EFFECTS OF BLUE AND RED LIGHT ON STEM ELONGATION AND FLOWERING OF TOMATO SEEDLINGS

Year-round tomato cultivation needs high-quality seedlings of which stem length and node position of the first flower truss are controlled at suitable values. The objective of this study was to develop a method for controlling stem elongation and flowering of tomato seedlings in a closed seedling production system using light emitting diodes (LEDs). To this end, we measured the growth parameters of tomato seedlings grown under a mixture of blue (B; peak wavelength, 450 nm) and red (R; peak wavelength, 660 nm) LEDs to determine the optimum light conditions. We conducted two experiments to investigate whether the blue/red (B/R) ratio or blue and red light quantity affects morphogenesis. In experiment 1, the stem length at a 1.0 B/R ratio was shorter than that at a 0.1 B/R ratio. The node position of the first flower truss at the 0.1 B/R ratio was lower than that at the 1.0 B/R ratio. The net photosynthetic rate increased upon the decrease of the B/R ratio. Therefore, it appeared that flowering promotion was affected by photosynthesis. In experiment 2, stem elongation under 25, 50, and 75 μmol m^-2 s^-1 of red light with a constant quantity of blue light remained similar regardless of red light quantity. These results suggested that stem elongation depended on blue light quantity. Thus, LED irradiation at 75 μmol m^-2 s^-1 of blue light and a B/R ratio of <1.0 would suppress spindly growth and promote flowering during tomato seedling growth.