David R. Parker

Reevaluating the free-ion activity model of trace metal availability to higher plants

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) of metal bioavailability to higher plants. Binding by an apoplastic functional group is often envisioned as a requisite step in the absorption or toxicity of a metal, and can be modeled in a variety of ways. Typically, however, speciation of the bulk solution is calculated without regard to such surface binding, even though it could influence the pertinent mass balance expressions. A more thorough treatment considers simultaneous formation of both the metal-ligand complex in solution (ML) and the metal-cell surface complex (M-X). Here, empirical conformity to the FIAM can be expected, but only under pivotal assumptions about the relative sizes of the test solution and the root biomass, and about the relative binding strength of L and -X. Moreover, empirical conformity to the FIAM does not preclude cell-surface binding of the complexed metal followed by ligand exchange (ML + -X ⇆ M-X + L), so that correlations between biological response and free metal-ion activity imply nothing about the molecular species that actually interacts with the cell surface. Computer simulations of Cu (II) binding by a model apoplastic ligand are used to illustrate these and other key features of the FIAM. Departures from the FIAM seem most likely when (i) the quantity of the metal-complexing ligand is limited (as may be the case in soil solution or in the rhizosphere), and/or (ii) the solution ligand is very weak.

ZINC-PHOSPHORUS INTERACTIONS IN TWO CULTIVARS OF TOMATO (LYCOPERSICON ESCULENTUM L.) GROWN IN CHELATOR-BUFFERED NUTRIENT SOLUTIONS

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 μM P, and 0 to 91 μM total Zn. Computed free Zn2+ activities were buffered at ≤10-10.3M by inclusion of a 100-μM 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 Zn2+ activities ≤10-10.6M resulted in Zn deficiencies ranging from mild to severe, but activities ≤10-11.2 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-1, which may be an artifact of the selection of DTPA for buffering free Zn2+. 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 ZnSO4 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 Zn2+ levels in the root apoplasm.

Advances in Solution Culture Methods for Plant Mineral Nutrition Research

Publisher Summary This chapter reviews some of the advances in solution culture methodology for plant mineral nutrition research. It focuses primarily on methodological advancements for the specific study of mineral nutrition, but recognizes that the methods may be useful for other types of physiological research as well. The methods are described for overcoming one of the most fundamental limitations of traditional solution culture, the absence of chemical “buffering” of nutrient concentrations. In soils, the soil solution is depleted of nutrients by plant uptake, but continuously replenished via chemical and biological reactions of the solid phase. The uses of flowing solution cultures are discussed in detail. The chapter provides an overview of methods wherein nutrients are continuously or intermittently added in proportion to plant growth. For convenience, abbreviations and chemical names for the chelating agents and pH buffers discussed throughout this paper are summarized.

Phytosiderophore release in relation to micronutrient metal deficiencies in barley

Phytosiderophore release occurs under both iron and zinc deficiencies in representative Poaceae and has been speculated to be a general adaptive response to enhance the acquisition of micronutrient metals. To test this hypothesis, phytosiderophore (PS) release rates from barley (Hordeum vulgare cv. CM72) subjected to deficiencies of Fe, Zn, Mn, and Cu were compared using chelator-buffered nutrient solutions. PS release rates were determined at two day intervals during onset and development of deficiency symptoms. Plant dry matter yields and nutrient concentrations, measured at three time points were used to construct growth curves for calculation of PS release per unit root mass and estimation of critical internal nutrient levels associated with PS release. In comparison to trace metal-sufficient control plants, dry matter production was markedly reduced in the Zn, Mn, and Cu deficiency treatments, with final relative yields of 49, 61, and 34%, respectively. Relative yields for Fe-deficient plants grown at three suboptimal Fe levels ranged from 95 to 33% of control, and provided a basis for comparison of PS release rates by Zn-, Mn-, and Cu-deficient plants at similar levels of growth inhibition. Under Fe deficiency, PS release increased with severity of the deficiency as measured by foliar Fe concentration, yield reduction, and chlorosis. Changes in PS release rates over time suggested a cyclical pattern that may be regulated by Fe concentration in the plant shoot. The highest rate of PS release (35 μmol g−1 root dw 2 h−1) was measured after 10 days of growth at pFe 19, whereas control plants adapted for growth at pFe 17 released only 2 to 3 μmol g−1 root dw 2 h−1. In a second experiment, maximum PS release rates for barley subjected to Zn, Mn, and Cu deficiencies were only 2.6, 2.5 and 1 μmol g−1 2 h−1, respectively and were only slightly elevated over those of the control plants (ca. 1 μmol g−1 root dw 2 h−1) grown at pFe 16.5. Moreover, enhanced PS release under Zn deficiency occurred much later, after the deficiency had already caused severely reduced growth. The results suggest that phytosiderophore release in this barley cultivar is a specific response to Fe deficiency and is not significantly induced in response to deficiencies of other trace metals.

Uptake and accumulation of four PPCP/EDCs in two leafy vegetables

Many pharmaceutical and personal care products (PPCPs) and endocrine-disrupting chemicals (EDCs) are present in reclaimed water, leading to concerns of human health risks from the consumption of food crops irrigated with reclaimed water. This study evaluated the potential for plant uptake and accumulation of four commonly occurring PPCP/EDCs, i.e., bisphenol A (BPA), diclofenac sodium (DCL), naproxen (NPX), and 4-nonylphenol (NP), by lettuce (Lactuca sativa) and collards (Brassica oleracea) in hydroponic culture, using (14)C-labeled compounds. In both plant species, plant accumulation followed the order of BPA > NP > DCL > NPX and accumulation in roots was much greater than in leaves and stems. Concentrations of (14)C-PPCP/EDCs in plant tissues ranged from 0.22 ± 0.03 to 927 ± 213 ng/g, but nearly all (14)C-residue was non-extractable. PPCP/EDCs, particularly BPA and NP, were also extensively transformed in the nutrient solution. Dietary uptake of these PPCP/EDCs by humans was predicted to be negligible.

Zinc deficiency-induced phytosiderophore release by the Triticaceae is not consistently expressed in solution culture.

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 μM 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.

Probing the malate hypothesis of differential aluminum tolerance in wheat by using other rhizotoxic ions as proxies for Al

Abstract. An Al-stimulated efflux 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 suffers from some significant 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 specifically probed whether the quantities of malate released by tolerant genotypes could effectively detoxify Al. Experiments with exogenous additions of malate to solution showed that ≥200 μM malate is required to account for the difference 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 μM) to induce malate efflux, 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 significance of malate efflux 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 efflux plays at most a minor role in the differential Al tolerance of wheat, and that a more integrative, multifaceted model of tolerance is needed.

Perchlorate in the environment: the emerging emphasis on natural occurrence.

Environmental context. Perchlorate is an emerging environmental contaminant that has a unique ability to interfere with normal iodine uptake by the human thyroid gland, and thus has the potential to adversely affect normal growth and development of infants and children. In the last decade, perchlorate’s environmental behaviour has been intensely studied in the United States, but has received little attention elsewhere. Recent evidence strongly suggests that perchlorate occurs at low levels naturally, and is ubiquitously present in the human diet. An atmospheric source for this natural occurrence is strongly implicated, and the naturally occurring isotopes of oxygen and chlorine offer considerable promise for unravelling the chemical mechanisms responsible. Abstract. Salts of perchlorate (ClO4–) are widely used in solid rocket propellants, and in a variety of munitions, explosives, and pyrotechnics; it is an emerging environmental pollutant that has caused widespread water contamination in the United States and probably other locales worldwide. Perchlorate interferes with normal iodine uptake by the human thyroid, and may thus lead to a lowered production of key hormones that are needed for proper growth and development. Debate about ‘safe’ levels of perchlorate is being fuelled by considerable evidence of declining iodine intake in many western nations. With the advent of more sensitive analytical methods, perchlorate is being found as a nearly ubiquitous contaminant in water, beverages, fresh produce, and other sources of human exposure. Recent evidence, including isotopic forensics, makes a strong case for more widespread natural occurrence of perchlorate, outside of the long-established occurrence in caliches of the Atacama Desert in Chile. Many questions about this low-level occurrence remain, including the role of microbial metabolism in attenuating the concentrations typically found in surface- and groundwaters.

Selenium phytoremediation potential of Stanleya pinnata

Disposal of saline irrigation wastewater in hydrologically closed sinks in the semi-arid western U.S. has concentrated selenium-rich salts to hazardous levels and phytoextraction, along with plant-enhanced volatilization of methyl-selenides, is an active area of research. Here, we provide an overview of our ongoing studies of Stanleya pinnata (Brassicaceae), a previously unstudied candidate that is a primary accumulator (hyperaccumulator) of Se that is widespread and broadly adapted in the western U.S. When grown in sand culture under uniform greenhouse conditions, 16 populations representing S. pinnatas broad biogeographical range differed in shoot Se concentration by 1.4- to 3.6-fold, and the shoot concentrations were positively correlated with the indigenous soil Se levels at the collection sites. Thus, S. pinnata exhibits significant ecotypic variation in Se accumulation. All populations accumulated SeO42- preferentially over SO42- consistent with S. pinnatas classification as a primary Se accumulator, while hydroponically-grown Brassica juncea consistently accumulated sulfate preferentially over selenate. The Se in S. pinnata shoots was predominately in the soluble amino-acid pool, which may serve as direct precursor to volatile forms such as dimethyldiselenide; inorganic forms (e.g. selenate) dominated in B. juncea. Preliminary results suggest that S. pinnata may volatilize unusually large quantities of Se when grown at high sulfate concentrations, an unexpected result not heretofore reported in any species. In a sand–culture experiment, S. pinnata exhibited excellent tolerance of excess boron, but only moderate tolerance of salinity, and superior genotypes will likely be needed for phytoremediation of highly salinized soils and sediments. Stanleya pinnata is a perennial that responded favorably to repeated cuffing in the greenhouse, a trait that could prove valuable in field-scale phytoremediation. Environmental concerns about Se are common in the western USA, and S. pinnata is a potentially useful species for phytoremediation due to its broad adaptation to western soils and environments, and its uptake, metabolism and volatilization of Se.

Effect of transpiration on plant accumulation and translocation of PPCP/EDCs.

The reuse of treated wastewater for agricultural irrigation in arid and hot climates where plant transpiration is high may affect plant accumulation of pharmaceutical and personal care products (PPCPs) and endocrine disrupting chemicals (EDCs). In this study, carrot, lettuce, and tomato plants were grown in solution containing 16 PPCP/EDCs in either a cool-humid or a warm-dry environment. Leaf bioconcentration factors (BCF) were positively correlated with transpiration for chemical groups of different ionized states (p < 0.05). However, root BCFs were correlated with transpiration only for neutral PPCP/EDCs (p < 0.05). Neutral and cationic PPCP/EDCs showed similar accumulation, while anionic PPCP/EDCs had significantly higher accumulation in roots and significantly lower accumulation in leaves (p < 0.05). Results show that plant transpiration may play a significant role in the uptake and translocation of PPCP/EDCs, which may have a pronounced effect in arid and hot climates where irrigation with treated wastewater is common.