Trace metal phytotoxicity in solution culture: a review.

Solution culture has been used extensively to determine the phytotoxic effects of trace metals. A review of the literature from 1975 to 2009 was carried out to evaluate the effects of As(V), Cd(II), Co(II), Cu(II), Hg(II), Mn(II), Ni(II), Pb(II), and Zn(II) on plants grown in solution. A total of 119 studies was selected using criteria that allowed a valid comparison of the results; reported toxic concentrations varied by five orders of magnitude. Across a range of plant species and experimental conditions, the phytotoxicity of the trace metals followed the trend (from most to least toxic): Pb approximately Hg >Cu >Cd approximately As >Co approximately Ni approximately Zn >Mn, with median toxic concentrations of (muM): 0.30 Pb, 0.47 Hg, 2.0 Cu, 5.0 Cd, 9.0 As, 17 Co, 19 Ni, 25 Zn, and 46 Mn. For phytotoxicity studies in solution culture, we suggest (i) plants should be grown in a dilute solution which mimics the soil solution, or that, at a minimum, contains Ca and B, (ii) solution pH should be monitored and reported (as should the concentrations of the trace metal of interest), (iii) assessment should be made of the influence of pH on solution composition and ion speciation, and (iv) both the period of exposure to the trace metal and the plant variable measured should be appropriate. Observing these criteria will potentially lead to reliable data on the relationship between growth depression and the concentration of the toxic metal in solution.

Localization and chemical speciation of Pb in roots of signal grass (Brachiaria decumbens) and Rhodes grass (Chloris gayana).

Lead (Pb) contamination of soils is of global importance but little is known regarding Pb uptake, localization, or the chemical forms in which Pb is found within plants, or indeed how some plants tolerate elevated Pb in the environment. Two grasses, signal grass (Brachiaria decumbens Stapf) (Pb-resistant) and Rhodes grass (Chloris gayana Kunth)(Pb-sensitive), were grown for 14 d in dilute nutrient solutions before examination of roots using transmission electron microscopy (TEM) to determine the distribution and speciation of Pb in situ. In both grasses, Pb was initially present primarily in the cytoplasm of rhizodermal and cortical cells before being sequestered within vacuoles as the highly insoluble (and presumably nontoxic) chloropyromorphite (Pb5(PO4)3Cl). In signal grass, Pb also accumulated within membranous structures (perhaps the Golgi apparatus), prior to apoplastic sequestration as chloropyromorphite. These findings suggest that the ability of signal grass to sequester insoluble Pb in the cell wall represents an additional and potentially important mechanism of Pb tolerance not possessed by the Pb-sensitive Rhodes grass.

Effects of lanthanum and cerium on the growth and mineral nutrition of corn and mungbean.

BACKGROUND AND AIMS: Plant growth responses to the rare earth elements lanthanum (La) and cerium (Ce) have been reported, but little is known about the effects of these two elements on plant mineral nutrition. METHODS: Corn (Zea mays 'Hycorn 82') and mungbean (Vigna radiata 'Berken') were grown in continuous flowing nutrient solutions containing 0, 0.2, 1.0 and 5.0 microm La or Ce. At harvest plants were divided into roots and shoots, dried, weighed and analysed for macro- and micronutrients, as well as for La and Ce. KEY RESULTS: La and Ce did not increase the growth of corn or mungbean. The dry weight of corn shoots was decreased by 32 % in the presence of 5.0 microm Ce; the other La and Ce concentrations had no effect. La and Ce concentrations of 0.9 and 5.0 microm decreased the shoot dry weight of mungbean by 75 or 95 %, the two elements having closely similar effects. Decreases in the uptake of Ca, Na, Zn and Mn by corn were observed with increases in solution La and Ce. For mungbean, the uptake rates of all measured elements decreased with increases in solution La and Ce. The concentrations of La and Ce in the roots of both species were higher than in the shoots and increased strongly with increasing concentrations of La or Ce in solution. The La and Ce concentrations in mungbean shoots were always higher than in corn shoots. CONCLUSIONS: La and Ce did not enhance the growth of corn or mungbean, but decreased the growth, root function and consequently the nutritional status of mungbean at concentrations >0.2 microm in solution. It is concluded that if La or Ce have positive effects on corn and mungbean growth, they can only occur at solution concentrations below 0.2 microm.

Toxic effects of Pb2+ on growth of cowpea (Vigna unguiculata).

A concentration as low as 1 microM lead (Pb) is highly toxic to plants, but previous studies have typically related plant growth to the total amount of Pb added to a solution. In the present experiment, the relative fresh mass of cowpea (Vigna unguiculata) was reduced by 10% at a Pb2+ activity of 0.2 microM for the shoots and at a Pb2+ activity of 0.06 microM for the roots. The primary site of Pb2+ toxicity was the root, causing severe reductions in root growth, loss of apical dominance (shown by an increase in branching per unit root length), the formation of localized swellings behind the root tips (due to the initiation of lateral roots), and the bending of some root tips. In the root, Pb was found to accumulate primarily within the cell walls and intercellular spaces.