Zinc-phosphorus interactions have been frequently studied using a diverse number of crop species, but attainment of reproducible Zn deficiencies, especially severe ones, has been hampered by the use of conventional hydroponic solutions wherein contaminating levels of Zn are often near-adequate for normal growth. We utilized novel, chelator-buffered nutrient solutions for precise imposition of Zn deficiencies. Tomato (Lycopersicon esculentum L. cv. Jackpot or Celebrity) seedlings were grown for 15 to 18 d in nutrient solutions containing 200, 600, or 1200/xM P, and 0 to 91/zM total Zn. Computed free Zn 2÷ activities were buffered at ~< 10 -l°3 M by inclusion of a 100-/zM excess (above the sum of the micronutrient metal concentrations) of the chelator DTPA. At total added Zn = 0, acute Zn deficiency resulted in zero growth after seedling transfer, and plant death prior to termination. Free Zn 2* activities ~< 10-1°6 M resulted in Zn deficiencies ranging from mild to severe, but activities <~ 10-~2 were required to cause hyperaccumulation of shoot P to potentially toxic levels. Despite severe Zn deficiency (i.e. ca. 20% of control growth), tissue Zn levels were usually much higher than the widely reported critical value of 20 mg kg-~, which may be an artifact of the selection of DTPA for buffering free Zn 2÷. Across Zn treatments, increasing solution P depressed growth slightly, especially in Celebrity, but corresponding increases in tissue P (indicative of enhanced P toxicity) or decreases in tissue Zn (P-induced Zn deficiency) were not observed. The depressive effect of P was also not explained by reductions in the water-soluble Zn fraction. Within 40 h, restoration of Zn supply did not ameliorate high leakage rates (as measured by K + efflux) of Zn-deficient roots. Similarly, transfer of Zn-sufficient plants to deficient solutions did not induce leakiness within 40 h. Foliar sprays of ZnSO 4 almost completely corrected both Zn deficiency and membrane leakiness of plants grown in low-Zn solutions. Hence, maintenance of root membrane integrity appears to depend on the overall Zn nutritional status of the plant, and not on the presence of certain free Zn 2÷ levels in the root apoplasm.

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.

Agricultural drainage water from the west side of the San Joaquin Valley, CA is highly salinized, and is often contaminated with an assortment of metals and metalloids, including Se. Among proposed disposal options, vegetation management may be a critical component of remediation strategies designed to reduce soil or sediment concentrations of Se to safe levels. Soil salinity (mostly sodium sulfate) and B pose serious limitations to the use of many plant species. We screened a number of cultivars or lines of species from the genera Astragalus, Leucaena, Medicago, Trifolium, Elymus, Elytrigia, Festuca, Leymus, Oryzopsis, Psathyrostachys, Puccinellia, and Sporobolus for tolerance to salinity and B using solution culture methods. Considerable variation in tolerance to salinity, both within and across species, was observed during seed germination. Electrical conductivities required to produce a 50% reduction in germination (EC50) ranged from 5 to 30 dS m−1. Boron levels up to 4.0 mM had only minimal effects on germination. The most promising genotypes, representing some 15 species, were then tested for salinity and B tolerance during the seedling growth stage. Lines of five species (Astragalus bisulcatus, A. racemosus, Elytrigia pontica, Puccinellia dlstans, and Sporobolus airoides) appeared most promising; all exhibited EC50 values > 20 dS m−1 and were unaffected by B concentrations up to 4.0 mM during seedling growth. Astragalus bisulcatus and A. racemosus are considered primary accumulators of Se; their tolerance of high salinity and B daring seedling growth make them particularly good candidates for remediation of Se‐enriched soils and sediments.

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Abstract-The effects of solution speciation on the bioavailability of trace metals are well documented, but the role of speciation in the bioavailability of oxyanionic trace elements that may form significant ion pairs with Ca and Mg in saline media has not been investigated. We assessed the effects of such ion pairing on the availability of selenate to representative monocotyledonous and dicotyledonous higher plants. Formation constants for the and ion pairs were experimentally determined using 0 0CaSO CaSeO 4 4 a Ca 2ϩ ion-selective electrode. The published value of 10 2.3 for formation was confirmed, but the value of 10 2.7 for 0 CaSO 4 was found to be in error; a value of 10 2.0 is proposed here as the correct formation constant. Five solution culture experiments 0 CaSeO 4 were conducted using alfalfa (Medicago sativa L.) or tall wheatgrass (Elytrigia pontica [Podp.] Holub) with treatments consisting of NaSeO 4 levels in combination with various levels of MgCl 2 or CaCl 2 . Both shoot Se concentrations and whole-plant Se contents were highly correlated with the free activity but were poorly correlated with the sum of the free ion plus Ca and Mg ion 2ϪSeO 4 pair species. Thus, we have shown, for the first time, that the free ion model of trace metal bioavailability is also valid for oxyanions that form complexes with Ca and Mg in saline media but that this conclusion hinges critically on the accuracy of the pertinent formation constants.