Monitoring of aluminium‐induced inhibition of root elongation in four maize cultivars differing in tolerance to aluminium and proton toxicity

Four maize cultivars, which differ in tolerance to acid soils under field conditions (Zea mays L., acid soil‐tolerant C 525 M, BR 201 F and Adour 250, and acid soil‐sensitive HS 7777) were used to study the influence of pH (4.3 and 6.0) and Al (0, 20 and 50 μM) on the elongation of seminal roots in nutrient solution. Root elongation was inhibited by high H⁺ concentrations (pH 4.3) in cultivars C 525 M, Adour 250 and HS 7777 but not in BR 201 F. After 20 h exposure to Al, root elongation rates were more inhibited in cultivars BR 201 F and HS 7777 than in C 525 M and Adour 250. The use of a computerized linear displacement transducer system with high resolution (1 μm) allowed the monitoring of short‐term responses of root elongation to Al. In the three cultivars affected by H⁺ toxicity, but not in the acid‐tolerant BR 201 F, Al supply caused an immediate, but transient increase of relative root elongation rates. This result supports the hypothesis that Al‐induced growth stimulation is caused by amelioration of proton toxicity. The time required for 20 μM Al to induce a 5% decrease of root elongation rates was shorter in the Al‐sensitive BR 201 F (33 min) and HS 7777 (86 min) than in the Al‐tolerant C 525 M (112 min) and Adour 250 (146 min) cultivars. However, the response‐time to Al may be overestimated in the proton‐sensitive cultivars, due to the transient stimulation of root elongation rates induced by Al. According to our results, experiments intended to investigate primary mechanisms of Al toxicity should be started after less than 30 min exposure to toxic Al concentrations, using pH conditions which avoid Al‐induced growth stimulation due to amelioration of proton toxicity.