Na+ Tolerance and Na+ Transport in Higher Plants

Organic acids, such as malate, citrate and oxalate, have been proposed to be involved in many processes operating in the rhizosphere, including nutrient acquisition and metal detoxification, alleviation of anaerobic stress in roots, mineral weathering and pathogen attraction. A full assessment of their role in these processes, however, cannot be determined unless the exact mechanisms of plant organic acid release and the fate of these compounds in the soil are more fully understood. This review therefore includes information on organic acid levels in plants (concentrations, compartmentalisation, spatial aspects, synthesis), plant efflux (passive versus active transport, theoretical versus experimental considerations), soil reactions (soil solution concentrations, sorption) and microbial considerations (mineralization). In summary, the release of organic acids from roots can operate by multiple mechanisms in response to a number of well-defined environmental stresses (e.g., Al, P and Fe stress, anoxia): These responses, however, are highly stress- and plant-species specific. In addition, this review indicates that the sorption of organic acids to the mineral phase and mineralisation by the soil's microbial biomass are critical to determining the effectiveness of organic acids in most rhizosphere processes.

Organic acids in the rhizosphere: a critical review

Response to cadmium in higher plants

Toxic metal pollution of waters and soils is a major environmental problem, and most conventional remediation approaches do not provide acceptable solutions. The use of specially selected and engineered metal-accumulating plants for environmental clean-up is an emerging technology called phytoremediation. Three subsets of this technology are applicable to toxic metal remediation: (1) Phytoextraction--the use of metal-accumulating plants to remove toxic metals from soil; (2) Rhizofiltration--the use of plant roots to remove toxic metals from polluted waters; and (3) Phytostabilization--the use of plants to eliminate the bioavailability of toxic metals in soils. Biological mechanisms of toxic metal uptake, translocation and resistance as well as strategies for improving phytoremediation are also discussed.

Phytoremediation: A Novel Strategy for the Removal of Toxic Metals from the Environment Using Plants

In most soils, inorganic phosphorus occurs at fairly low concentrations in the soil solution whilst a large proportion of it is more or less strongly held by diverse soil minerals. Phosphate ions can indeed be adsorbed onto positively charged minerals such as Fe and Al oxides. Phosphate (P) ions can also form a range of minerals in combination with metals such as Ca, Fe and Al. These adsorption/desorption and precipitation/dissolution equilibria control the concentration of P in the soil solution and, thereby, both its chemical mobility and bioavailability. Apart from the concentration of P ions, the major factors that determine those equilibria as well as the speciation of soil P are (i) the pH, (ii) the concentrations of anions that compete with P ions for ligand exchange reactions and (iii) the concentrations of metals (Ca, Fe and Al) that can coprecipitate with P ions. The chemical conditions of the rhizosphere are known to considerably differ from those of the bulk soil, as a consequence of a range of processes that are induced either directly by the activity of plant roots or by the activity of rhizosphere microflora. The aim of this paper is to give an overview of those chemical processes that are directly induced by plant roots and which can affect the concentration of P in the soil solution and, ultimately, the bioavailability of soil inorganic P to plants. Amongst these, the uptake activity of plant roots should be taken into account in the first place. A second group of activities which is of major concern with respect to P bioavailability are those processes that can affect soil pH, such as proton/bicarbonate release (anion/cation balance) and gaseous (O2/CO2) exchanges. Thirdly, the release of root exudates such as organic ligands is another activity of the root that can alter the concentration of P in the soil solution. These various processes and their relative contributions to the changes in the bioavailability of soil inorganic P that can occur in the rhizosphere can considerably vary with (i) plant species, (ii) plant nutritional status and (iii) ambient soil conditions, as will be stressed in this paper. Their possible implications for the understanding and management of P nutrition of plants will be briefly addressed and discussed.

Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: A review

GEOCHEM-PC—A Chemical Speciation Program for IBM and Compatible Personal Computers

High Cd content in durum wheat (Triticum turgidum L. var durum) grain grown in the United States and Canada presents potential health and economic problems for consumers and growers. In an effort to understand the biological processes that result in excess Cd accumulation, root Cd uptake and xylem translocation to shoots in seedlings of bread wheat (Triticum aestivum L.) and durum wheat cultivars were studied. Whole-plant Cd accumulation was somewhat greater in the bread wheat cultivar, but this was probably because of increased apoplastic Cd binding. Concentration-dependent109Cd2+-influx kinetics in both cultivars were characterized by smooth, nonsaturating curves that could be dissected into linear and saturable components. The saturable component likely represented carrier-mediated Cd influx across root-cell plasma membranes (Michaelis constant, 20–40 nm; maximum initial velocity, 26–29 nmol g−1 fresh weight h−1), whereas linear Cd uptake represented cell wall binding of109Cd. Cd translocation to shoots was greater in the bread wheat cultivar than in the durum cultivar because a larger proportion of root-absorbed Cd moved to shoots. Our results indicate that excess Cd accumulation in durum wheat grain is not correlated with seedling-root influx rates or root-to-shoot translocation, but may be related to phloem-mediated Cd transport to the grain.

Characterization of Cadmium Binding, Uptake, and Translocation in Intact Seedlings of Bread and Durum Wheat Cultivars

This study attempts to identify the soil properties controlling the fractionation of copper into various soil pools and determine the influence of pH and metal loading on soil-solution free copper activity (pCu2+). The pCu2+ was determined in 0.01 M CaCl2 soil extracts using a copper ion selective electrode. We analyzed a wide variety of soils: urban, agricultural and forest soils from the Province of Quebec, New York State and Denmark. The pCu2+ ranged from 12.21 to 6.18. The relationships among pCu2+, total soil copper, total dissolved copper and soil pH are studied for their variability within and between sites as well as for the whole data set. Regression equations are presented for predicting soluble copper as a function of total soil copper and also for predicting pCu2+ as a function of total soil copper and soil pH.

Copper solubility and speciation of in situ contaminated soils : Effects of copper level, pH and organic matter

Field studies have shown that the addition of Zn to Cd-containing soils can help reduce accumulation of Cd in crop plants. To understand the mechanisms involved, this study used 109 Cd and 65 Zn to examine the transport interactions of Zn and Cd at the root cell plasma membrane of bread wheat (Triticum aestivum L.) and durum wheat (Triticum turgidum L. var. durum). Results showed that Cd 2+ uptake was inhibited by Zn 2+ and Zn 2+ uptake was inhibited by Cd 2+ . Concentration-dependent uptake of both Cd 2+ and Zn 2+ consisted of a combination of linear binding by cell walls and saturable, Michaelis-Menten influx across the plasma membrane. Saturable influx data from experiments with and without 10 μm concentrations of the corresponding inhibiting ion were converted to double reciprocal plots. The results revealed a competitive interaction between Cd 2+ and Zn 2+ , confirming that Cd 2+ and Zn 2+ share a common transport system at the root cell plasma membrane in both bread and durum wheat. The study suggests that breeding or agronomic strategies that aim to decrease Cd uptake or increase Zn uptake must take into account the potential accompanying change in transport of the competing ion.

Transport interactions between cadmium and zinc in roots of bread and durum wheat seedlings.

We report the soil solution speciation of Cd in 64 field-collected contaminated soils containing between 0.1 and 38 mg Cd kg-1. The soils were analyzed for pH (3.5−8.1), soil organic matter (8.0−108 g C kg-1), total dissolved Cd (0.03−182 μg Cd L-1), dissolved organic carbon (1.5−12 mg C L-1), and free Cd2+ (10-10−2 × 10-7 M). Free Cd2+ activity in solution was determined using differential pulse anodic stripping voltammetry (DPASV), assuming DPASV is sensitive to easily dissociated inorganic ion-pairs and free Cd2+ while excluding organic complexes. The solid/liquid partition coefficient (Kd) varied over a range from 10 to 100 000, and the fraction of the dissolved Cd present in solution as the estimated free Cd2+ species varied between 0 and 60% but averaged about 20%. The dissolved concentrations of Cd and the free Cd2+ activity in the soil solutions of contaminated soils of different origins can be predicted with reasonable accuracy using a simple competitive adsorption model dependent on pH and total...

Wendell A. Norvell

Papers

GEOCHEM-PC—A Chemical Speciation Program for IBM and Compatible Personal Computers

Characterization of Cadmium Binding, Uptake, and Translocation in Intact Seedlings of Bread and Durum Wheat Cultivars

Copper solubility and speciation of in situ contaminated soils : Effects of copper level, pH and organic matter

Transport interactions between cadmium and zinc in roots of bread and durum wheat seedlings.

Speciation and Complexation of Cadmium in Extracted Soil Solutions