Effect of LED light with different ratios of red to blue light on photosynthesis and energy use efficiency for lettuce

Abstract: Compared with traditional plant production systems, the advantage of plant factory with LED (light emitting diode) lighting for plant production is annually planned plant production with high quality/quantity and less energy consumption. However, commercial promotion of plant factory with artificial lighting is still limited since LED is used as the sole light source for plant growth and the operation cost is quite high. To reduce the operation cost of the plant factory with artificial lighting, improving electric-energy use efficiency (EUE) and/or light use efficiency (LUE) by optimizing the ratio of red to blue light (R/B) was considered as a feasible way since the red and blue light are mainly absorbed by photosynthetic pigments and more effective for plant production than other wavelengths. In this study, the effects of different R/B on EUE and LUE were revealed by investigating the effects of different R/B on photosynthesis from the aspects of Rubisco carboxylation rate, partitioning of electron flow and leaf nitrogen in photosynthetic apparatus. In this experiment, lettuce plants (Lactuca sativa L.) were exposed to 200 μmol/(m2·s) irradiance for a 16 h/d photoperiod under the following 7 treatments: monochromatic red light (R, peak wavelength: 657 nm), monochromatic blue light (B, peak wavelength: 450 nm) and mixture of R and B with different R/B of 12, 8, 4, and 1, fluorescent lamps (FL, R/B/G=1.2:1.0:1.3, G was green light). Lettuce plants under FL treatment were set as the control. The results showed that: 1) Decreasing R/B until 8 significantly decreased the partitioning of electron flow to photorespiration and increased the partitioning of leaf nitrogen in carboxylation and bioenergetics system, resulting in increasing photosynthesis among the treatments with R/B of higher than 8. 2) No significant differences were found in Rubisco carboxylation rate, the partitioning of electron flow and leaf nitrogen in photosynthetic apparatus among the treatments with R/B of lower than 8, except FL treatment. 3) EUE and LUE both increased with R/B increasing until up to 12 with the maximum values of 0.494% and 1.773%, respectively; there were no significant differences for EUE among the treatments with R/B of higher than 8; however, LUE under the treatment with R/B of 12 was 12.5% higher than that under the treatment with R/B of 8. Although photosynthesis decreased with the increasing of R/B until 8, leaf area index (LAI) increased with the increasing of R/B until 8. Increase in LAI offset the negative effect of increasing R/B on the leaf photosynthesis, resulting in capturing much more photosynthetic active radiation. 4) Leaf photosynthetic performance under FL treatment was the nearest to that under the treatment with the R/B of 12, which could be explained from the aspects of no significant differences between 2 treatments in Rubisco carboxylation rate, the partitioning of electron flow and leaf nitrogen in photosynthetic apparatus; however, although lettuce plants under FL treatment had high LAI, EUE was the lowest among all the treatments because of the lowest electro-optical conversion efficiency of fluorescent lamp. Based on the above results, R/B of 8 is the turning point for photosynthesis and EUE; it is concluded that R/B of lower than 8 is suitable for energy-saving and high-efficient production when lettuce plants are exposed to the combination of red light and blue light with light intensity of 200 μmol/(m2·s) in plant factory with LED lighting.