Narrowband blue LEDs with different peak wavelengths similarly promote seedling elongation and have greater promotion effects than ultraviolet A and far-red in two species of Brassicaceae

Previous studies showed pure blue (B) spectra from light emitting diodes (LEDs) can promote shade avoidance responses (SARs). However, it is unknown whether SARs vary among different peak wavelengths (λpeak) of B or how they compare with other photomorphologically-important wavebands. To answer these questions, mustard and arugula seedlings were grown to the cotyledon-unfolding stage under the following narrowband spectrum treatments: ultraviolet A (UVA) (λpeak = 385 nm), B1 (λpeak = 405 nm), B2 (λpeak = 440 nm), B3 (λpeak = 455 nm), and far-red (FR) (λpeak = 730 nm). Both red (R, λpeak = 660 nm) and dark (D) were used as control treatments. The spectrum treatments were provided at 50 µmol m −2 s −1 on a continuous basis. There were no differences among the three B treatments except for reduced mustard fresh weight in B3. Compared with R, the B treatments promoted hypocotyl elongation, reduced cotyledon size, and increased petiole length in arugula and B1 increased petiole length in mustard. Compared with the B treatments, UVA inhibited hypocotyl and petiole elongation, with responses that were similar to or greater than R in both species. Compared with the B treatments, seedlings grown under FR generally had the lowest hypocotyl and petiole elongation and the smallest cotyledons. Among the LED treatments, the three B treatments had the greatest promotional effects on plant elongation. Therefore, low-intensity B with different λpeak can similarly promote hypocotyl elongation as SARs during de-etiolation stage, and have greater promotion effects on elongation than UVA and FR.