Uptake of Chemicals into Plants Lectures by Dr. Stefan Trapp

Stefan Trapp CV 1962 * Germany 1986 dipl geoecology 1992 PhD botany 1998 habil mathematics 1998 DTU applied ecology Modeling of plant uptake and phytoremediation

Lecture today Part 1: Standard Model Part 2: Dynamic Cascade Model Part 3: Cell Model Part 4: Translaminar Leaf Model if time: Standard model for ionics

Part 1 Standard Model for Plant Uptake of Organic Compounds I Concepts II Uptake into Vegetation III Exercises

How plants function Roots take up water and solutes Stems transport water and solutes Xylem = water pipe Phloem = sugar pipe Leaves transpire water and take up gas Fruits are sinks for phloem and xylem

Definition “BCF” BCF is “bioconcentration factor” Concentration in plants [mg/kg] BCF = ―――――――――――――――――― Concentration in soil [mg/kg] Take care! BCF differs for - dry weight versus wet weight - with uptake from air - for roots, leaves, fruits, wood

Advective uptake with water Diffusion Direct soil contact Translocation in xylem Soil – air plant Particle deposition Xylem & Phloem transport Exchange with air

Some measured BCF (Organics) CompoundPropertiesmean BCFRangePlant part PAH, BaPlipophil to 0.01roots, leaves TCEvolatile< fruits, leaves metabolites of TCE polar, non- volatile 0.01fruits, leaves Pesticidespolar, non- volatile 1<1 to 10roots, leaves, fruits Explosives (TNT, RDX) polar, non- volatile to 29roots, leaves, fruits POPs (DDT, lindane, PCB) lipophil to 0.2roots, leaves “dioxins” TCDD/F lipophil10 -2 to to roots, leaves, fruits Sulfolane (detergent) Polar, non- volatile 680leaves

Regression with log K OW for C vegetation to C soil (dry wt.) BCF: Empirical regression by Travis & Arms Easy to use Gives good results Old (ex-RISK) Problem: only uptake from soil; no air

Principles of plant uptake models

Crop specific models

Root model mass balance Change of mass in roots = +uptake with water – transport to shoots dm R /dt = C W Q – C Xy Q where m is mass of chemical (mg) C is concentration [mg/kg, mg/L] Q is water flow [L/d] index R is roots, W is water and Xy is xylem

From mass to concentration m is chemicals’ mass (mg) M is root mass (kg) C is concentration (mg/kg) C = m / M dm R /dt = d(C R M R )/dt The root grows – integration for C and M required (oh no...!)

Dilution by exponential growth Chemical mass: m = constant Plant mass: M(t) = M(0) x e +kt m/M = Concentration in plant: C(t) = C(0) x e -kt

Root model concentration Change of concentration in roots = + uptake with water – transport to shoots – dilution by growth (rate k) dC R /dt = C W Q/M – C Xy Q/M – kC R where k is growth rate [d -1 ] C Xy is concentration in xylem = C R /K RW C W is concentration in soil pore water

Partition constant Root to Water K RW = equilibrium root to water K RW = W + L x K OW 0.77 W ca log Kow K RW Data by Briggs et al. (1982) for barley

Root model solution Mass balance: change = flux in – flux out Set to steady-state and solve for C R Concentration: divide by plant mass M

For lipophilic compounds: growth dilution. BCF > factor 100 below equilibrium Root Model result for roots to soil (Csoil = 1 mg/kg) TCE BaP

Translocation Upwards

Transpiration of plants in Europe Type mm/yearmm/d Broad-leaf trees Needle trees Corn fields Pasture, meadows General rule: About 2/3rd of precipitation is transpired by plants. 1 mm = 1 L/m 2

Translocation upwards in the xylem A ”standard plant” transpires 500 L water for the production of 1 kg dry weight biomass! = approx. 50 L per 1 kg fresh weight = approx. 1 L/day for 1 kg plant mass

Translocation upwards in the Xylem For translocation upwards, the chemical must cross the root and come into the xylem. “TSCF” = transpiration stream concentration factor = C Xylem /C Water

Definition TSCF TSCF = ”Transpiration stream concentration factor” [mg/L : mg/L] If TSCF is high, good translocation upwards. Two methods: 1) Regression to log K OW (Briggs et al., Dettenmaier et al.) 2) Calculation from root model

Briggs et al. (1982) = optimum curve Method 1: Regression for TSCF by Briggs (1982)

Method 2: Regression for TSCF by Dettenmaier (2009) Dettenmaier et al. = sigmoidal curve

Method 2: Calculation of TSCF with Root Model Model: Lipophilic chemicals (high log Kow) are adsorbed in the root and not translocated

Test of TSCF-Methods Compilation of data from literature Predicted TSCF

So which TSCF is best?

Uptake of contaminants into leaves and fruits Leaves and fruits are highly exposed to air Additionally high water flux to leaves (xylem) plus phloem flux (sugar) to fruits  Contamination possible from soil and air

Model for uptake into leaves + - exchange with air (+ spray application) + influx with xylem - dilution by growth - metabolism Mass balance: uptake from soil and air

Outflux from roots is influx to leaves and fruits Remember: high for polar compounds (low log Kow)

Leaves – exchange with air Stomata  Cuticle 

Equilibrium between leaves and air Leaves are plant material, like roots. But they do not hang in soil, and not in water. Leaves hang in air. The concentration ratio between air and water is The concentration ratio between leaves and air is then Because K AW 1  K LA >> 1

The model for leafy vegetables Adapted by the EU in the Technical Guidance Documents for Risk Assessment  ”TGD model” Used also by many soil risk assessment models Uptake from soil (via xylem) and from air (or loss to...) + Exponential growth

Mass balance for the leafy vegetables The change of mass in leaves = + translocation from roots + uptake from air - loss to air from rootsfrom airto air easy to solve: linear diff. eq. of the type growth & degradation

g Conductance leaf - air Estimation of g can be quite complex. It is convinient to use a default value of 1 mm s -1 = 86.4 m d -1. cuticle waystomata way

Mass Balance of Fruits essentially identical to the mass balance in leaves + - exchange with air ( + spray application) + influx with xylem and phloem - dilution by growth - metabolism

Mass balance for Fruits The change of mass in fruits = + flux from xylem and phloem + uptake from air - loss to air from rootsfrom airto air easy to solve: linear diff. eq. of the type growth & degradation

Summary: "Standard Model" where index R is root, W is water, L is soil, F is fruit and A is air. C is concentration (mg/kg), Q is water flux (L/d), M is plant mass (kg), K is partition coefficient (L/kg or kg/kg), A is area (m 2 ), g is conductance (m d -1 ) and k is rate (d -1 ). A system of coupled linear differential equations

Standard Model in excel – free for all

Uptake from soil into leaves partitioning air-water Accumulation in leaves: polar, non-volatile compounds (such as pesticides, detergents, pharmaceuticals)

Uptake from soil into fruits Accumulation in fruits: less than in leaves, but also polar and non-volatile compounds

Uptake into fruits from air “the usual candidates”: semivolatile lipophilic organic compounds such as PCB, DDT, PAH, PCDD/F

Bioaccumulation of lipophilic chemicals We learned at university (did you ???): ”Lipophilic chemical accumulate via the food-chain” high log K OW  high bioaccumulation this is only one out of two mechanisms

Bioaccumulation of hydrophilic compounds from soil in plants A typical plant transpires 500 L water for the production of 1 kg dry weight biomass! = ~ 50 L per 1 kg fresh weight = ~ 1 L/day for 1 kg plant mass The chemical comes with the water, the water evaporates, the chemical remains. This can lead to a bioaccumulation plant to soil of >> factor 100

Transfer to leaves with attached soil Soil on plant surfaces (Li et al. 1994) [g soil/kg plant dw] Lettuce260 Wheat 4.8 Cabbage 1.1 Default value: 1% attached soil (wet weight) BCF(leafy vegetables to soil) = BCF model

A ”standard” child eats 200 mg soil a day ”Pica child: 10 grams (acute effects) How much soil do you eat? More than you think... (1% of 500 g is 5000 mg/d) Direct Soil Uptake

Application of the Standard Model The "Standard Model" is the easiest way to calculate the dynamic system soil-plant-air in a "correct way". That's why it is rather popular. It is used by ● EU Chemical risk assessment (TGD, REACH) ● CLEA Contaminated Land Exosure Assessment (UK) ● Csoil (NL) ● RISK (USA) and also ● Teaching at DTU ● Teaching here and now ☺

Limitations of the Standard Model The "Standard Model" is only applicable ● for neutral organic compounds ● for exponentially growing plants ● for steady state Thus it is difficult to simulate real scenarios. It is more a "generic" model. More realistic scenarios can be simulated using the "dynamic cascade model" (see next section).

End of part 1. Any questions?