Interaction of the Far-red Radiation Intensity and CO2 Concentration during Early Indoor Production Stages of Red Leaf Lettuce

Although far-red (FR) radiation (700–750 nm) is beyond the traditional photosynthetically active radiation (PAR) waveband (400–700 nm), recent findings suggest equivalent or similar photosynthetic action when applied along with shorter PAR wavelengths. Potential interactive effects of FR with other environmental factors that affect photosynthesis, such as CO 2 , have not been widely established. In the present study, the effects of four FR photon flux densities (0, 20, 40, or 60 µmol·m −2 ·s −1 ) substituting for red (R; 600–700 nm) in combination with three CO 2 concentrations (400, 800, or 1200 µmol·mol −1 ) were investigated at three distinctive stages of young red lettuce production. We tested whether the photomorphogenic response of young plant leaves to FR would result in a higher light interception area, as well as whether interactive effects of CO 2 with FR might modify the effects of FR alone. The total photon flux density (TPFD) was maintained at a low limiting intensity of 200 µmol·m −2 ·s −1 (daily light integral: 11.5 mol·m −2 ·d −1 ), consistent with indoor commercial setpoints for sole-source lighting. Although blue (B; 400–500 nm) was held constant at 20 µmol·m −2 ·s −1 for all treatments, different combinations of R and FR were adjusted. Red oakleaf lettuce ( Lactuca sativa ‘Rouxai’) used as a model test crop was harvested at 14 days (small baby greens), 18 days (standard baby greens), and 22 days (teen greens) after sowing. Crop productivity was highest at 800 µmol·mol −1 CO 2 for small and standard baby stages. At the teen green stage, plant productivity was similar for the two elevated CO 2 concentrations. The FR substitution for R did not affect dry biomass accumulation at all CO 2 concentrations tested, thus supporting equivalent photosynthetic action. A photomorphogenic effect of FR that elongated but did not increase the area of leaves persisted from the earliest to the most mature stage of crop development tested. The FR inclusion in the light recipe resulted in longer, thinner leaves with lower biomass compared to those without FR. At higher FR fluxes, purple pigmentation reduction occurred. However, the interaction of elevated CO 2 with FR radiation counteracted purple pigment reduction caused by FR alone. Substitution of FR in the light recipe for indoor production of young ‘Rouxai’ lettuce at low limiting light intensity did not improve productivity, even in combination with elevated CO 2 .