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Nine crop species (rice, wheat, soybean, lupin, azuki bean, sugar beet, tomato, cabbage and radish) were grown in phosphorus-sufficient and phosphorusdeficient nutrient solutions. Then, the activities of acid phosphatase secreted by the roots for 24 h and of that eluted by 100 mM NaC1 were investigated. Both activities of acid phosphatase increased under low phosphorus conditions in all the crop species. The activity of acid phosphatase secreted by the roots under phosphorus-deficient conditions was remarkably high in lupin and tomato, high in cabbage and radish, moderate in soybean and sugar beet, and low in wheat and azuki bean. The increased rates of activity of acid phosphatase (on a root dry weight basis) secreted under phosphorus-deficient conditions when compared with those under phosphorus-sufficient conditions ranged from 1.5 times in azuki bean to 19.9 times in lupin. The activity of acid phosphatase in the NaC1 eluates was also higher in all the crop species under phosphorus-deficient conditions than that under phosphorus-sufficient conditions. Rice and lupin were grown in the nutrient solution with four phosphorus levels, and the activities of acid phosphatase secreted and eluted by 100 mM NaC1 were determined during the growth. The activities of acid phosphatase in lupin were much higher than those in rice. Both activities of acid phosphatase increased with the decrease of the phosphorus concentration in the nutrient solution, and were substantially high at the initial growth stage. Significance of the secretion of acid phosphatase under phosphorus-deficient conditions was discussed.Key Words: acid phosphatase, phosphorus deficiency, rhizosphere, secretion by roots.Plant roots secrete several organic compounds into the rhizosphere along with the uptake of water and nutrients. Both the kind and the amount of organic compounds secreted by plant roots are dependent on various factors, such as plant species, plant age, temperature, light, microorganism status in the rhizosphere, soil moisture, and root damage, etc. (Rovira 1969). For example, mycorrhizal plant species exude a larger amount of carbohydrates and amino acids from the roots than nonmycorrhizal plant species (Schwab et al. 1984).The nutritional status of plants also has a great influence on the root secretion. When plants grow under nutrient stress conditions, such as phosphorus deficiency, iron deficiency, and aluminum excess, plant roots secrete functional organic compounds in order to avoid these conditions. Therefore, the root secretion ability of plants plays an important role in growth under nutrient stress conditions. It is considered that root secretion is one of the adaptive mechanisms in response to nutrient deficiencies. 130T. TADANO and H. SAKAI Large amounts of citric acid have been shown to be secreted by lupin roots under phosphorus-deficient conditions, and it has been indicated that citric acid was able to release inorganic phosphate from phosphated ferric hydroxide (Gardner et al. 1983). It has also been reported that the s...

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Summary• Common reed ( Phragmites australis ) is a well known salt-tolerant plant and it is suggested that reeds recover Na + in the xylem sap of the shoot base (basal part of the shoot), store it temporarily in the shoot base, release it into the phloem sap, and then retranslocate it to the roots.• To investigate whether Na + is retained in the shoot base of reeds, confocal laser scanning microscope (CLSM) observations were conducted using an intracellular Na + -specific fluorescent probe. The CLSM observations revealed that reeds produced a large number of the starch granules at the shoot base when salt-stressed, and that the fluorescence indicating the location of intracellular free Na + was observed in the same position as the starch granules.• The Na content of starch granules was considerably greater than that of the shoot base, whereas the potassium (K) contents of the granules was only slightly greater than that of the shoot base.• Reeds produced Na + -binding starch granules in the parenchyma cells of the shoot base when salt-stressed; these starch granules may decrease intracellular free Na + . It is proposed that the site-specific production of Na + -binding starch granules constitutes a novel salt tolerance mechanism.