Variability of bioaccessibility results using seventeen different methods on a standard reference material, NIST 2710

Bioaccessibility is a measurement of a substance's solubility in the human gastro-intestinal system, and is often used in the risk assessment of soils. The present study was designed to determine the variability among laboratories using different methods to measure the bioaccessibility of 24 inorganic contaminants in one standardized soil sample, the standard reference material NIST 2710. Fourteen laboratories used a total of 17 bioaccessibility extraction methods. The variability between methods was assessed by calculating the reproducibility relative standard deviations (RSDs), where reproducibility is the sum of within-laboratory and between-laboratory variability. Whereas within-laboratory repeatability was usually better than (<) 15% for most elements, reproducibility RSDs were much higher, indicating more variability, although for many elements they were comparable to typical uncertainties (e.g., 30% in commercial laboratories). For five trace elements of interest, reproducibility RSDs were: arsenic (As), 22-44%; cadmium (Cd), 11-41%; Cu, 15-30%; lead (Pb), 45-83%; and Zn, 18-56%. Only one method variable, pH, was found to correlate significantly with bioaccessibility for aluminum (Al), Cd, copper (Cu), manganese (Mn), Pb and zinc (Zn) but other method variables could not be examined systematically because of the study design. When bioaccessibility results were directly compared with bioavailability results for As (swine and mouse) and Pb (swine), four methods returned results within uncertainty ranges for both elements: two that were defined as simpler (gastric phase only, limited chemicals) and two were more complex (gastric + intestinal phases, with a mixture of chemicals).

The effect of pharmacologically altered gastric pH on cadmium absorption from the diet and its accumulation in murine tissues

Solubility of Cd in Cd-amended mouse chow in water was reduced by increased pH; even less Cd was solubilized by simulated digestion in vitro, where increased gastric phase pH decreased solubility, an effect that persisted following intestinal digestion at pH 5.5. These data suggested that increasing gastric pH in vivo pharmacologically would reduce Cd accumulation in target organs of mice treated with omeprazole (a proton-pump inhibitor) or cimetidine (a H2-receptor antagonist). This expectation was mostly not realized. Gastric pH in animals receiving Cd-amended diet was increased by omeprazole, but not cimetidine, relative to animals receiving no drugs, and Cd-amended diet. Tissue concentrations of Cd were similar among the three groups receiving Cd-amended diet, for liver, kidney and testes. Small intestine Cd concentration was lower for omeprazole-treated animals than for those receiving neither drug and Cd-amended diet, suggesting that omeprazole decreased Cd absorption by this organ. This effect may have been compensated for by increased uptake of complexed Cd by the large intestine, as accumulation in the liver, kidney and testes was not reduced. In vitro determinations of bioaccessible Cd in food may not predict in vivo bioaccumulation in all target organs.

Differential accumulation of Cd in durum wheat cultivars: uptake and retranslocation as sources of variation

Durum wheat (Triticum turgidum L. var. durum) accumulates Cd from the soil depending on various factors. When grown in hydroponic solution containing Cd (20 microg l(-1)), roots had higher tissue Cd concentrations than shoots or heads. Kyle (the higher grain-Cd accumulating cultivar) had lower root-Cd, and greater shoot-Cd and head-Cd concentrations than Arcola (the lower grain-Cd accumulating cultivar). These cultivar differences were greater at flowering and ripening than at tillering. Much of the root-Cd was lost between the flowering and ripening stages of development. Distribution of (106)Cd among plant parts, after a single 24 h feeding, demonstrated that root-to-shoot transfer of Cd in Arcola was similar to that of Kyle at tillering, but it had ceased at flowering in Arcola but not Kyle. None of the Cd in wheat heads at ripening originated from (106)Cd exposure in the previous 24 h, suggesting that grain-Cd is a function of total shoot accumulation. Both cultivars demonstrated basipetal translocation of Cd; Arcola at tillering translocated more Cd from shoots to roots than Kyle. The proportion of Cd(2+)/Cd(total) in the nutrient solution decreased with time, suggesting that plant activity altered the solution chemistry. The alteration probably resulted from either preferential depletion of solution Cd(2+) and/or addition of root exudates. Lower grain-Cd accumulation in Arcola possibly resulted from a combination of reduced root-to-shoot transfer of Cd at flowering, as well as enhanced shoot-to-root retranslocation of Cd, at least in younger plants. Plant-mediated changes in solution-Cd speciation did not play a role.

Accumulation of dietary cadmium (Cd) in rabbit tissues and excretions: a comparison of lettuce amended with soluble Cd salt and lettuce with plant-incorporated Cd

Quantifying the transfer of Cd from foods to mammalian target organs is key to estimating the health risk from this exposure; however, the bioaccumulation of Cd from foods is modified by many dietary components. Studies of dietary Cd absorption would be simpler if it were known that Cd added to foods as a soluble salt was as bioavailable as Cd incorporated during growth of the food species. Rabbits were fed, for 16 d, fresh lettuce containing cadmium incorporated into the lettuce during growth or added to the lettuce before feeding, or lettuce with no Cd but soluble Cd administered to the animals by gavage. There was a marked positive relationship between increased Cd dose and its accumulation in kidney; the slopes for the gavage and added treatments were not clearly different from the incorporated treatment; liver data were highly variable. In a 10-wk study of Cd-incorporated and -amended lettuce diets, for the incorporated and control diets there was less Cd accumulation in the kidneys, but not liver, per unit cumulative dose, than for the amended diet. Cd accumulation in the small intestine and Cd concentration in feces, both per unit daily dose, were smaller for the incorporated than for the control and amended diets; Cd concentrations in bile, urine, and serum, per unit daily dose, were higher in the control diet than values in the amended diet, which were higher than the incorporated diet. These differences could not be accounted for by variation in Fe or Zn contents of the diets. Thus, data suggest that Cd-amended diets overestimate bioaccumulation in kidney, an important target organ, by up to one-third, and that studies of short duration are not adequate to evaluate Cd bioavailability from food.

Cadmium accumulation by durum wheat roots in ligand-buffered hydroponic culture: uptake of Cd–ligand complexes or enhanced diffusion?

The objectives of this study were to test the free ion model of Cd phytoavailability in solutions where the dissolved free ion was buffered by SO4 or EDTA and to examine if Cd2+ diffusion to roots might be limiting to its accumulation in roots. At similar Cd2+ activities, solutions that contained EDTA or a higher nominal SO4 concentration (with SO4 supplied as K2SO4, and more total Cd) resulted in greater accumulation of Cd by roots than predicted by the Cd2+ activity. These solutions were predicted using MINEQL+ (chemical speciation software) to contain greater concentrations of Cd complexes (CdEDTA2– or CdSO04 (aq)) in solution; when SO4 was supplied by MgSO4, the increase in Mg2+ suppressed enhanced accumulation of Cd. Estimates of diffusion of Cd to the roots through the boundary layer and flux of Cd across the root surface, for this study, were similar in magnitude across the range of solution Cd concentrations studied, particularly when Cd uptake was assumed to occur at the root tips only. We conclude that CdEDTA2– and CdSO04 (aq) could have alleviated this diffusion limitation by buffering Cd2+ at the root surface and (or) been taken up directly as a complex.Key words: bioavailability, cadmium, EDTA, SO4, durum wheat, speciation.

Effects of pre-exposure to ultraviolet-B radiation on responses of tomato (Lycopersicon esculentum cv. New Yorker) to ozone in ambient and elevated carbon dioxide

Patterns of environmental change in the biosphere include concurrent and sequential combinations of increasing ultraviolet (UV-B) and ozone (O(3)) at increasing carbon dioxide (CO(2)) levels; long-term changes are resulting mainly from stratospheric O(3) depletion, greater tropospheric O(3) photochemical synthesis, and increasing CO(2) emissions. Effects of selected combinations were evaluated in tomato (Lycopersicon esculentum cv. New Yorker) seedlings using sequential exposures to enhanced UV-B radiation and O(3) in differential CO(2) concentrations. Ambient (7.2 kJ m(-2 )day(-1)) or enhanced (13.1 kJ m(-2) day(-1)) UV-B fluences and ambient (380 microl l(-1)) or elevated (600 microl l(-1)) CO(2) were imposed for 19 days before exposure to 3-day simulated O(3) episodes with peak concentrations of 0.00, 0.08, 0.16 or 0.24 microl l(-1) O(3) in ambient or elevated CO(2). CO(2) enrichment increased dry mass, leaf area, specific leaf weight, chlorophyll concentration and UV-absorbing compounds per unit leaf area. Exposure to enhanced UV-B increased leaf chlorophyll and UV-absorbing compounds but decreased leaf area and root/shoot ratio. O(3) exposure generally inhibited growth and leaf photosynthesis and did not affect UV-absorbing compounds. The highest dose of O(3) eliminated the stimulating effect of CO(2) enrichment after ambient UV-B pre-exposure on leaf photosynthesis. Pre-exposure to enhanced UV-B mitigated O(3) damage to leaf photosynthesis at elevated CO(2).

The influence of photosynthetically active radiation on the effects of ultraviolet-B radiation on Arabidopsis thaliana

Ultraviolet-B (UVB; 280-320 nm) radiation is a small but biologically significant portion of the solar spectrum reaching the earth's surface. Research interests have been fostered because UVB has been increasing in recent years due to depletion of stratospheric ozone. Ultraviolet-B that penetrates into plant tissue may damage important cellular macromolecules. Although there has been considerable research on the effects of UVB on plants, the influence of the level of photosynthetically active radiation (PAR; 400-700 nm) on effects of UVB requires further definition as a prelude to studies of UVB sensitivity and defense mechanisms. Arabidopsis thaliana wild-type ecotype Landsberg erecta (LER), which is relatively insensitive to UVB, and the relatively sensitive LER-based mutant transparent testa-5 (tt5), were grown under 100 or 250 mumol m-2 s-1 PAR and then exposed to (zero) or 7 kJ m-2 day-1 UVBBE under these PAR levels. Plants exposed to UVB had reduced dry weight and leaf area and higher levels of UV-absorbing compounds in leaf tissue. The level of PAR did influence the effects of UVB, with the higher level of PAR prior to UVB exposure reducing sensitivity of LER to UVB. In contrast to other studies, higher PAR supplied simultaneously with UVB increased rather than decreased sensitivity of both genotypes to UVB. These results demonstrate the importance of controlling and comparing PAR levels when undertaking studies of UVB sensitivity, as effects of UVB on plants are influenced by the PAR levels plants are growing under prior to and during exposure to UVB.

Responses of greening bean seedling leaves to nitrogen dioxide and nutrient nitrate supply

Phaseolus vulgaris cv. Kinghorn Wax seedlings grown in darkness at 25 degrees C for 7 days with half strength Hoagland's nutrient solution containing no nitrogen, were transferred to lit continuous stirred tank reactors (CSTRs) in atmospheres containing 0 or 0.3 ppm NO(2) and irrigated with a nutrient solution containing 0 or 5 mm nitrate as sole nitrogen source and allowed to grow for a period of up to 5 days in a 14 h photoperiod. Exposure to NO(2) increased total Kjeldahl nitrogen in the leaves. Further, the exposure to NO(2) increased chlorophyll content from day 3 onwards and inhibited the leaf dry weight substantially on days 4 and 5. The primary leaves of the seedlings exposed to 0.3 ppm NO(2) and supplied with nitrate accumulated some nitrite after 5 days of exposure. Some of the seedlings were returned from CSTRs to growth chambers and allowed to grow for a further period of 5 days in a 14 h photoperiod without NO(2). The growth which developed after the NO(2) exposure growth period, as measured by fresh and dry weights of the leaves, was significantly less in NO(2)-exposed plants than in nitrate-grown plants. The experiments demonstrate that the leaves of greening seedlings are able to assimilate NO(2) and that a reduction in leaf dry weight by prolonged NO(2) exposure in the presence of nutrient nitrate can be associated with nitrite accumulation, and that NO(2) has a carry-over effect beyond the duration of NO(2) exposure. It is apparent that NO(2) induces some durable biochemical or cytological aberration in the presence of nutrient nitrate, which adversely affects subsequent leaf growth.