Aluminum Toxicity in Roots

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 Ca2" and Cl fluxes. In 50% of experiments, Al began to inhibit the net Ho influx in Scout 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.Al2 toxicity has been identified as one of the most important factors limiting plant growth in acid soils (5). Although a wide range of Al-related effects have been described in plants ' P.R.R.

Genotypical Differences in Aluminum Resistance of Maize Are Expressed in the Distal Part of the Transition Zone. Is Reduced Basipetal Auxin Flow Involved in Inhibition of Root Elongation by Aluminum?

“…an H ϩ /anion symport, which is in agreement with the findings of Collings et al (1992) and Ryan et al (1992) comparing two wheat cultivars differing in Al resistance; (b) enhanced respiration and thus bicarbonate production; and (c) enhanced release of organic acid anions, which at the low ambient pH in the apoplast will bind free protons. However, the release of organic anions appears unlikely to account for alkalization in the Al-sensitive cultivar, because it is well established that this will lead to Al resistance through complexation, as shown for malate in wheat (Delhaize et al, 1993b;Basu et al, 1994;Ryan et al, 1995aRyan et al, , 1995b, citrate in maize (Pellet et al, 1995), and oxalate in buckwheat (Zheng et al, 1998).…”

“…Collings et al (1992) discussed the role of the currents observed in transducing the gravitropic stimulus from the root cap to the EZ. By removing K ϩ , Na ϩ , Ca 2ϩ , or Cl Ϫ ions from the bathing solution or by applying the Ca 2ϩ channel blocker lanthanum, they could demonstrate that the currents measured were primarily composed of protons, a finding that is in accordance with Ryan et al (1992). Ca 2ϩ influx and Cl Ϫ efflux contribute to these currents to a lesser extent (Iwabuchi et al, 1989;Ryan et al, 1992).…”

“…Hence, gating of anion channels permeable for these anions would allow considerable efflux within a short time (Schmidt and Schroeder, 1994;Papernik and Kochian, 1997;Ryan et al, 1997). Even though there is agreement about the role of adequate cell wall pH in root elongation (Rayle and Cleland, 1992;Zieschang et al, 1993;Kutschera, 1994;Felle, 1998), a causal relation between the changes in pH profile observed and Al-induced inhibition of root growth remains a matter of debate (Ryan et al, 1992).…”

“…This view is in agreement with findings by Yan et al (1998) showing that maize roots are capable of coping with extremely low external pH if carefully adapted. Miyasaka et al (1989), Ryan et al (1992), andCollings et al (1992) explained the pH pattern along the rhizoplane as resulting from endogenous ionic currents traversing the root tip. Collings et al (1992) discussed the role of the currents observed in transducing the gravitropic stimulus from the root cap to the EZ.…”

Evaluation of aluminum resistance in hybrids of Brachiaria decumbens Stapf.

“…High levels of toxic aluminum in the soil solution impair root growth rate, and also reduce root numbers and length, which interferes in the absorption, transportation and use of nutrients (Ca 2+ , Mg 2+ , K 2+ e P) (Foy 1978, Kochian 1995, Ryan et al 1995. Acid soils often require lime and fertilizer application to overcome nutrient deficiencies and metal toxicity, as well as to increase crop yield.…”