The Cd content of wheat ( Triticum aestivum L.) grain samples collected representatively from the major wheat growing regions of Britain in 1982, 1992, and 1993 was measured. Geographical distributions of grain Cd content were compared with the total Cd concentration in soils of England and Wales. The ranges and mean grain Cd concentrations for 1982, 1992, and 1993 were (mg kg −1 dry weight, DW) 0.016 to 0.18, mean 0.052; 0.008 to 0.27, mean 0.042; and 0.004 to 0.31, mean 0.038, respectively. There was about a 20% decrease in mean grain Cd concentration from 1982 to 1992–1993. The proportion of samples above the World Health Organization's (WHO) limit of 0.12 mg Cd kg −1 DW was 4% for 1982 and 1992 and 2% for 1993. The geographical distribution of these high Cd grain samples corresponded with areas where the total soil Cd concentrations were significantly above background concentrations. Differences between wheat varieties were small and significant ( P < 0.001) only for 1992. The concentration of Cd in white flour was significantly correlated with that in wholemeal flour ( r = 0.95), and bran Cd was significantly correlated with white flour Cd ( r = 0.87). On average, the concentration of Cd in bran was twice that in wholemeal flour, and white flour contained about 31% less Cd than wholemeal flour. The estimated daily dietary Cd intake in the United Kingdom (UK) through the consumption of wheat‐based products was about 8 μg Cd per person or about 11% of the WHO's limit of 70 μg Cd d −1 . The Cd content of the vast majority of British wheat grain would not constitute a health hazard to the population of the UK or elsewhere.
• Several fern species can hyperaccumulate arsenic, although the mechanisms are not fully understood. Here we investigate the roles of root absorption, translocation and tolerance in As hyperaccumulation by comparing the hyperaccumulator Pteris vittata and the nonhyperaccumulator Pteris tremula. • The two species were grown in a pot experiment with 0–500 mg As kg−1 added as arsenate, and in a short-term (8 h) uptake experiment with 5 µM arsenate under phosphorus-sufficient conditions. • In the pot experiment, P. vittata accumulated up to 2500 mg As kg−1 frond d. wt and suffered no phytotoxicity. P. tremula accumulated < 100 mg As kg−1 frond d. wt and suffered severe phytotoxicity with additions of ≥ 25 mg As kg−1. In the short-term uptake experiment, P. vittata had a 2.2-fold higher rate of arsenate uptake than P. tremula, and distributed more As taken up to the fronds (76%) than did P. tremula (9%). • Our results show that enhanced root uptake, efficient root-to-shoot translocation, and a much elevated tolerance through internal detoxification all contribute to As hyperaccumulation in P. vittata.
Elevated atmospheric carbon dioxide concentration ([CO2]) can stimulate crop growth and increase yield, but the effect may be constrained by soil contamination with heavy metals. In a free-air carbon dioxide enrichment experiment over three seasons, we found that soil heavy metal contamination can constrain or even reverse the projected CO2 fertilization effect on rice yield. Elevated [CO2] produced opposing effects on the accumulation of arsenic and cadmium in rice grain. Breeding crops for heavy-metal resistance and low arsenic accumulation may become necessary with continuing climate change.
Microbes detoxify arsenate by reduction and efflux of arsenite. Plants have a high capacity to reduce arsenate, but arsenic efflux has not been reported. Tomato (Lycopersicon esculentum) and rice (Oryza sativa) were grown hydroponically and supplied with 10 microm arsenate or arsenite, with or without phosphate, for 1-3 d. The chemical species of As in nutrient solutions, roots and xylem sap were monitored, roles of microbes and root exudates in As transformation were investigated and efflux of As species from tomato roots was determined. Arsenite remained stable in the nutrient solution, whereas arsenate was rapidly reduced to arsenite. Microbes and root exudates contributed little to the reduction of external arsenate. Arsenite was the predominant species in roots and xylem sap. Phosphate inhibited arsenate uptake and the appearance of arsenite in the nutrient solution, but the reduction was near complete in 24 h in both -P- and +P-treated tomato. Phosphate had a greater effect in rice than tomato. Efflux of both arsenite and arsenate was observed; the former was inhibited and the latter enhanced by the metabolic inhibitor carbonylcyanide m-chlorophenylhydrazone. Tomato and rice roots rapidly reduce arsenate to arsenite, some of which is actively effluxed to the medium. The study reveals a new aspect of As metabolism in plants.
SUMMARY Field experiments were conducted to test the seed yield responses of winter oilseed rape ( Brassica napus L., cvs Libravo or Falcon) to the addition of different rates of S fertilizer, at three N application rates, on a sandy loam at Woburn, Bedfordshire, in 1990/91, 1991/92 and 1993/94. Large increases in seed yields, ranging from 0·7 to 1·6 t/ha, or 42–267% on a relative scale, were obtained in response to the application of 40 kg S/ha with 180 and 230 kg N/ha treatments. The effects of S were highly significant in 1991/92 ( P < 0·01) and 1993/94 ( P < 0·001) and close to significant ( P = 0·053) in 1990/91. The yield benefits were obtained mainly from the application of the first 10 kg S/ha and further yield increases were unlikely above 40 kg S/ha. Increasing N application from 180 to 230 kg/ha decreased seed yield in 1990/91 and 1993/94, when no S was applied. In contrast, seed yield was not increased by S at zero or low (50 or 100 kg/ha) N rates. The interactions between N and S on seed yield were significant ( P < 0·05) in 1990/91 but not in the other two seasons. Application of S also increased seed oil content in 1993/94, when the degree of S deficiency was particularly severe. With an application of 230 kg N/ha, the crops took up 5–22 kg S/ha at maturity when no S was applied and 26–51 kg S/ha when 40 kg S/ha was applied. The utilization efficiency of the fertilizer S ranged from 50 to 73% in the three seasons. Although the concentrations of total N in plants were largely unaffected by S treatments, large amounts of NO 3 -N accumulated in the leaves of S-deficient plants in 1993/94. This indicates that N metabolism was disrupted by S deficiency. The concentrations of S and the N: S ratios in different tissues and the whole plant changed considerably with time. The concentration of S in leaves at early flowering was found to be the best index in predicting S deficiency in terms of seed yield, and a critical value of 3·8 mg/g was obtained. In comparison, the N: S ratio in leaves at early flowering was a much poorer predictor of S deficiency.
No abstract is provided for this article.
Quantum technologies based on quantum point defects in crystals require control over the defect charge state. Here we tune the charge state of shallow nitrogen-vacancy and silicon-vacancy centers by locally oxidizing a hydrogenated surface with moderate optical excitation and simultaneous spectral monitoring. The loss of conductivity and change in work function due to oxidation are measured in atmosphere using conductive atomic force microscopy (C-AFM) and Kelvin probe force microscopy (KPFM). We correlate these scanning probe measurements with optical spectroscopy of the nitrogen-vacancy and silicon-vacancy centers created via implantation and annealing 15-25 nm beneath the diamond surface. The observed charge state of the defects as a function of optical exposure demonstrates that laser oxidation provides a way to precisely tune the Fermi level over a range of at least 2.00 eV. We also observe a significantly larger oxidation rate for implanted surfaces compared to unimplanted surfaces under ambient conditions. Combined with knowledge of the electron affinity of a surface, these results suggest KPFM is a powerful, high-spatial resolution technique to advance surface Fermi level engineering for charge stabilization of quantum defects.
Ethylenediaminetetraacetic acid (EDTA) extraction of Zn, Cd, Cu and Pb from four contaminated soils was studied using batch and column leaching experiments. In the batch experiment, the heavy metals extracted were virtually all as 1:1 metal-EDTA complexes. The ratios of Zn, Cd, Cu and Pb of the extracted were similar to those in the soils, suggesting that EDTA extracted the four heavy metals with similar efficiency. In contrast, different elution patterns were obtained for Zn, Cd, Cu and Pb in the column leaching experiment using 0.01 M EDTA. Cu was either the most mobile or among the most mobile of the four heavy metals, and its peak concentration corresponded with the arrival of full strength EDTA in the leachate. The mobility of Zn and Cd was usually slightly lower than that of Cu. Pb was the least mobile, and its elution increased after the peaks of Cu and Zn. Sequential fractionations of leached and un-leached soils showed that heavy metals in various operationally defined fractions contributed to the removal by EDTA. Considerable mobilisation of Fe occurred in two of the four soils during EDTA leaching. Decreases in the Fe and Mn oxide fraction of heavy metals after EDTA leaching occurred in both soils, as well as in a third soil that showed little Fe mobilisation. The results suggest that the lability of metals in soil, the kinetics of metal desorption/dissolution and the mode of EDTA addition were the main factors controlling the behaviour of metal leaching with EDTA.
