Change in apoplastic aluminum during the initial growth response to aluminum by roots of a tolerant maize variety

Root elongation, hematoxylin staining, and changes in the ultrastructure of root-tip cells of an Al-tolerant maize variety (Zea mays L. C 525 M) expos ed to nutrient solutions with 20 mu M Al (2.1 mu M Al3+ activity) for 0, 4, and 24 h were investigated in relation to the subcellular distribution of Al using scanning transmission electron microscopy and energy-dispersive x- ray microanalysis on samples fixed by different methods. Inhibition of root -elongation rates, hematoxylin staining, cell wall thickening, and disturba nce of the distribution of pyroantimoniate-stainable cations, mainly Ca, wa s observed only after 4 and not after 24 h of exposure to Al. The occurrenc e of these transient, toxic Al effects on root elongation and in cell walls was accompanied by the presence of solid Al-P deposits in the walls. Where as no Al was detectable in cell walls after 24 h, an increase of vacuolar A l was observed after 4 h of exposure. After 24 h, a higher amount of electr on-dense deposits containing Al and P or Si was observed in the vacuoles. T hese results indicate that in this tropical maize variety, tolerance mechan isms that cause a change in apoplastic Al must be active. Our data support the hypothesis that in Al-tolerant plants, Al can rapidly cross the plasma membrane; these data clearly contradict the former conclusions that Al main ly accumulates in the apoplast and enters the symplast only after severe ce ll damage has occurred.