Interactions between Blue Light and Far-Red Radiation on Growth of Culinary Herb Seedlings

The photon spectrum (light quality) can regulate growth and quality characteristics of young plants, but responses of culinary herb transplants are not well described. Blue light (400 to 499 nm) generally inhibits extension growth while far-red radiation (700 to 750 nm) promotes stem elongation and leaf expansion. The objective of this study was to investigate the potential interaction between blue light and far-red radiation on six culinary herb species, basil ( Ocimum basilicum ), cilantro ( Coriandrum sativum ), parsley ( Petroselinum crispum ), sage ( Salvia officinalis ), mint ( Mentha spicata ), and oregano ( Oreganum vulgare ), with the goal of producing high-quality transplants with compact growth. Six sole-source lighting treatments were tested with blue photon flux densities (PFDs) of 20, 60, or 100 µmol·m −2 ·s −1 and far-red radiation of 0 or 60 µmol·m −2 ·s −1 , with red light (600 to 699 nm) added so that the total PFD was 210 µmol·m −2 ·s −1 in all treatments. Seeds were sown in 72-cell trays at a constant 23 °C under a 16-h photoperiod and grown for 28 to 44 days until harvest. Far-red radiation increased stem length in all species, while blue light up to 60 µmol·m −2 ·s −1 inhibited stem elongation. Interestingly, the promotive effects of far-red radiation on stem length increased as the B PFD increased in basil and cilantro, but the opposite occurred in sage. In all herbs except oregano, far-red radiation decreased the root dry-mass fraction and increased the leaf area ratio. Blue light generally decreased shoot dry mass, while far-red radiation had the opposite effect. However, by increasing shoot growth disproportionally to root growth, far-red radiation decreased the quality of the transplants. We conclude that blue light and far-red radiation interact, sometimes in contrasting ways across species, to regulate stem length, leaf area, and biomass accumulation. The species-specific responses to the photon spectrum indicate that while far-red radiation is generally a growth promoter, it can negatively affect transplant quality, and in some species, its effect depends on the B PFD.