Quantifying and understanding uptake of plant protection products from soil into plants

This project brings together the Product Safety Division of Syngenta and Soil Research Centre at the University of Reading to address the uptake of pesticides by plants from soil solution. The collaboration addresses issues raised at the EUregPUF workshop, York, 2013 regarding a standardized protocol for the determination of the plant uptake factor (PUF) and subject areas identified in the Syngenta call for proposals i.e. Pesticide metabolism, transport and uptake in fungi, plants and insects (inc.modelling & prediction) and Movement of chemicals in soil and uptake into roots.

Aims
To establish the relationship between plant uptake in hydroponic solution against that in soil.
To improve environmental fate models by quantifying field variability and streamlining model structure.

Experimental
The project will use the protocol developed by Sweeney et al. to determine the PUF of a range of pesticides with varying physicochemical characteristics from hydroponic solution. There is some debate whether the PUF can be greater or less than 1 and whether hydroponic systems are a realistic surrogate for uptake by plants from soil. The protocol will be marginally altered in order to provide more realism i.e. sand as the substrate to improve root growth with nutrients added to ensure plant vitality.
In lysimeters/small field plots selected active ingredients will be examined to measure the plant uptake and fate in soil. Soil pore water concentration will be measured both in-situ (PECAM and TECAM) and ex-situ (centrifuge technique). The measurements will be taken at a range of distances from the plant to determine the root influence across a fixed area of soil enabling an integrated value for a whole field to be determined. Data from an on-site meteorological station will be used to calculate the plant transpiration which when multiplied by the PUF determines the mass of pesticide removed by the crop.
The field and laboratory values will be compared and the factors responsible for the primary differences in plant uptake identified, their variability will be recorded for the modelling phase.

Modelling
There are considerable gaps to be addressed when modelling the fate of pesticides in soils. For example; model complexity is frequently not justified, the validation rarely uses an independent data set and single parameter values are used without knowledge of their variability. Frequently, models used for regulatory purposes within the EU such as PEARL and PELMO use a single parameter to represent plant uptake as a result of abstraction of water from soil to satisfy evapotranspiration demand. Such a simple approach does not take into consideration the pH of soil pore water (pH is known to affect uptake of ionisable compounds) and the split between adsorption to roots and translocation into shoots. Of particular interest are soil sorption and pesticide transformation parameters which are an order of magnitude more sensitive than water flow parameters in pesticide leaching fluxes.
The project will use the existing Syngenta suite of models and determine the sensitive parameters within these. The data from the experimental studies will be used to establish the extent to which existing models of environmental fate accurately represent the observed uptake of pesticides from soil in the field. Sensitivity and uncertainty analyses will be used to improve the models.

Outcome
At the conclusion of the project Syngenta will have an independently validated system for the determination of pesticide PUFs and the robustness of these values will have been tested against field data. The latter will underpin a model validation and calibration exercise to improve Syngenta's capability to predict the fate of field applied pesticides.