Quercus species differ in water and nutrient characteristics in a resource-limited fall-line sandhill habitat.

We compared co-occurring mature Quercus laevis Walt. (turkey oak), Q. margaretta Ashe (sand post oak) and Q. incana Bartr. (bluejack oak) trees growing in resource-limited sandhill habitats of the southeastern United States for water and nutrient characteristics. The Quercus spp. differed in their distribution along soil water and nutrient gradients, and in their access to and use of water, even though the study year was wetter than average with no mid-season drought. Quercus laevis had the greatest access to soil water (least negative pre-dawn water potential, psi(pd)) and the most conservative water-use strategy based on its relatively low stomatal conductance (g(s)), high instantaneous water-use efficiency (WUE), least negative midday water potential (psy(md)) and high leaf specific hydraulic conductance (K(L)). Quercus margaretta had the least conservative water-use characteristics, exhibiting relatively high g(s), low instantaneous WUE, most negative psi(md), and low K(L). Quercus margaretta also had a low photosynthetic nitrogen-use efficiency (PNUE), but a high leaf phosphorus concentration. Quercus incana had the poorest access to soil water, but intermediate water-use characteristics and leaf nutrient characteristics more similar to those of Q. laevis. There were no species differences for photosynthesis (A), leaf nitrogen on an area basis, or seasonally integrated WUE (delta13C). Both A and g(s) were positively correlated for each species, but A and g(s) were generally not correlated with psi(pd), psi(md) or delta psi(pd-md). Although we found differences in resource use and resource status among these sandhill Quercus spp., the results are consistent with the interpretation that they are generally drought avoiders. Quercus laevis may have an advantage on xeric ridges because of its greater ability to access soil water and use it more conservatively compared with the other Quercus spp.

Atmospheric deposition, resuspension, and root uptake of Pu in corn and other grain-producing agroecosystems near a nuclear fuel facility.

Plutonium released to the environment may contribute to dose to humans through inhalation or ingestion of contaminated foodstuffs. Plutonium contamination of agricultural plants may result from interception and retention of atmospheric deposition, resuspension of Pu-bearing soil particles to plant surfaces, and root uptake. Plutonium on vegetation surfaces may be transferred to grain surfaces during mechanical harvesting. Data obtained from corn grown near the U.S. Department of Energy's H-Area nuclear fuel chemical separations facility on the Savannah River Site were used to estimate parameters of a simple model of Pu transport in agroecosystems. The parameter estimates for corn were compared to those previously obtained for wheat and soybeans. Despite some differences in parameter estimates among crops, the relative importances of atmospheric deposition, resuspension, and root uptake were similar among crops. For even small deposition rates, the relative importances of processes for Pu contamination of corn grain should be: transfer of atmospheric deposition from vegetation surfaces to grain surfaces during combining greater than resuspension of soil to grain surfaces greater than root uptake. Approximately 3.9 X 10(-5) of a year's atmospheric deposition is transferred to grain. Approximately 6.2 X 10(-9) of the Pu inventory in the soil is resuspended to corn grain, and a further 7.3 X 10(-10) of the soil Pu inventory is absorbed and translocated to grains.

Mass loading of soil particles on plant surfaces.

Radionuclide-bearing soil particles on plant surfaces can be ingested and contribute to human dose, but evaluating the potential dose is limited by the relatively few data available on the masses of soil particles present on plant surfaces. This report summarizes mass loading data (i.e., mass of soil per unit of vegetation) for crops in the southeastern United States and compares these data to 1) those from other regions and 2) the mass loadings used in radionuclide transfer models to predict soil contamination of plant surfaces. Mass loadings were estimated using the 238Pu content of crops as an indicator of soil on plant surfaces. Crops were grown in two soils: a sandy clay loam soil and a loamy sand soil. Concentrations of soil on southeastern crops (i.e., mg soil g-1 plant) differed by more than a factor of 100 due to differences in crop growth form and biomass. Mean concentrations ranged from 1.7 mg g-1 for corn to 260 mg g-1 for lettuce. Differences in mass loadings between soils were less than those among crops. Concentrations differed by less than a factor of two between the two soil types. Because of 1) the differences among crops and 2) the limited data available from other systems, it is difficult to draw conclusions regarding regional or climatic variation in mass loadings. There is, however, little evidence to suggest large differences among regions. The mass loadings used to predict soil contamination in current radionuclide transfer models appear to be less than those observed for most crops.

The accuracy of some simple models for predicting particulate interception and retention in agricultural systems.

The accuracy of three radionuclide transfer models for predicting the interception and retention of airborne particles by agricultural crops was tested using Pu-bearing aerosols released to the atmosphere from nuclear fuel facilities on the U.S. Department of Energy's Savannah River Plant, near Aiken, SC. The models evaluated were: 1) NRC, the model defined in U.S. Nuclear Regulatory Guide 1.109; 2) FOOD, a model similar to the NRC model that also predicts concentrations in grains; and 3) AGNS, a model developed from the NRC model for the southeastern United States. Plutonium concentrations in vegetation and grain were predicted from measured deposition rates and compared to concentrations observed in the field. Crops included wheat, soybeans, corn and cabbage. Although predictions of the three models differed by less than a factor of 4, they showed different abilities to predict concentrations observed in the field. The NRC and FOOD models consistently underpredicted the observed Pu concentrations for vegetation. The AGNS model was a more accurate predictor of Pu concentrations for vegetation. Both the FOOD and AGNS models accurately predicted the Pu concentrations for grains.

Sigmoid growth and the assessment of hormesis: a case for caution.

Recent advances in procedures for the analysis of sigmoid curves have provided some sensitive methods of detecting and evaluating hormesis in the growth responses of organisms exposed to a variety of stressors. Based on a reparameterized Richards process error model, these procedures allow the quantification and independent evaluation of the three major properties of a sigmoid growth curve: size: a measure of the asymptote approached by the growth process, rate: a measure of the approximate amount of time required to complete growth, and shape: a quantity which indicates the specific path or trajectory taken by the growth process to approach the asymptote within the time constraints of the growing period. When applied to growth data for cypress tree seedlings and two species of waterfowl exposed chronically to low levels of a variety of stressors, these analyses revealed that curve shape was more likely to change in response to stress than were either asymptotic size or growth rate. The types of changes observed suggested that growth size, rate and curve shape may respond independently and in some cases, in opposite directions. Thus, while one aspect of growth may change in a fashion suggestive of hormesis (e.g. larger asymptote or faster growth rate), other aspects of the same growth function may be changing in a way suggestive of a stress response. Thus, studies designed to reveal growth hormesis should be specific with respect to which particular mathematical model is chosen, as well as with respect to which aspect of the growth response is being considered.

Effects of anaerobic growth conditions on phosphorus tissue concentrations and absorption rates of southern pine seedlings.

A non-circulating, continuously flowing solution culture was used to examine the long- and short-term effects of anaerobic growth conditions on phosphorus uptake in 12-week-old seedlings of three pine species. Sand pine (Pinus clausa (Engelm.) Sarg.) and a drought-hardy loblolly pine (P. taeda L.) had the largest reductions in biomass after 8 weeks in anaerobic solution, whereas the more flood-tolerant pond pine (P. serotina Michx.) and wet-site loblolly pine seedlings were least affected. Anaerobic growth conditions reduced P concentrations in the shoot and increased P concentrations in the root. Short-term (32)P-uptake experiments were conducted with intact seedlings to determine absorption rates (influx) of inorganic phosphate (Pi) by aerobically and anaerobically grown pine seedlings. Influx of Pi was weakly correlated with shoot fresh weight, root fresh weight, root P and shoot P per gram fresh weight root, suggesting that internal P levels and biomass influenced Pi absorption. The highest Pi absorption rates were found in anaerobically grown seedlings under anaerobic (32)P-uptake conditions. Of these, sand pine had the highest absorption rate in 50 microM KH(2)PO(4) (0.96 micromol Pi g(-1) FW root h(-1)), and the wet-site loblolly pine exhibited the lowest rate (0.24 micromol Pi g(-1) FW root h(-1)). Aerobically grown seedlings had similar Pi absorption rates that were not significantly affected by O(2) concentration in the (32)P-uptake solution.

Uptake of 244Cm, 238Pu and other radionuclides by trees inhabiting a contaminated flood plain.

The plant uptake of 244Cm, 137Cs, 238Pu and 90Sr was measured for trees in a flood plain forest whose soils were contaminated by aqueous discharges from a nuclear-fuel chemical separations facility. Uptake of the naturally occurring radionuclide 226Ra was also measured. The relative availability of the nuclides was 238Pu less than 244Cm less than 137Cs less than 226Ra less than or equal to 90Sr. The concentration ratios for 238Pu and 244Cm, 3 X 10(-4) and 3.6 X 10(-3), respectively, were similar to those reported for other plant-soil systems. The ratios for 137Cs and 90Sr, 0.11 and 3.9, were similar to those reported for other southeastern soils. However, the ratio for 226Ra, 2.1, was greater than that normally reported. These ratios, which were determined in the field, were generally similar to those reported for greenhouse studies on the same soil.

Contribution of a nuclear fuel chemical separations facility to the plutonium content of a tobacco crop.

Tobacco, an important crop in the southeastern United States, can potentially contribute via the inhalation pathway to the dose-to-man from radionuclides. To evaluate this potential dose-to-man from the interception and retention of Pu aerosols, a tobacco crop was grown near a chemical separations facility at the Savannah River Plant (SRP) which releases Pu to the atmosphere. Average leaf 238Pu and 239, 240Pu concentrations were 9.8 and 5.1 fCi /g X dry wt, respectively. These concentrations indicate that 2.5% of the deposition occurring during the tobacco growth period was on merchantable leaves. Leaf Pu concentrations were slightly greater than Pu concentration of stem tissue. Tobacco grown near the facility had 10 times higher Pu concentrations than that grown off the SRP. Pu concentrations of tobacco were similar to other broadleaf crops. Dose commitment would be 1000 times greater for tobacco usage than wheat or soybean consumption when these crops were all grown under identical Pu deposition situations.

Plutonium contents of broadleaf vegetable crops grown near a nuclear fuel chemical separations facility.

Among agricultural crops, broadleaf vegetables are particularly prone to intercept and retain aerially released contaminants. The plutonium concentration of four broadleaf crops (broccoli, cabbage, lettuce and turnip greens) was determined, when grown in close proximity to a nuclear-fuel chemical-separations facility. Concentrations varied among species, apparently influenced by the crop morphology, with Pu concentrations increasing in the sequence: cabbage less than broccoli less than turnip greens less than lettuce. Washing of the crops significantly reduced the Pu concentration of lettuce, but had no effect on Pu concentration of broccoli and cabbage. The vast majority of Pu found in the crops was due to direct deposition of recently released Pu and resuspension of Pu-bearing soil particles, and was not due to root uptake. Resultant doses from consumption are small relative to the annual background dose.

The relative importance of uptake and surface adherence in determining the radionuclide contents of subterranean crops.

The adherence of Pu-bearing particles to external surfaces of carrots, turnips, red potatoes, and sweet potatoes accounted for greater than or equal to 93% of their total Pu content. Uptake, which was measured as Pu content in peeled crops, accounted for less than or equal to 7%. Plutonium concentrations in most peeled crops were below background, and consequently, uptake could not be conclusively demonstrated. However, uptake accounted for most of the 137Cs, 40K, and 226Ra contents of subterranean crops. Concentration ratios for total radionuclide contents (i.e. surface adherence plus uptake) ranged from 3.9 X 10(1) for 40K, to 1.1 X 10(-2) for 239, 240Pu. Approximately 1.5 X 10(-3) pCi 239, 240Pu adhered per cm2 of subterranean crop surface per 1 pCi Pu/g of soil.