AL3-CA2+ INTERACTIONS IN ALUMINUM RHIZOTOXICITY .1. INHIBITION OF ROOT-GROWTH IS NOT CAUSED BY REDUCTION OF CALCIUM-UPTAKE()

The cation Al3+ is toxic to plants at micromolar concentrations and ca n severely inhibit root growth in solution experiments.1 The inhibitio n of Ca2+ uptake in roots by Al3+ has been proposed as a possible mech anism for Al3+ toxicity, and in this study the hypothesis was tested d irectly. Root growth and Ca2+ uptake were measured in 5-d-old seedling s of wheat (Triticum aestivum L. Thell) during exposure to Al3+ in a l ow-Ca2+ basal medium, and to Al3+ in the presence of added cations. Up take of Ca2+ in whole roots and translocation to the shoot were measur ed using Ca-45(2+), and localized measurements of net Ca2+ flux were a lso made at the root apex using the technique of microelectrode ion-fl ux estimation. Treatment with 2.64 muM AlCl3 in 226 muM CaCl2, at pH 4 .5, severely inhibited root growth without affecting Ca2+ uptake. Addi tion of 30 mM Na+, 3 mM Mg2+ or 50 muM tris(ethylenediamine)cobalt(III ) to this Al3+ treatment restored root growth but significantly reduce d Ca2+ uptake measured over the entire root system and at the root ape x. The Al3+ and Ca2+ concentrations were adjusted so that the activiti es of the Al3+ and Ca2+ ions were constant in all solutions (1.5 muM a nd 200 muM, respectively). Root growth can be severely inhibited by Al 3+ concentrations that do not affect Ca2+ uptake, while the addition o f ameliorating cations depresses Ca2+ uptake. These results argue agai nst the hypothesis that Al3+ inhibits root growth by reducing Ca2+ upt ake.