Responses of aquicultured lupin and tomato plants to low P concentrations in the growth media were compared. Under the low P condition, the lupin and tomato plants exhibited similar responses though the rates of the responses were different. These responses included an increased secretion of organic acids, Apase and phytase by roots. However, the following differences were apparent. (1) The increase of Apase secretion was higher in lupin roots than in tomato roots while the increase of phytase secretion was reverse. (2) The increase of organic acid secretion was considerably higher in lupin than that in tomato. (3) The external threshold P concentration in growth media to induce the responses was considerably lower in lupin (approximately 10 μmol/L) than in tomato (300 μmol/L P). (4) The physiological threshold P content (PTPC), an internal P content to induce the response, was considerably higher in tomato (4.2 mg P.g-1) than in lupin (1.5 mg P.g-1), indicating that the P requirement of tomato for normal growth was quite higher than that of lupin plant. It was considered that the high P requirement of tomato for normal growth was the most important factor in the low adaptive ability to P deficient conditions. The secretion of Apase, phytase and organic acids by roots provides a set of adaptive mechanisms by which plants can enhance the availability of P and therefore alter the nutrient status in the rhizosphere.