Chlorophyll fluorescence has been known as one of the indicators of photosynthetic status to various environmental stresses. The aims of this study were to assess the effects of environmental factors on lettuce chlorophyll fluorescent responses (Fv/Fm) and to develop an environment optimization model for lettuce growth using a simple genetic algorithm. High values of Fv/Fm were observed when environmental factors were 22-26ºC ambient temperature, 15-23ºC root zone temperature, 900-1,600 ppm CO 2 concentration, 0.4-1.3 m⋅s -1 air current speed, and 65-85% relative humidity. As photosynthesis photon flux (PPF) increased over 150 μmol⋅m PPF, and 75% relative humidity. The Fv/Fm value can be a good indicator of plant stress level and thus a useful parameter to optimize the environment for plant growth.

Evaporative cooling systems have been developed to provide the desired conditions for plant growth in the greenhouses during hot seasons. The fog system performs better than the pad-and-fan system with respect to uniform distribution of temperature and relative humidity in the greenhouse. The appropriate combination of air and water supply depends on the environmental conditions, such as solar radiation, ambient temperature and relative humidity, and is essential for maintaining the desired conditions in the greenhouse 3 . The fog-cooling system supplies water droplets in diameters of 2-60 µm so as to enhance the heat and mass exchange between the water and the air 5 . Ventilation is an important factor since the water spray evaporates along with the movement by airflow. Most studies on fog-cooling systems used forced ventilation systems 3,4 . Natural ventilation is normally achieved by air exchanges through multiple controlled openings. Wind is generally the primary driving force in natural ventilation systems. System design and subsequent field-testing are very difficult due to the variations of environmental factors, wind velocity and direction, solar radiation, outside temperature, etc. AbstractA CFD model was developed to simulate the air temperature and relative humidity distribution in greenhouses adopting fog-cooling systems using FLUENT. The developed model was validated using the data from a fog-cooling experiment in a single-span greenhouse without plants. The measured and simulated air temperatures varied from 0.1 to 1.4ºC and the differences of relative humidity varied 0.3-6.0%. The validated model was then used to evaluate the design of a fog-cooling system in a multi-span glasshouse. The optimal system design was determined in terms of the cooling efficiency and the special uniformity of air temperature and relative humidity. The simulations demonstrated that the best performance of the cooling system occurred when the fog nozzles were located at the height of 2.3 m above the floor and at a distance of 1.9 m from the sidewalls with uniform row-to-row spacing of 3.7 m. The most effective location of the nozzles was within the air entry from the sidewall ventilator inlets of the greenhouse. However, it was important not to wet the sidewalls with the fog. This study suggested that the CFD model developed could be a useful tool to design and evaluate the fog-cooling systems in greenhouses with various configurations. Discipline: Agricultural facilitiesAdditional key words: CFD, fog-cooling system, glasshouse, natural ventilation, wind speed JARQ 41 (4), 283 -290 (2007) http://www.jircas.affrc.go.jp This research was carried out under the US-Japan collaborative project on analysis and control of greenhouse environment for efficient crop production under semiarid climate.