2017 ASHS Annual Conference

By definition, far-red radiation (FR, 700–800 nm) is outside the photosynthetically active waveband, but it has a significant role in plant growth and development. Adding FR to blue (400–500 nm) and red (600–700 nm) light has the potential to produce desirable growth attributes. We grew green butterhead lettuce (Lactuca sativa) ‘Rex’ and red oakleaf lettuce ‘Rouxai’ from seed at 20 °C for 3 days before transferring them to six different light-emitting diode treatments in a growth chamber at 22 °C. Plants received FR (peak wavelength = 732 nm) at 0, 30, or 75 µmol∙m^–2∙s^–1 in addition to equal amounts of blue (peak wavelength = 447 nm) and red (peak wavelength = 661 nm) light, each at 90 (low light) or 180 (high light) µmol∙m^–2∙s^–1. Growth, morphological, and coloration data were collected on day 8 for ‘Rex’ and day 10 for ‘Rouxai’. We then switched the light treatments for some plants to assess adaptive growth in the following 5–7 days. Under low light, the addition of FR increased the shoot dry weight of ‘Rex’ by up to 36% and that of ‘Rouxai’ by up to 52%. The extent of this increase under high light was less than that under low light. Under the same FR intensity, high light increased the shoot dry weight of ‘Rex’ by 43–55% and that of ‘Rouxai’ by 19–67% compared with low light. Although high light increased chlorophyll concentrations by 16–29%, FR enrichment decreased it under low and high light by up to 23% in ‘Rex’ and 10% in ‘Rouxai’, and the percentage reduction was positively correlated with FR intensity. Low light or FR enrichment promoted leaf expansion and hypocotyl elongation. The leaf coloration of ‘Rouxai’ was greater under high light than low light. However, with increasing FR intensities, the red pigmentation of ‘Rouxai’ diminished under low light but intensified under high light. Plants that received the same two light treatments exhibited similar growth patterns irrespective of the light sequence. We conclude that FR enrichment improves photosynthetic light capture and thus promotes lettuce growth in light-limiting conditions. Growers need to assess which spectral compositions produce desirable growth attributes and whether including FR in sole-source lighting is economically viable.