED; amber light; controlled environment agriculture; morphology; plant growth; red light.

Figure 1

Simplified schematic diagram of the LED lighting system showing the power supply, power distribution panel, voltage drivers, voltage regulators, and LED assemblies.

Figure 2

Relative spectral irradiance compositions of the LED spectra that were obtained with filters and used as light treatments on lettuce plants. The arrow in the upper figure indicates the wavelength of the valley using the amber LED assembly with the notch filter.

Figure 3

Plant morphology of lettuce (Lactuca sativa, cv. Breen) plants grown under 595 + 655-nm, 595-nm, 633-nm, and 613-nm light (from left to right) for two weeks. Wavelength values in the figure indicate the peak wavelengths of the light treatments.

Figure 4

Representative pigmentation of a lettuce (Lactuca sativa, cv. Breen) leaf grown under 595 + 655-nm and 595-nm light treatments for two weeks.