Far-red radiation interacts with relative and absolute blue and red photon flux densities to regulate growth, morphology, and pigmentation of lettuce and basil seedlings

Abstract

Although outside the photosynthetic photon flux density (PPFD) waveband (400–700 nm), far-red (FR; 700–800 nm) radiation regulates photomorphogenesis and photochemistry, thereby meriting consideration in sole-source plant-lighting applications. We investigated how FR radiation interacted with the ratio of blue (B; 400–500 nm) to red (R; 600–700 nm) radiation (B:R) (experiment I) and PPFD (experiment II) to regulate seedling growth under sole-source lighting. We postulated that adding FR radiation to B + R radiation would increase leaf expansion and thus radiation capture to promote whole-plant photosynthesis, but that FR effects would depend on B:R and PPFD. In experiment I, lettuce (Lactuca sativa) ‘Rex’ and ‘Cherokee’ and basil (Ocimum basilicum) ‘Genovese’ were continuously irradiated by 180 μmol m–2 s–1 of B and/or R radiation [B30R150 (low B:R), B90R90 (high B:R), R180, or B180, where subscripts indicate respective photon flux densities in μmol m–2 s–1] with or without 30 μmol m–2 s–1 of FR radiation. Twelve and 16 days after seed sow for lettuce and basil, respectively, the addition of FR radiation increased leaf length and shoot weight of all crops with more pronounced impacts under high B:R than low B:R. It also increased root dry weight of basil and lettuce ‘Cherokee’. Adding FR to B + R radiation reduced specific chlorophyll content in lettuce by 10–20%, but not in basil. Red pigmentation of lettuce ‘Cherokee’ increased with increasing B:R but decreased with the inclusion of FR radiation. In experiment II, we grew lettuce ‘Rex’ (for 8 days) and ‘Rouxai’ (for 10 days) under B90R90 (low PPFD) or B180R180 (high PPFD) at the same B:R with or without FR radiation at 30 or 75 μmol m–2 s–1. Additional FR radiation increased lettuce shoot weight and extension growth but reduced relative specific chlorophyll content under both PPFDs, although FR effects were attenuated under the high PPFD. Shoot dry weight, relative specific chlorophyll content, and red foliage pigmentation increased with PPFD. We conclude that FR enrichment improves photosynthetic radiation capture and thus promotes crop growth under sole-source lighting, and that its effects are especially pronounced under high B:R and a low PPFD.