Formation of Organic Acids in the Root Apices of Rice Plants (Oryza sativa L.) Grown in Acidic Nutrient Solution Containing Aluminum

Abstract

Aluminum (Al) is a major phytotoxic mineral in the soil at low pH conditions (Jones and Kochian, 1996). Pellet et al. (1995) reported that Al would damage the roots by making the plants more sensitive to other abiotic stresses such as defi ciency of water and nutrients, and eventually reduce the crop yield. One of the Al-tolerance mechanisms in plants would be Al exclusion from the root apices. In several upland plants, Al tolerance is closely associated with an increased ability of roots to release organic acids, such as citrate (de la Fuente et al., 1997; Ma et al., 1997a; Pineros et al., 2002), malate (Osawa and Matsumoto, 2001; Tesfaye et al., 2001), and oxalate (Ma et al., 1997b; Wenzl et al., 2001), which may prevent Al uptake by chelating Al. Rice, a lowland plant, has been identifi ed as a high Al-excluder at root apex (Osaki et al., 1997), but the mechanism of Al exclusion in rice plants has not been fully elucidated. The purpose of the present study is to confi rm that the varietal difference of the Al tolerance in rice is associated with organic acid formation in the intact root apices when Al was added to the low pH nutrient solution, and furthermore, to identify the sort of organic acid formed in the root exposed to Al.