Aluminum Toxicity in Roots

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Abstract
The inhibition of root growth by aluminum (Al) is well established, yet a unifying mechanism for Al toxicity remains unclear. The association between cell growth and endogenously generated ionic currents measured in many different systems, including plant roots, suggests that these currents may be directing growth. A vibrating voltage microelectrode system was used to measure the net ionic currents at the apex of wheat (Triticum aestivum L.) roots from Al-tolerant and Al-sensitive cultivars. We examined the relationship between these currents and Al-induced inhibition of root growth. In the Al-sensitive cultivar, Scout 66, 10 micromolar Al (pH 4.5) began to inhibit the net current and root elongation within 1 to 3 hours. These changes occurred concurrently in 75% of experiments. A significant correlation was found between current magnitude and the rate of root growth when data were pooled. No changes in either current magnitude or growth rate were observed in similar experiments using the Al-tolerant cultivar Atlas 66. Measurements with ion-selective microelectrodes suggested that H(+) influx was responsible for most of the current at the apex, with smaller contributions from Ca(2+) and Cl(-) fluxes. In 50% of experiments, Al began to inhibit the net H(+) influx in Scott 66 roots at the same time that growth was affected. However, in more than 25% of cases, Al-induced inhibition of growth rate occurred before any sustained decrease in the current or H(+) flux. Although showing a correlation between growth and current or H(+) fluxes, these data do not suggest a mechanistic association between these processes. We conclude that the inhibition of root growth by Al is not caused by the reduction in current or H(+) influx at the root apex.