Effect of Far-Red Light on Growth and Photosynthetic Parameters of Lactuca sativa L. at Different Light Intensities

The effect of different light intensities (60, 80, 180, and 240 μmol photon/(m2 s)) and two RL/FRL ratios of 0.4 and 1.3 in the spectrum of white LEDs on growth, photosynthetic parameters and the content of photosynthetic pigments in 33-day-old Lactuca sativa L. plants was studied. It was shown that at light intensities of 60, 80, and 180 μmol photon/(m2 s) and an RL/FRL ratio of 0.4 compared to 1.3 ratio, higher values of wet and dry biomass, as well as leaf area, were observed. Moreover, the greatest increase was noted at an intensity of 60 μmol photon/(m2 s), and the smallest at 180 μmol photon/(m2 s). At light intensity of 240 μmol photon/(m2 s), no difference was found among these parameters at ratios of 0.4 and 1.3. The rate of photosynthesis did not differ significantly at the same light intensity but different RL/FRL ratios. However, in all treatments except the treatment with a light intensity of 240 μmol photon/(m2 s), the rate of photosynthesis per plant was higher at an RL/FRL ratio of 0.4 than at the ratio of 1.3. At a light intensity of 60 μmol photon/(m2 s), we observed an increased content of Chl (a + b) and carotenoids, as well as an increased Chl a/Chl b ratio at an RL/FRL ratio of 1.3 compared to the ratio of 0.4. Apparently, the higher growth and photosynthesis rates in plants at lower RL/FRL ratios, and at low and moderate light intensities are a consequence of the larger leaf area, which, at the same Pn value per unit leaf area, causes positive effects of FRL on biomass under these conditions.