Spectral Effects of Light-emitting Diodes on Plant Growth, Visual Color Quality, and Photosynthetic Photon Efficacy: White versus Blue plus Red Radiation

Arrays of blue (B, 400-500 nm) and red (R, 600-700 nm) light-emitting diodes (LEDs) used for plant growth
applications make visual assessment of plants difficult compared to a broad
(white, W) spectrum. Although W LEDs are sometimes used in horticultural lighting
fixtures, little research has been published using them for sole-source
lighting. We
grew seedlings of begonia (Begonia ×semperflorens), geranium (Pelargonium ×horturum), petunia (Petunia ×hybrida), and snapdragon (Antirrhinum majus) at 20 °C under six sole-source LED lighting
treatments with a photosynthetic photon flux density (PPFD) of 160 µmol∙m–2∙s–1
using B (peak= 447 nm), green (G, peak= 531 nm), R (peak= 660 nm), and/or mint W
(MW, peak= 558 nm) LEDs that emitted 15% B, 59% G, and 26% R plus 6 µmol∙m−2∙s−1
of far-red radiation. The lighting treatments (with percentage from each LED in
subscript) were MW100, MW75R25, MW45R55,
MW25R75, B15R85, and B20G40R40.
At the transplant stage, total leaf area, and fresh and dry weight were similar
among treatments in all species. Surprisingly, when petunia seedlings were
grown longer (beyond the transplant stage) under sole-source lighting
treatments, the primary stem elongated and had flower buds earlier under MW100
and MW75R25 compared to under B15R85.
The color rendering index of MW75R25 and MW45R55
were 72, and 77, respectively, which was higher than those of other treatments,
which were ≤64. While photosynthetic photon efficacy of B15R85
(2.25 µmol∙J–1) was higher than the W light treatments (1.51-2.13
µmol∙J–1), the dry weight gain per unit electric energy consumption
(in g∙kWh–1) of B15R85 was similar to those of
MW25R75, MW45R55, and MW75R25
in three species. We conclude that compared to B+R radiation, W
radiation had generally similar effects on seedling growth at the same PPFD
with similar electric energy consumption, and improved the visual color quality of sole-source lighting.