ARS | Publication request: DO ROOTS OF ALUMINUM TOLERANT PLANTS RELEASE ENOUGH ORGANIC ACIDS TO ACCOUNT FOR THE OBSERVED AL TOLERANCE?

Submitted to: Plant Physiology
Publication Type: Abstract Only
Publication Acceptance Date: June 18, 2000
Publication Date: N/A

Technical Abstract: Root exposure to Al3+ has been shown to trigger an Al-tolerance mechanism that involves excretion of organic acids from the root apex into the rhizosphere. Consequently Al3+ is complexed, preventing its entry into the root and minimizing phytotoxicity. This response has been shown in many important crop plants, such as wheat, maize, rye, and soybean, where the measurements of organic release in hydroponic culture have been made in th bulk solution, not in the rhizosphere. This has left this research somewhat open to controversy, since one can only estimate root surface concentrations of organic acids and Al3+-organic acid interactions. Here, we have used novel techniques to sample nanoliter volumes in the unstirred layer at the root surface and then to measure organic acid concentrations in these samples with a specially adapted capillary zone electrophoresis system, allowing better resolution of the spatial and temporal aspects of organic acid release. Roots from the Al-tolerant wheat cultivar 'Atlas 66', which releases malate when exposed to Al3+, were exposed to various levels of Al3+ and their rhizospheres microsampled with a glass microelectrode along the first cm of the root tip. In 50mM Al3+ (32mM activity), malate exudation was reasonably uniform in pattern and the steady-state malate concentration at the root surface averaged 91mM along the first five millimeters. The released malate reduced Al3+ activity to 400 nM, a 98% reduction in activity according to the Geochem-PC program. Control plants (0 mM Al3+ activity) had much lower levels of malate exudation. Results were compared to the output from existing mathematical models that have been used to simulate organic acid release and rhizosphere Al3+ activities, and these findings will also be presented.