August 18, 2016
The plant uptake factor (PUF) is typically used in regulatory environmental fate modeling of pesticides such as PEARL, PELMO and MACRO to simulate uptake of chemicals from soil solution. The magnitude of the PUF value can significantly affect the mass of chemical available in the soil pore water and subsequently the predicted concentrations in environmentally relevant compartments. Yet, there is no regulatory accepted test guidance for determining PUF values.
In the absence of experimentally measured PUF values a default of 0.5 is recommended for systemic, non-ionic chemicals and 0 for all others (FOCUS 2009). However, this default probability of uptake value was based on the empirical relationship developed by Briggs et al (1982, 1983) between the octanol-water partition coefficient (Kow) and measured concentrations in the plant. An exact expression for the PUF has since been derived from a theoretical treatment of plant abstraction of chemicals from solution that is relevant to how this process is simulated by environmental fate models (Sweeney et al. 2014, submitted):
The parameters in the above equation can be generated from a carefully designed study protocol, and specific PUF value calculated for a given pesticide. In the absence of such a protocol, a new study design was developed to derive PUF values for regulatory leaching models.
Last year, the Industrieverband Agrar e.V. and the European Crop Protection Association (IVA/ECPA) PUF working group organized a round robin test to evaluate this new protocol. Ten laboratories participated in the round robin test for [14C]-1,2,4-Triazole uptake in wheat by means of a hydroponic test system. The results of the test demonstrated that robust and reliable experiments could be performed to calculate plant uptake parameters for leaching simulations. The results of this test were presented at the 2015 SETAC Europe in Barcelona, Spain and the Piacenza Pesticide Chemistry Symposium, Italy. An update was recently presented at the Fresenius conference in Mainz, Germany, and was presented at the ACS National Meeting, August 21-25, 2016. Additional experiments were conducted with wheat, potato and tomato using up to six different pesticides.
It has been demonstrated that this test protocol is robust, reliable and can be used for different crops and compounds and therefore, providing a plant uptake parameter calculated from test results instead of using a default value of 0 or 0.5. This protocol could soon become a standardized study design for the determination of PUF values to be used in regulatory environmental fate modeling of pesticides.
Briggs G. G., Bromilow R.H. and Evans A.A. (1982) Relationships between lipophilicity and root uptake and translocation of non-ionized chemicals in barley. Pesticide Science 13:495-504
Sweeney, 2014, submitted: PUF formula
FOCUS (2009). Assessing Potential for Movement of Active Substance and their Metabolites to Ground Water in the EU. In: Report of the FOCUS Ground Water Work Group, EC Document Reference Sanco/13144/2010 version 1, 604pp.
To discuss these and other testing strategies in detail, contact:
Dr. Kalumbu Malekani