No abstract

The common “hardness cations”, Ca and Mg, are at least partially capable of alleviating the toxicities of metal ions, but quantitative data for their effect on Cu rhizotoxicity are lacking. We used 2‐d root elongation of wheat (Triticum aestivum L., cv. Yecora Rojo) in a simple medium (≥0.2 mM CaCl2) as a sensitive bioassay for examining how Ca, Mg, and pH affect Cu rhizotoxicity. A fourfold increase in solution CaT from 1 to 4 mM showed a slight alleviation of toxicity, but the effect could be completely ascribed to changes in ionic strength and thus in Cu2+ activity. Subsequently, a 25‐fold range in CaT (17‐fold range in Ca2+ activity) was found to improve Cu‐inhibited root elongation by some 30 to 35%. Increases in solution Mg2+ activity were equally or slightly more effective than comparable increases in free‐ion activity [denoted by parentheses, i.e., (Ca2+)] of Ca in alleviating Cu stress, an unexpected result because of reports of selective binding of Ca by apoplastic sites. Neither cation alleviated Cu rhizotoxicity as profoundly as they do with other metals such as Al. Lowering pH from 6.5 to 5.5 alleviated Cu stress slightly; a further decrease to pH 4.5 was more effective. Finally, seedlings were reared en masse in nine treatments selected from the previous root elongation studies, and root tips were analyzed for operationally defined apoplastic and symplastic Cu. Growth inhibition was only weakly correlated with symplastic Cu (or with apoplastic or total Cu), and the results suggested that Ca and Mg (but not H) afford some physiological protection against Cu that is not explained by simple competitive inhibition of Cu accumulation in the apical tissues.

Across a diverse spectrum of organisms, the absorption and toxicity of trace elements are usually correlated with the activity of the free metal ion, but reported exceptions to this generalization are increasing. For the first time, we tested the validity of the free-ion activity model (FIAM) in the case of terrestrial plants and organic acids that may be abundant in the soil solution and rhizosphere. Short-term (48-h) root elongation of wheat (Triticum aestitvum L.) in a simple medium (2 mM CaCl2, pH 6.0) was used to probe the toxicity of Cu and Zn in the presence of malonate, malate, and citrate. Precautions were taken to prevent biodegradation of the organic acids, and its absence was confirmed by ion chromatography. Copper speciation was verified using a Cu-selective ion electrode, and published stability constants were modified to improve agreement between measured and calculated Cu2+ activities. With additions of both malonate and malate, Cu toxicity was alleviated but not to the extent predicted by the FIAM; the Cu-ligand complexes seemingly contributed to the toxicity. No such departures were observed with citrate and Cu nor with any of the three ligands in combination with Zn. Thus, exceptions to the FIAM occur with higher plants as well as with aquatic biota but do not seem to occur in a predictable or systematic fashion with respect to metal or organic acid under investigation. Several possible explanations for the observed departures from the FIAM are discussed, including the possibility of accidental cotransport of metal and ligand into the cytoplasm.

No abstract

The effects of zinc (Zn) and iron (Fe) deficiencies on phytosiderophore (PS) exudation by three barley (Hordeum vulgare L.) cultivars differing in Zn efficiency were assessed using chelator-buffered nutrient solutions. A similar study was carried out with four wheat (Triticum aestivum L. and T. durum Desf.) cultivars, including the Zn-efficient Aroona and Zn-inefficient Durati. Despite severe Zn deficiency, none of the barley or wheat cultivars studied exhibited significantly elevated PS release rates, although there was significantly enhanced PS exudation under Fe deficiency. Aroona and Durati wheats were grown in a further study of the effects of phosphate (P) supply on PS release, using 100 microM KH2PO4 as high P, or solid hydroxyapatite as a supply of low-release P. Phytosiderophore exudation was not increased due to P treatment under control or Zn-deficient conditions, but was increased by Fe deficiency. Accumulation of P in shoots of Zn- and Fe-deficient plants was seen in both P treatments, somewhat more so under the KH2PO4 treatment. Zinc-efficient wheats and grasses have been previously shown to exude more PS under Zn deficiency than Zn-inefficient genotypes. We did not observe Zn-deficiency-induced PS release and were unable to replicate the results of previous researchers. The tendency for Zn deficiency to induce PS release seems to be method dependent, and we suggest that all reported instances may be explained by an induced physiological deficiency of Fe.

No abstract

An Al-stimulated eux of malate from the root apex has been proposed as the primary mechanism whereby some wheat (Triticum aestivum L.) genotypes demonstrate marked resistance to the rhixotoxic metal Al. Appealing in its simplicity, the model has not been unequivocally validated, and suers from some signi®-cant discrepancies between estimated, steady-state concentrations of malate at the root surface and concentrations that are necessary to explain the resistance of the superior genotypes. Using two other rhizotoxic ions that are also chelated by malate, Cu(II) and La(III), we speci®cally probed whether the quantities of malate released by tolerant genotypes could eectively detoxify Al. Experiments with exogenous additions of malate to solution showed that ³200 lM malate is required to account for the dierence between Scout 66 (Al-sensitive) and Atlas 66 (Al-tolerant) wheats, and that this level of malate can also partially alleviate the toxicities of Cu and La. When simultaneously exposed to a mildly rhizotoxic level of Al (25 lM) to induce malate eux, Atlas exhibited a pronounced reduction in sensitivity to Cu. When, La was used as the proxy ion, however, no such Al-induced tolerance to La was observed, a result that refutes the signi®cance of malate eux to Al tolerance. Additional experiments using Al, Cu, and La in combination suggested that a trivalent ion can alleviate Cu toxicity directly (i.e. via competition for apoplastic binding), providing an alternative explanation for the ability of Al to detoxify Cu in Atlas. Using a weight-of-evidence approach, we argue that malate eux plays at most a minor role in the dierential Al tolerance of wheat, and that a more integrative, multifaceted model of tolerance is needed.

No abstract