Separate mechanisms of aluminium toxicity for nitrate uptake and root elongation

Abstract

Aluminium toxicity has been shown to decrease NO₃ ^- uptake in Zea mays seedlings during the first 30 minutes after addition of Al³⁺. This suggests that NO₃ ^- uptake inhibition could be a primary response to Al addition. We therefore tested the hypothesis that NO₃ ^- 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 NO₃ ^--N 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 Al³⁺ 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 NO₃ ^- was not, it was concluded that Al³⁺ toxicity decreased root growth and NO₃ ^- uptake by different mechanisms.