Arsenate uptake consists of a rapid, apparently irreversible, initial phase followed by a less rapid 'steady state' phase, both of which are temperature sensitive and strongly inhibited by phosphate. By comparison, 'steady state' uptake of phosphate was only weakly inhibited by arsenate. The initial and 'steady state' phases of arsenate uptake were highly correlated (R = 0.958, P < 0.001). Arsenite also appeared to exhibit initial and 'steady state' uptake phases but uptake was slower by a factor of 3-4 than for arsenate at the same concentration. The results are discussed in relation to recent work on a DNP-sensitive initial binding step in the uptake of other elements.

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.

A concentration as low as 1 μM 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 Pb 2+ activity of 0.2 μM for the shoots and at a Pb 2+ activity of 0.06 μM for the roots. The primary site of Pb 2+ 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 swelling 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.Abbreviations: CDTA -cyclohexane-1,2-diaminetetra-acetic acid; DAP -days after planting; DIdeionised; EC -electrical conductivity; I -ionic strength; ICPOES -inductively coupled plasma optical emission spectrometry; ICPMS -inductively coupled plasma mass spectrometry; TEMtransmission electron microscopy Kopittke PM, Asher CJ, Kopittke RA, Menzies NW, 2007. Environmental Pollution 150, 280-287.

Colorimetric procedures were developed for the determination of inorganic monomeric aluminium (Al) in the presence of organic ligands including high molecular weight fulvic acids and short chain carboxylic acids. The procedures take advantage of marked differences in reaction rate between the chromogens aluminon and pyrocatechol violet (PCV) and aluminium in either free inorganic forms or organically complexed forms. The aluminon method uses a 30 s reaction and has a working range for total Al concentrations between 20 and 250 PM. The more sensitive PCV method uses a reaction time of 60 s and provides a linear calibration line in the range 0.2-25 VM. Both methods are calibrated by using standard solutions containing known total A1 concentration prepared with varying ratios of inorganic monomeric and organically complexed Al. The determination of A1 in soil solution and its fractionation into inorganic monomeric and organically complexed forms requires the measurement of total Al in filtered soil solutions by inductively coupled plasma atomic emission spectroscopy (ICPAES) and the use of one of the short reaction time colorimetric procedures.Limited testing on soil solutions from acid, Al toxic soils showed better agreement between inorganic monomeric Al concentration determined by the new procedures and relative yield data than that obtained by existing colorimetric methods. * Part I, Aust. J. Soil Res., 1989, 2 7 , 79-90.

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