SEPARATE MECHANISMS OF ALUMINUM TOXICITY FOR NITRATE UPTAKE AND ROOT ELONGATION

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 mu M 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.