Effect of cooling the root zone with a duct of microporous film on the cultivation of spinach

To investigate the growth of spinach by root-zone cooling, we made a cooling tube consisting of a duct of microporous film and a polyethylene tube. Gas is able to permeate the microporous film, but the film is impervious to water. A duct made from a nylon mesh tube (3 cm diam.) that was covered with a cylindrical microporous film was passed through a polyethylene tube 18 cm in circumference. The cooling tube was 5 m long and bent up 50 cm from both ends; the space between the duct and the polyethylene tube was filled with 6 L of water. The relationship between the cooling tube temperature and air flow rate through the duct above the foam polystyrene boards was investigated. Air flows of 1, 4 and 8 m·s^-1 decreased the cooling tube temperature at 1 m from the air intake, whereas treatments above 4 m·s^-1 decreased the temperatures at 2.5 and 4 m from the air intake. The cooling tube, which was placed in the center of the row, decreased the soil temperature just under it. The shoot fresh weights of spinach growing near the cooling tube were higher than those grown in the absence of the tube from August 20 to October 5, 2004. These results indicate that the use of the cooling tube with a duct of microporus film improved spinach growth compared with those exposed to a higher root-zone temperature.