MEASUREMENTS OF THE PHOTOSYNTHETIC RATES IN VEGETABLES UNDER VARIOUS QUALITIES OF LIGHT FROM LIGHT-EMITTING DIODES

Here, we evaluated the effect of light quality on vegetable growth to determine the efficiency of light energy utilization on photosynthesis in plant factories. Specifi-cally, we measured the gross photosynthetic rate (P) of butterhead lettuce, green perilla, red perilla, and tomato seedlings under a range of different LED peak wave-lengths using the assimilation chamber technique. The light-source system included 6 peak LED wavelength types: 405, 465, 530, 595, 660, and 735 nm. The system produces light composed of different combinations of the 6 wavelength ranges. Photo¬synthetic photon flux (PPF) values of 160 and 320 μmol•m^-2•s^-1 with 4 peak wavelengths (465, 530, 595, and 660 nm of 40 and 80 μmol•m^-2•s^-1, respectively) were set as the controls. Then, the PPF conditions of 120 and 240 μmol•m^-2•s^-1 were determined by turning off 1 of the 4 peak LEDs. By using the difference between P at 120 and 160 μmol•m^-2•s^-1 and 240 and 320 μmol•m^-2•s^-1, at a given wavelength, an incremental increase in P at low and high PPF levels was calculated, and expressed as absorbed CO₂ per added photon. The results showed that the highest incremental increase was obtained at 660 nm for all 4 vegetables. For red perilla and tomato, a significantly low incremental increase was obtained at 465 and 530 nm. The anthocyanin-rich leaves of these 2 plants are known to absorb large quantities of blue and green light in the epidermis, which led to lower P values at these wavelengths. The difference in the incremental increase was smaller at 320 μmol•m^-2•s^-1 than at 160 μmol•m^-2•s^-1 for all 4 vegetables. In conclusion, we suggest that the suitable wavelength to increase P varies depending on crop type, light quality history, and light intensity.