Separate mechanisms of aluminium toxicity for nitrate uptake and root elongation

Aluminium toxicity has been shown to decrease $NO_3^-$ uptake in Zea mays seedlings during the first 30 minutes after addition of Al³⁺. This suggests that $NO_3^-$ uptake inhibition could be a primary response to Al addition. We therefore tested the hypothesis that $NO_3^-$ uptake and root elongation are affected differently by Al³⁺. Eight-day old seedlings were exposed to 100 µm Al³⁺ in the presence of 1 or 10 mM Ca²⁺, added as either CaSO₄ or CaCl₂. In the presence of Al³⁺, cumulative uptake of $NO_3^-$ during an 8 h period was not affected by Ca²⁺ level (1 or 10 mM). Root elongation at 1 mM Ca²⁺ was decreased to 63% of the control by the presence of Al³⁺. Raising ambient Ca²⁺ from 1 to 10 mM in the presence of A1³⁺ restored elongation rates to 78% (CaCl₂) and 88% (CaSO₄) of elongation without Al. Because reductions in root elongation were partially overcome by added Ca²⁺, but lowered uptake of $NO_3^-$ was not, it was concluded that Al³⁺ toxicity decreased root growth and $NO_3^-$ uptake by different mechanisms.