Optimization of Photosynthetic Photon Flux Density and Light Quality for Increasing Radiation-Use Efficiency in Dwarf Tomato under LED Light at the Vegetative Growth Stage

Abstract

Dwarf tomatoes are advantageous when cultivated in a plant factory with artificial light because they can grow well in a small volume. However, few studies have been reported on cultivation in a controlled environment for improving productivity. We performed two experiments to investigate the effects of photosynthetic photon flux density (PPFD; 300, 500, and 700 μmol m^-2 s^-1) with white light and light quality (white, R3B1 (red:blue = 3:1), and R9B1) with a PPFD of 300 μmol m^-2 s^-1 on plant growth and radiation-use efficiency (RUE) of a dwarf tomato cultivar ('Micro-Tom') at the vegetative growth stage. The results clearly demonstrated that higher PPFD leads to higher dry mass and lower specific leaf area, but it does not affect the stem length. Furthermore, high PPFD increased the photosynthetic rate (Pn) of individual leaves but decreased RUE. A higher blue light proportion inhibited dry mass production with the same intercepted light because the leaves under high blue light proportion had low Pn and photosynthetic light-use efficiency. In conclusion, 300 μmol m^-2 s^-1 PPFD and R9B1 are the recommended proper PPFD and light quality, respectively, for 'Micro-Tom' cultivation at the vegetative growth stage to increase the RUE.

Keywords: blue light; chlorophyll concentration; leaf optical properties; light interception; photosynthetic light-use efficiency; projected leaf area; red light; tomato; vertical farming; white light.

Figure 1

Effects of PPFD on the spectra of reflectance and transmittance of leaves in ‘Micro-Tom’ 10 DAT in Experiment 1. The range of measured light spectrum was 400–700 nm. W300, W500, and W700 denote 300, 500, and 700 μmol m⁻² s⁻¹ PPFD, respectively. Each value represents the average of the values of eight plants.

Figure 2

Effect of PPFD on net photosynthetic rate (Pn) of leaves in ‘Micro-Tom’ 10 DAT in Experiment 1. Four plants were measured for each PPFD. Solid point denotes the measured value for one plant. X-mark represents the average Pn of four plants in each treatment. Error bars represent ± standard error. Different letters indicate significant differences among the treatments based on Tukey−Kramer’s test at p < 0.05 (n = 4). W300, W500, and W700 denote 300, 500, and 700 μmol m⁻² s⁻¹ PPFD, respectively.

Figure 3

Light response curve of net leaf photosynthetic rate in ‘Micro-Tom’ 11 DAT in Experiment 1. The Pn to PPFD was determined on the third leaf (counted from top, fully expanded, and unshaded leaf). Each value represents the average of three plants. Error bars represent ± standard error. Different letters indicate significant differences among the treatments based on Tukey−Kramer’s test at p < 0.05. W300, W500, and W700 denote PPFDs of 300, 500, and 700 μmol m⁻² s⁻¹, respectively.

Figure 4

Effects of light quality on daily average of intercepted PPFD and proportion of the canopy in ‘Micro-Tom’ in Experiment 2. The intercepted PPFD of the canopy was calculated as the difference between the average PPFD on the top and bottom of the canopy. The intercepted PPFD proportion was calculated as the ratio of the intercepted PPFD to the average PPFD above the top of the canopy. The PPFD of the three treatments was set at 300 μmol m⁻² s⁻¹. W: white light; R3B1: red/blue ratio = 3; R9B1: red/blue ratio = 9.

Figure 5

Effect of light quality on Pn of leaves in ‘Micro-Tom’ 10 DAT in Experiment 2. Four plants were measured for each PPFD. Solid point denotes the measured value for one plant. X-mark represents the average Pn of four plants in each treatment. Error bars represent ± standard error. Different letters indicate significant differences among the treatments based on Tukey−Kramer’s test at p < 0.05 (n = 4). W: white light; R3B1: red/blue ratio = 3; R9B1: red/blue ratio = 9.

Figure 6

Spectral photon flux distributions of white LED lamp (customized lamp) in Experiment 1 (a), white LED lamp (LDL40S-N19/21) in Experiment 2 (b), as well as red and blue LED lamps in R3B1 (c) and R9B1 (d) in Experiment 2. R3B1 and R9B1 represent photon flux ratios (red to blue light) of 3:1 and 9:1, respectively. The peak wavelengths of the white lamp were 446 nm and 592 nm in Experiment 1 and 454 nm and 593 nm in Experiment 2. The peak wavelengths of red and blue light were 667 and 450 nm, respectively, in Experiment 2. The maximum value of photon flux was converted to 1.0.

Figure 6

Spectral photon flux distributions of white LED lamp (customized lamp) in Experiment 1 (a), white LED lamp (LDL40S-N19/21) in Experiment 2 (b), as well as red and blue LED lamps in R3B1 (c) and R9B1 (d) in Experiment 2. R3B1 and R9B1 represent photon flux ratios (red to blue light) of 3:1 and 9:1, respectively. The peak wavelengths of the white lamp were 446 nm and 592 nm in Experiment 1 and 454 nm and 593 nm in Experiment 2. The peak wavelengths of red and blue light were 667 and 450 nm, respectively, in Experiment 2. The maximum value of photon flux was converted to 1.0.