AOBPreview originally published online on November 16, 2004
Annals of Botany 2005 95(2):379-385; doi:10.1093/aob/mci032
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Annals of Botany 95/2 © Annals of Botany Company 2004; all rights reserved
Interactive Effects of Al, Ca and Other Cations on Root Elongation of Rice Cultivars Under Low pH
1 Graduate School of Agriculture, Hokkaido University, Kita 9, Nishi 9, Kitaku, Sapporo 060-8589, Japan and 2 National Agricultural Center, 3-1-1 Kannondai, Tsukuba, Ibaraki 305-8666, Japan
* For correspondence. E-mail nabe{at}chem.agr.hokudai.ac.jp
Received: 11 August 2004 Returned for revision: 27 August 2004 Accepted: 30 September 2004 Published electronically: 16 November 2004
• 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 tolerances 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 root 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 disappears 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.
Key words: Aluminium tolerance, aluminium toxicity, barium toxicity, Ca2+-displacement hypothesis, Oryza sativa L., root cell walls, strontium toxicity, Triticum aestivum L
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