DIRECT EVALUATION OF THE CA2-DISPLACEMENT HYPOTHESIS FOR AL TOXICITY()

One explanation for Al toxicity in plants suggests that Al displaces C a2+ from critical sites in the apoplasm. We evaluated the Ca2+-displac ement hypothesis directly using near-isogenic lines of wheat (Triticum aestivum L.) that differ in Al tolerance at a single locus. We measur ed both the growth and total accumulation (apoplasmic plus symplasmic) of Ca-45 and Al into roots that had been exposed to Al alone or to Al with other cations. Root growth in the Al-sensitive line was found to be severely inhibited by low activities of Al, even though Ca2+ accum ulation was relatively unaffected. In solutions containing the same ac tivity of the Al3+ and Ca2+ ions as above, but also including either 3 .0 mm Mg2+, 3.0 nm Sr2+, or 30 mm Na+, growth improved, whereas Ca-45( 2+) significantly decreased. Since most of the Ca-45(2+) accumulated b y roots during short-term treatments will reside in the apoplasm, thes e results indicate that displacement of Ca2+ from the apoplasm by Al c annot account for the Al-induced inhibition of root growth and, theref ore, do not support the Ca2+-displacement hypothesis for Al toxicity. We also show that total accumulation of Al by root apices is greater i n the Al-sensitive genotype than the Al-tolerant genotype and suggest that cation amelioration of Al toxicity is caused by the reduction in Al accumulation.