S08 - Session P5 - Supplemental far-red radiation influences lettuce growth and quality parameters

Authors: Ellen Van de Velde *, Marie-Christine Van Labeke, Kathy Steppe

In indoor farming, artificial lighting is required to drive plant growth and development. To this end, Light Emitting Diode (LED) lighting is especially suitable because of its high luminous efficacy, long life expectancy and limited heat generation. Moreover, LEDs allow the application of specific wavelengths of light, so plant growth and quality parameters can be controlled. Although many authors reported on the effect of R:B ratios on lettuce, effects of far-red are less studied. Far-red is of particular interest, as it is known to induce shade avoidance responses and thus might affect the commercial diameter of butterhead lettuce. In this study, 3-week-old butterhead lettuce plants were grown in a growth chamber under different R:FR ratios, while PAR of 90% red and 10% blue was kept at 200 µmol m -2 s -1 . In a first experiment, two far-red treatments (18 and 90 µmol m -2 s -1 ) were applied for three weeks and in a second experiment, a gradient of 0-9-18-36-72 µmol m -2 s -1 was applied for four weeks. In both experiments, FR had limited effects on fresh and dry weight. Leaf area and head diameter were determined using image analysis. FR increased leaf expansion, but length/width ratio was not influenced. Specific leaf area was increased by FR addition. Furthermore, leaf anatomy was studied by bright-field microscopy on microsections of the distal end of the midrib. Leaf colour was assessed using spectral reflectance measurements and higher FR resulted in paler green leaves. To assess internal quality, the content of chlorophyll and carotenoids, nitrate, soluble sugars (glucose, fructose and sucrose) and total phenolics was also measured.