Mechanism Of Effects Of Light Intensity And Light Quality Under The Combination Of Red Light And Blue Light On Photosynthetic Capacity Of Lettuce

Cultivation with artificial lighting has become an important way of industrialized production for plant. However, high electric-energy consumption of artificial lighting limits popularization and application. LEDs have been considered as ideal light sources for plant growth in the future due to its narrow wavelength and high energy-efficiency. Hence, to explore optimal light recipe supplied with LEDs, as the main light sources for plant growth in plant factory with artificial lighting, is the key to improving production capacity and reducing energy-consumption of light source. The current studies mostly focused on the effects of light intensity and different ratios of red light to blue light（R/B） on plant growth, morphology and quality under the combination of red light and blue light. There were few studies about the quantitative effects of light intensity and different ratios of red light to blue light on leaf photosynthetic rate. Moreover, few studies about reducing the electric-energy consumption in plant factory with LED from light use efficiency and electric-energy use efficiency’s point of view have been reported. The objectives of this study was to improve light use efficiency and reduce the operation costs of plant factory with artificial lighting by increasing leaf photosynthetic capacity. Lettuce was chosen as the experimental material due to its wide cultivation and sensitive response to light. Lettuces（Lactuca sativa L.） were grown in hydroponic system under the combination of red light and blue light. The mechanism of effects of different R/B ratios and light intensities on leaf photosynthetic capacity was analyzed from the following respects: leaf morphology, photosynthetic characteristics, light response curve, CO₂ response curve, fluorescence characteristics, photosynthetic electron flow distribution, stomatal characteristics, nitrogen concentration and light use efficiency. The main results and conclusions are as follows:（1） To investigate the suitable light intensity for lettuce growth, lettuces were exposed to light intensity of 200, 300 and 400 μmol·m^-2·s^-1 and described as L200, L300 and L400, respectively. The ratio of red light to blue light was set as 1. The maximum light use efficiency（LUE） and electric-energy use efficiency（EUE） were found under L200 and L300 treatments, respectively. However, for EUE, no significant difference was found under L200 and L300 treatments. It was concluded that light intensity of 200 μmol·m^-2·s^-1 was suitable for lettuce growth in the view of saving-energy and high efficiency.（2） Lettuce plants were exposed to 200 μmol·m^-2·s^-1 irradiance for a 16 h·d^-1 photoperiod under the following seven treatments: fluorescence light（FL）, monochromatic red light（R）, monochromatic blue light（B） and the mixture of R and B with different R/B ratios of 12, 8, 4, and 1. Leaf photosynthetic capacity（Amax） and photosynthetic rate（Pn） increased with decreasing R/B ratio until 1, Pn and Amax under B treatment had 7.6 and 11.8 % reduction in comparison with those under R/B=1 treatment, respectively. Pn under FL treatment was higher than those under R and R/B=12 treatments and lower than that under R/B=1treatment. However, shoot dry weight increased with increasing R/B ratio with the greatest value under R/B=12 treatment. Therefore, addition of blue light could improve photosynthetic capacity, but be adverse to shoot dry weight accumulation by inhibiting leaf expansion. LUE and EUE were both the highest under R/B=12 treatment. In conclusion, the suitable ratio of red light to blue light was 12 under light intensity of 200 μmol·m^-2·s^-1.（3） Lettuce plants were exposed to 200 and 400 μmol·m^-2·s^-1 irradiance for a 16 h·d^-1 photoperiod under the following six treatments: monochromatic red light（R）, monochromatic blue light（B） and the mixture of R and B with different R/B ratios of 12, 8, 4, and 1. The Pn and Amax under 400 μmol·m^-2·s^-1 treatment with high R/B ratio could be achieved by decreasing R/B ratio under 200 μmol·m^-2·s^-1 treatment. In addition, variation trends of Pn and Amax with decreasing R/B ratio under 400 μmol·m^-2·s^-1 treatment were consistent with those under 200 μmol·m^-2·s^-1 treatment. It can be concluded that enhancement of Pn and Amax by increasing light intensity could be achieved by decreasing R/B ratio under the combination of red light and blue light.（4） Lettuce plants were exposed to R/B=12 for a 16 h·d^-1 photoperiod under the following four light intensity treatments: 100, 150, 200 and 300 μmol·m^-2·s^-1. Leaf photosynthetic capacity（Amax） and photosynthetic rate（Pn） increased with increasing light intensity by increasing stomatal conductance and carboxylation capacity of Rubisco, along with reducing the ratio of oxidation rate to carboxylation rate and the allocation proportion of photosynthetic electron transport to photorespiration. Meanwhile, shoot dry weight increased with increasing light intensity by capturing much more photosynthetic active radiation achieved by increasing Pn and leaf area. LUE and EUE both reached the maximum values under 200 μmol·m^-2·s^-1 treatment as compared with those under 100, 150 and 300 μmol·m^-2·s^-1 treatments with R/B ratio of 12. However, the highest shoot dry weight was observed under 300 μmol·m^-2·s^-1 treatment. Therefore, compromise should be considered in practical production due to the conflict relationship between high yield and high efficiency.