Interactive effects of Al, Ca and other cations on root elongation of rice cultivars under low pH

Watanabe T, Okada K

ANNALS OF BOTANY 95 (2): 379-385 JAN 2005

Abstract: Background and Aims As with other crop species, Al tolerance in rice (Oryza sativa) is widely different among cultivars. and the mechanism for tolerance is unknown. The Ca²⁺-displacement hypothesis. that is. Al displaces Ca²⁺ from critical sites in the root apoplast, was predicted to be the essential mechanism for causing Al toxicity in rice cultivars. If displacement of Ca is an essential cause of Al toxicity in rice, Al toxicity may Show the same trend as toxicities of elements such as Sr and Ba that are effective in displacing Ca.

Methods The interactive effects of Al, Ca, Sr and Ba on root elongation of rice cultivars with different Al tolerance were evaluated in hydroponic culture. Al and Ca accumulation in root tips was also investigated.

Key Results and Conclusions Not only Al but also Sr and Ba applications inhibited toot growth of rice cultivars under low Ca conditions. As expected, rice cultivars more tolerant of Sr and Ba were also tolerant of Al (japonica > indica). Although Mg application did not affect Sr or Ba toxicity Mg alleviated Al toxicity to the same level as Ca application. In addition. Ca application decreased the Al content in root tips without displacement. These results suggest that Ca does not have a specific, irreplaceable role in Al toxicity, unlike Sr and Ba toxicities. Alleviation of Al toxicity with increasing concentrations of Ca in rice cultivars is due to increased ionic strength, not due to decreased Al activity. The difference in Al tolerance between indica and japonica cultivars disappear under high ionic strength conditions, suggesting that different electrochemical characteristics of root-tip cells are related to the significant difference in Al tolerance under low ionic strength conditions. (C) 2001 Annals of Botany Company.

Keywords: aluminium tolerance; aluminium toxicity; barium toxicity; Ca²⁺-displacement hypothesis; Oryza sativa L.; root cell walls; strontium toxicity; Triticum aestivum L.