Studies on Shade-net Characters and Covered Cultivation of Summer Pak-choi

Test the shade-net characters and its effect on the yield and quality of summer Pak-choi in continuous three summers. Results show that different shade-nets are different in shading rate, quality of transmissive light and illuminance. Reduced rate of transpiration roughly synchronizes with the shading rate. The character of temperature lowering is proved by the significant drop in the highest surface temperature, the temperature at 20 cm above and under the earth, and the leaf temperature. In the fine and hot summer with the highest air temperature over 35C the maximum surface temperature is 56. 2 C and the minimum is 41C , while the temperature will drop 9～13C after being covered by shade-net. The maximum difference will be 19. 9C, and 63～435% increase of summer Pak-choi production will be resulted (1992). When the highest air temperature is between 30'C and 35 C , the highest surface temperature ranges from 33. 7 C to 44. 5 C. In the cool, overcast and rainy summers (1991 and 1993), black shade-net decreases the yield, while the grey shade-net of 45%-50% shading rate increases the yield. When the air temperature is between 25C and 30 C the shade-net makes little temperature drop. The exterior commercial quality of covered summer Pak-choi is better than that of open-air product, but the interior nutrient quality is worse, and the nitrate accumulation raises. Revealed 5～7 days before harvest, each nutrient quality is equal to or over that of open-air one, and the nitrate is lower. Therefore, revealing before harvest is a necessary cultivation measure.