1. Translaminar leaf model - Approach Source Sink Approach of the translaminar leaf model: ► Source is at the surface of the leaf, sink is at the bottom. ► The red arrow symbolises transport through the leaf.

2. Translaminar leaf model - basic equation where m is mass (mg), A is surface area, V is volume and P is permeability (m/d) unknown: P Integration using a symmetry condition gives The change of the mass in (1) is - loss from (1) + gain from (2) Source m 1 Sink m 2

3. Translaminar leaf model - basic equation Typical result. The shape of the curve is always the same. What changes is the time until m(1) = m(2) Source m 1 Sink m 2 Wanted: half-time t12

4. apoplastic transport symplastic transport Transport across the leaf P apoplast Diffusion between cells P symplast Across biomembranes and cell wall Assumptions: Only the fraction of chemical present in apoplast (f apo ) and Symplast (f Cyt ) moves. Movement is passiv (diffusion) Cuticle taken out

5. Fractions in Apoplast and Symplast are simply calculated using the Cell Model (for t = 1 h, see there)

6. Results from the translaminar leaf model Fractions: How much is in cytosol, vacuole and apoplast ? Half-time: How fast crosses a compound the leaf?

7. Sensitivity study Translaminar Leaf Model Neutral Compounds: Low half-time until log Kow 2 to 3 Distribution of polar neutral compound (log Kow = 0) in the leaf: Most is in vacuole and apoplast

8. Sensitivity study Translaminar Leaf Model Acids (log Kow = 0) Reasonable half-times, maximum at pKa 5 Acids distribution in the leaf: Most in Cytosol for 4 < pKa < 7 Most in vacuole and apoplast for pKa > 8 (like neutral polar cmpd.)

9. Sensitivity study Translaminar Leaf Model Acids (log Kow = 3) High half-times for pKa > 7 Acids (log Kow = 3) distribution in the leaf Most in Vacuole pKa > 5 (because acid stays neutral and thus lipophilic in vacuole at pH 5.5)

10. Sensitivity study Translaminar Leaf Model Bases (log Kow = 0) Low half-times Bases distribution in the leaf Very little in Cytosol Substance in vacuole (pKa < 8) or apoplast (pKa > 8)

11. Sensitivity study Translaminar Leaf Model Bases (log Kow = 3) Acceptable half-times, in particular at medium pKa Bases distribution in the leaf Most of substance now located in vacuoles for all pKa.

12. Summary - The Translaminar Leaf Model is a draft (can still be improved). - The approach is a combination between two-layer transport model and cell model. - It calculates the velocity of transport across a leaf (with/out cuticle) for neutral or ionizable (acids/bases) compounds. - It calculates the distribution within the leaf (cytosol/vacuole/apoplast). - All results are semi-quantitative only (model not tested nor calibrated). - All results are preliminary (changes can be done). - The outcome can be compared to experimental results, in order to improve the model, but also to improve the understanding of pesticide transport and action. Thank you for your attention.