A simulation model (PostPLANT-Soil) for predicting pesticide concentrations in succeeding leafy vegetables: II. Validation with experimental data on plant uptake in a growth chamber.

We validated a simulation model (PostPLANT-Soil) for predicting pesticide concentrations in succeeding leafy vegetables reported in our first paper in this series, which includes the pesticide sorption process into plant roots. As a result of the model validation with the measured data from a plant uptake study in a growth chamber, the model successfully simulated the concentration changes of pesticides in a plant shoot. However, the simulated shoot concentrations for several pesticides were overestimated compared to the measured values. The leafy vegetable (Brassica rapa) used in this study probably has a high metabolic ability for the fungicide flutolanil from the result of the uptake study under a hydroponic condition.

A simulation model (PostPLANT-Soil) for predicting pesticide concentrations in succeeding leafy vegetables: I. Validation with experimental data in a Japanese Andosol field.

We developed a simulation model for predicting pesticide concentrations in succeeding leafy vegetables (PostPLANT-Soil), which includes the process of pesticide uptake from plant roots. To validate the model, we compared pesticide concentrations simulated by the model with values measured from field experiments in an upland Andosol. The model validation showed that pesticide concentrations in the plant shoot were correlated with the concentrations in the soil solution rather than those of the water-extracted pesticides. The model successfully simulated the concentration changes in plant shoots when the simulated concentrations of the pesticides in the soil solution were fitted to the measured values by considering the key parameter - the corrective coefficient for the soil adsorption coefficient. However, the simulated shoot concentrations at the appropriate harvest period exceeded the measured values. This indicates that the leafy vegetable used in this study may have some metabolic capacity for the pesticides.

Studies on the sorption behavior and plant uptake of pesticides in Japanese soils.

To estimate pesticide residue levels in succeeding crops based on those in soils, the relationship between pesticide concentrations in komatsuna (Brassica rapa var. perviridis) and the concentrations extracted sequentially from soils using water and acetone was investigated. The concentrations of many pesticides in komatsuna shoots showed higher positive correlation with water-extractable concentrations (C W) than total-extractable concentrations in soils, so that the C W was available for evaluating the phytoavailability of pesticides in the soil. As a result of examining the dissipation behavior of the C W, the dissipation of the C W was able to be predicted by considering time-dependent soil sorption, which could be estimated using the sorption coefficients (K d) measured by a standard batch method. Furthermore, the present study showed that the properties of soil organic carbon such as black carbon content and the molecular structure of pesticides were important for estimating the K d values more accurately.

Effects of temperature on the dissipation of total- and water-extractable pesticides in Japanese soils.

We investigated the dissipation of 27 pesticides in five Japanese soils at three temperatures and the variability of activation energies (E a). The dissipation of total pesticides extracted sequentially using water and acetone was fitted to a single first-order (SFO) model. The E a values calculated from the dissipation rate constants of the SFO model showed a normal distribution with a median of 61.1 kJ mol-1. The dissipation of water-extractable pesticides (i.e., phytoavailable pesticides) was fitted to a double first-order in parallel model with two dissipation rate constants: k 1 and k 2. The E a values calculated from k 1 and k 2 showed normal or lognormal distribution, and the medians of the normal distribution calculated from k 1 and k 2 were 62.8 and 45.2 kJ mol-1, respectively. Furthermore, the method for estimating the biphasic dissipation of phytoavailable pesticides at different temperatures by using the median E a values of the laboratory experiment was demonstrated in a field experiment.

Relationship between growth stage of Brassica rapa var. perviridis and the abilities for uptake and translocation of pesticides in soil.

The relationships between plant growth stage and pesticide-uptake ability were investigated via cultivation of Brassica rapa L. var. perviridis in soil to which was added four pesticides of relatively high log K OW: fenobucarb, procymidone, flutolanil, and tolclofos-methyl. The root concentrations of pesticides were low in very young seedlings with undeveloped root systems, highest in seedlings with developed root systems, and tended to decrease until the usual harvesting stage. Additionally, the shoot concentrations of tested pesticides showed the same trends as the roots. The pesticide-uptake abilities of roots were lowest in very young seedlings and then constant for seedlings until the harvesting stage. In contrast, the pesticide-translocation abilities from root to shoot were constant regardless of growth stage. The results indicated that changes in shoot concentrations with growth stage were affected by the development of the root system and pesticide-uptake ability of roots.

The influence of Brassica rapa var. perviridis growth conditions on the uptake and translocation of pesticides.

We cultivated Brassica rapa var. perviridis in soil mixed with four pesticides (fenobucarb, procymidone, flutolanil, and tolclofos-methyl) at different temperatures, day lengths, and soil water contents. We compared plants' uptake and translocation abilities of the pesticides as affected by growth conditions. The root concentration factor (RCF) of pesticides tended to increase with rising temperature; however, but the influence of temperature on the transpiration stream concentration factor (TSCF) differed for each pesticide. The RCFs and TSCFs of pesticides were high for short days. The soil water content had little or no effect on the uptake and translocation of pesticides. These results showed that it is necessary to consider growth conditions, especially the temperature and day length in plant uptake models for these pesticides.

Comparison of soil sorption parameters of pesticides measured by batch and centrifugation methods using an andosol.

We compared the soil sorption coefficient (K d) measured by batch and centrifugation methods using a Japanese andosol and ten pesticides. Although the K d values measured by both methods increased with time, those obtained via the batch method tended to be higher during the test period. The difference in K d values between the two methods affected pesticide concentrations estimated in the soil solution, and the results estimated using K d values obtained via the batch method underestimated the observed trends.

Differential uptake and translocation of organic chemicals by several plant species from soil.

We performed uptake experiments with 12 different organic chemicals using 16 plant species and determined differences in the ability of plant species to take up and translocate these chemicals. There were differences among the plant species in the shoot and root concentrations of each organic chemical. The root concentration factor values increased with an increasing log of the n-octanol-water partition coefficient (log K OW) of organic chemicals. Thus, the concentrations in roots may be predicted to a certain extent because the root concentration factor values were related to the log K OW. The root-to-shoot translocation was related to the log K OW because the shoot-to-root concentration ratio decreased with an increasing log K OW; however, there was no clear relationship between the shoot concentration factor value and the log K OW, and this differed among plant species.

Potential application of immunoassays for simple, rapid and quantitative detections of phytoavailable neonicotinoid insecticides in cropland soils.

This study evaluated the applicability of commercially available kit-based enzyme-linked immunosorbent assay (ELISA) to simple, quick, and quantitative detection for three water-extractable (phytoavailable) neonicotinoid insecticides: dinotefuran, clothianidin, and imidacloprid in soils. ELISA showed excellent analytical sensitivity for determination, but with cross-reaction to structurally related neonicotinoid analogues, which might produce false positives. To analyze insecticides in soil samples of diverse physicochemical properties, they were extracted with water. The aqueous soil extracts were assayed directly with ELISA. No matrix interference was observed without additional dilution with water. Recovery experiments for the insecticides from aqueous soil extracts spiked at 2-10 ng/mL showed good accuracy (72-126%) and precision (<16%). Kit-based ELISAs were used to estimate soil-water distribution coefficients (Kd). Values estimated using this method showed positive correlation between organic carbon contents in soil and those for evaluated insecticides. Results indicate that the evaluated kit-based ELISA has applicability for simple, quick, and reliable detection of phytoavailable insecticides in soils and for estimating Kd values in soil.

Effect of Time-Dependent Sorption on the Dissipation of Water-Extractable Pesticides in Soils.

The dissipation behavior of water-extractable pesticides in soils is important when assessing the phytoavailability of pesticides in soils. This process is less understood than pesticide extraction with organic solvents. To elucidate the dissipation behavior of water-extractable pesticides in soils, we conducted an incubation study using 27 pesticides and five Japanese soils. The rate of decrease of the level of pesticides in water extracts was faster in soils than that of total extracts (water extracts and acetone extracts). This suggests that time-dependent sorption contributed to the difference in the dissipation between the pesticides in water and total extracts from soils. Increased apparent sorption coefficients (Kd,app) with time were positively and significantly correlated with Kd,app values of a 0 day incubation [Kd,app(t0)]. This empirical relationship suggests that Kd,app(t0) values can predict the time-dependent increase in Kd,app and the dissipation of water-extractable pesticides in soils.

A system for the directed evolution of the insecticidal protein from Bacillus thuringiensis.

Theoretically, the activity of AB-type toxin molecules such as the insecticidal toxin (Cry toxin) from B. thuringiensis, which have one active site and two binding site, is improved in parallel with the binding affinity to its receptor. In this experiment, we tried to devise a method for the directed evolution of Cry toxins to increase the binding affinity to the insect receptor. Using a commercial T7 phage-display system, we expressed Cry1Aa toxin on the phage surface as fusions with the capsid protein 10B. These recombinant phages bound to a cadherin-like protein that is one of the Cry1Aa toxin receptors in the model target insect Bombyx mori. The apparent affinity of Cry1Aa-expressing phage for the receptor was higher than that of Cry1Ab-expressing phage. Phages expressing Cry1Aa were isolated from a mixed suspension of phages expressing Cry1Ab and concentrated by up to 130,000-fold. Finally, random mutations were made in amino acid residues 369-375 in domain 2 of Cry1Aa toxin, the mutant toxins were expressed on phages, and the resulting phage library was screened with cadherin-like protein-coated beads. As a result, phages expressing abnormal or low-affinity mutant toxins were excluded, and phages with high-affinity mutant toxins were selected. These results indicate that a method combining T7 phage display with selection using cadherin-like protein-coated magnetic beads can be used to increase the activity of easily obtained, low-activity Cry toxins from bacteria.