Low water potential in saline soils enhances nitrate accumulation of lettuce

The relationship between nitrate accumulation of lettuce and soil salinity in plastic film house was investigated, and the effect of water stress on the nitrate accumulation in lettuce was further examined in nutrient solutions of different water potentials. Nitrate contents in fresh lettuce were in the range of 85-2510 mg kg(-1), and the content increased as soil electrical conductivity increased up to 5 dS m(-1). Nitrate content in lettuce increased significantly as water potential in the nutrient solution decreased under low light intensity and an ample supply of nitrate. Soluble sugar and oxalate contents also increased with water stress. However, the changes of nitrate contents in response to the water potential changes were more significant than those of organic compounds. These results confirm the osmotic regulatory function of nitrate in replacement of organic compounds in lettuce grown in plastic film house where light intensity and soil water potential are low.