Comparative toxicity of arsenite and arsenate to rice seedling under various levels of phosphate supply

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Abstract
Toxic effects of arsenite and arsenate on the elongation of rice radicle or plumule were compared under various levels of phosphate application. The toxicity of arsenite was almost independent of added phosphate and the lethal concentration was found to tend to be 10-4.0, M, whereas that of arsenate was antagonistically reduced by phosphate appIied in the range from a little lower than 10-4.5 to 10-2.5 M without supply of the other nutrients and to 10-2.0 M with it, though at 10-5.5 M of phosphate the growth response to arsenate was similar to that without phosphate and the antagonism seemed to be no longer brought about. The application of soluble phytate did not show such an effect as the phosphate. The above lethal concentration of arsenite was found to be equal to that of arsenate in the middle range of 10-6.5 and 10-3.5 m of phosphate irrespective of the co-existence of other nutrients. Below this phosphate level, therefore, arsenate came to exceed arsenite in toxicity to rice seedling. These observations lead to the tentative eonclusion that the common view that arsenite is more toxic than arsenate without regard to the status of co-existing phosphate is questionable.