David R. Parker

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The relative plant availability of selenate versus selenite depends on the concentrations of competing ions, specifically sulfate and phosphate, respectively. In solution culture, the concentration of phosphate is typically 100- to 1000-fold greater than in soil solution, an artifact that could lead to underestimation of the phytoavailability of selenite. A(More)
Root elongation in short-term experiments with wheat (Triticum aestivum L.) seedlings demonstrated that the following ions were rhizotoxic in the order Cu2+>Al 3+≫ H+>Zn2+≫ Na+. Additions of Ca2+ and Mg2+ alleviated the toxicity, but the relative ameliorative effectiveness of Ca2+ and Mg2+ depended upon the toxicant. The effectiveness of Mg2+ relative to(More)
The effects of excess zinc (Zn) on solution-cultured wheat (Triticum aestivum L., cv. Yecora Rojo) and radish (Raphanus sativus L., cv. Cherry Belle) were studied, using both short-term root elongation studies and longer term split-root experiments. Alleviation of Zn rhizotoxicity by Mg and K was observed, with especially dramatic alleviation of root(More)
The uptake of trace metals by plants is commonly assumed to depend on the free metal-ion activity, rather than on the total concentration of dissolved metal. Although this free-ion hypothesis has proved to be useful for the interpretation and prediction of metal uptake, several exceptions have been reported where metal complexes also affected metal uptake(More)
Knowledge of the mechanistic basis of differential aluminum (Al) tolerance depends, in part, on an improved ability to quantify Al located in the apoplastic and symplastic compartments of the root apex. Using root tips excised from seedlings of an Al-tolerant wheat cultivar (Triticum aestivum L. cv Yecora Rojo) grown in Al solutions for 2 d, we established(More)
Direct plant uptake of metals bound to chelating agents has important implications for metal uptake and the free-ion activity model. Uptake of hydrophilic solutes such as metal–EDTA complexes is believed to occur via bypass apoplastic flow, but many questions remain about the relative importance and selectivity of this pathway. In this study, Brassica(More)
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(More)
The resistance of some plants to Al (aluminium or aluminum) has been attributed to the secretion of Al(3+)-binding organic acid (OA) anions from the Al-sensitive root tips. Evidence for the 'OA secretion hypothesis' of resistance is substantial, but the mode of action remains unknown because the OA secretion appears to be too small to reduce adequately the(More)
The plant uptake and toxicity of many metals show a marked dependence on the aqueous speciation of the metal, and these responses often correlate best with the activity of the free metal ion. Exceptions to this generalization have been observed, however, and we sought to critically reexamine the theoretical foundation of the free-ion activity model (FIAM)(More)