We investigated an easy thermal insulation evaluation method for agricultural covering material and the thermal insulation effect of single and multilayer films. An air layer was maintained above and below a sheet of film using spacers consisting mainly of high heat insulating polystyrene and the heat transmission coefficient was measured using commercial thermal conductivity meter. Using this method, the measurement is finished in around one hour, and the reproducibility of the value was high. There was a (R²=0.910) high correlation with values reported in the literature, so it was considered a simple method of relative insulation evaluation of agricultural materials. Since agricultural covering materials have high radiation transmittance, it is necessary to consider radiative cooling similar to outdoor conditions. Because there is thermal radiation from the hot plate to the cold plate, this method simulates greenhouse conditions. It was confirmed that far-infrared transmittance significantly affects the heat transfer coefficient. In addition, in a single layer film house and a double layer film house of fluoro-resin film, the average minimum temperature in midwinter was 0.7 and 3.0 °C higher than outdoor, respectively, and improved heat retention was observed in the double layer house in particular.

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The aim of this study was to investigate optimum lighting conditions for growing red-leaf lettuce with commercial-grade fresh weight using red and blue light-emitting diodes (LEDs). Red-leaf lettuce plants were grown hydroponically for 21 d under 50, 100, 150, and 200μmol m^-2 s^-1 of red light (R) from red LEDs and blue light (B) from blue LEDs, respectively. Combinations of R and B with previously described intensities were also examined to give a total of 24 treatments. The effects of light quality and light intensity on the growth of red-leaf lettuce were investigated. Shoot fresh weight and dry weight increased as light intensity increased, regardless of light quality, with the highest rate of increase in both fresh and dry weights obtained under R. Red-leaf lettuce plants with commercial-grade fresh weight (› 80 g) could only be obtained in plants cultivated under 150 and 200μmol m^-2 s^-1 of R. Compared to R alone, the decrease in shoot biomass observed under B and all combinations of R and B could be attributed to a reduction in leaf area. However, the leaves of plants grown under R showed no red or purple coloration, which is an important characteristic of commercial red-leaf lettuce. On the other hand, red-leaf lettuce cultured under 50μmol m^-2 s^-1 or more of B, and all combinations of R and B, showed sufficient purple coloration. The results suggested that the production of red-leaf lettuce with a commercial-grade fresh weight and suitable color characteristics could be obtained under illumination by 100-200μmol m^-2 s^-1 of R and less than 50μmol m^-2 s^-1 of B.

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We investigated the effect of sugar and salt application to hydroponic culture solution for coloring red-leaf lettuce (Lactuca sativa L. ‘Prunai’ and ‘Rouxai’) under controlled culture conditions. In plant factories condition, red-leaf lettuce varieties frequently do not turn red. In order to solve this, we tested various stress conditions. Stresses were induced by addition of polyethylene glycol, sugar and salt to hydroponic culture solution, and using concentrated nutrient solution. The result showed that the combination of adding sugar and salt (5 and 0.02 % respectively in the nutrient solution at EC 0.6dSm^-1) accerelated coloring of red-leaf lettuce varieties. This technique might be efficient for growing red-leaf lettuce cultivation in plant factories.

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