OPTIMUM AIR CURRENT SPEED FOR RICE PLANT CANOPY IN A CLOSED PLANT-PRODUCTION SYSTEM

A completely closed plant production system was developed to produce genetically modified rice plants. Plant cultivation at high planting densities can some-times cause mutual shading of plant leaves and restrict air movement inside the canopy. In this study, we performed growth experiments to determine the optimum air current speed to enhance growth. A prototype closed plant production system was used; this system included a cultivation room and hydroponic containers. Rice plants were planted at a density of 40 m-2. The air from an air-conditioning system flowed through the plant canopy in a horizontal direction from the air supply outlet on the room wall. Several types of fans with different flow rates were placed at an upstream position to vary the air current speed inside the canopy. In experiment 1, the air current speed at 40 cm above the cultivation panel was set to 0.40-1.20 m•s^-1, and the plants were cultivated under these conditions until the heading stage. The dry weight and leaf area decreased with increase in air current speed. Thus, air current speed higher than 0.40 m•s^-1 was thought to cause excessive transpiration from the leaves. In experiment 2, the speed was set to 0.03-1.30 m•s^-1, and the plants were cultivated until the harvest stage. The rice yield was higher at 0.40 and 1.30 m•s^-1 than at other air current speeds. This was probably because low air current speed restricted vegetative growth and limited the translocation of photoassimilates in the leaves to the seeds. Our study showed the importance of controlling the air current speed inside a rice plant canopy and indicated that approximately 0.4 m•s^-1 of air speed was the most suitable to enhance growth and increase rice yield in a closed cultivation system.