Supplemental far-red light-emitting diode light increases growth of foxglove seedlings under sole-source lighting

Abstract

Seedlings may be grown indoors where environmental conditions can be precisely controlled to ensure consistent and reliable production. The optimal spectrum for production under sole-source lighting is currently unknown. Far-red light (λ = 700–800 nm) typically is not a significant part of the spectrum of light-emitting diode (LED) grow lights. However, far-red light is photosynthetically active and can enhance leaf elongation, which may result in larger leaves and increased light interception. We hypothesized that adding far-red light to sole-source lighting would increase the growth of ‘Dalmatian Peach’ foxglove (Digitalis pur-purea) seedlings grown under white LED lights, potentially shortening production times. Our objective was to evaluate the effect of far-red light intensities, ranging from 4.0 to 68.8 mmol•m-2•s-1, on the growth and morphology of foxglove seedlings. Foxglove seedlings were grown in a growth chamber with a photosynthetic photon flux density (PPFD) of 186 ± 6.4 mmol•m-2•s-1 and supplemental far-red light intensities ranging from 4.0 to 68.8 mmol•m-2•s-1. As far-red light increased, shoot dry weight, root dry weight, plant height, and plant height/ number of leaves increased by 38% (P = 0.004), 20% (P = 0.029), 38% (P = 0.025), and 34% (P = 0.024), respectively, while root weight fraction decreased 16% (P = 0.034). Although we expected supplemental far-red light to induce leaf and/or stem expansion, specific leaf area and compactness (two measures of morphology) were unaffected. Because a 37% increase in total photon flux density (PPFD plus far-red light) resulted in a 34.5% increase in total plant dry weight, the increased growth likely was due to increased photosynthesis rather than a shade-acclimation response. The growth response was linear across the 4.0 to 68.8 mmol•m-2•s-1 range of far-fed light tested, so we were unable to determine a saturating far-red photon flux density.