PLANT PHYSIOLOGY , Vol 113, Issue 4 1351-1357, Copyright © 1997 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Direct Evaluation of the Ca2+-Displacement Hypothesis for Al Toxicity
P. R. Ryan, R. J. Reid and F. A. Smith
Commonwealth Scientific and Industrial Research Organization Division of Plant Industry, G.P.O. Box 1600, Canberra, ACT 2601, Australia (P.R.R.)
One explanation for Al toxicity in plants suggests that Al displaces Ca2+
from critical sites in the apoplasm. We evaluated the Ca2+-displacement
hypothesis directly using near-isogenic lines of wheat (Triticum aestivum
L.) that differ in Al tolerance at a single locus. We measured both the
growth and total accumulation (apoplasmic plus symplasmic) of 45Ca 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+ accumulation was relatively
unaffected. In solutions containing the same activity of the Al3+ and Ca2+
ions as above, but also including either 3.0 mM Mg2+, 3.0 mM Sr2+, or 30 mM
Na+, growth improved, whereas 45Ca2+ accumulation was significantly
decreased. Since most of the 45Ca2+ accumulated by roots during short-term
treatments will reside in the apoplasm, these results indicate that
displacement of Ca2+ from the apoplasm by Al cannot account for the
Al-induced inhibition of root growth and, therefore, do not support the
Ca2+-displacement hypothesis for Al toxicity. We also show that total
accumulation of Al by root apices is greater in 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.
