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)

Compound Properties mean BCF Range Plant part

PAH, BaP lipophil 0.001 10-5 to 0.01 roots, leaves

TCE volatile < 10-3 < 10-3 fruits, leaves

metabolites of TCE

polar, non-volatile

0.01 fruits, leaves

Pesticides polar, non-volatile

1 <1 to 10 roots, leaves, fruits

Explosives (TNT, RDX)

polar, non-volatile

3 0.06 to 29 roots, leaves, fruits

POPs (DDT, lindane, PCB)

lipophil 0.01 0.02 to 0.2 roots, leaves

“dioxins” TCDD/F

lipophil 10-2 to 10-4 10-5 to 10-3 roots, leaves, fruits

Sulfolane (detergent)

Polar, non-volatile

680 leaves

Regression with log KOW for C vegetation to C soil (dry wt.)

588.1log578.0log OWKBCF

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

dmR/dt = CWQ – CXyQ

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

dmR/dt = d(CR MR)/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

0

25

50

75

100

0 24 48 72

Time

Pla

nt

mas

s,

con

cen

trat

ion

M (kg) m/M (mg/kg)

Root model concentration

Change of concentration in roots =

+ uptake with water

– transport to shoots

– dilution by growth (rate k)

dCR/dt = CWQ/M – CXyQ/M – kCR

where

k is growth rate [d-1]

CXy is concentration in xylem = CR/KRW

CW is concentration in soil pore water

Partition constant Root to Water KRW

= equilibrium root to water

KRW = W + L x KOW0.77

W ca. 0.85 log Kow

KR

W

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 CR

RRW

RW CkMK

QC

M

QC

0

QCQCdt

dmXyW

Concentration: divide by plant mass M

RXyW kCM

QC

M

QC

dt

dC

RW

RXy K

CC

d

SoilW K

CC

d

soil

RW

R K

C

kMK

QQ

C

For lipophilic compounds: growth dilution.

BCF > factor 100 below equilibrium

Root Model result for roots to soil (Csoil = 1 mg/kg)

0.0001

0.001

0.01

0.1

1

10

0 2 4 6 8

log Kow

C r

oo

t (m

g/k

g w

w)

T&A RCF root model

TCE

BaP

Translocation Upwards

Transpiration of plants in Europe

Type mm/year mm/d

Broad-leaf trees 500-800 4-5

Needle trees 300-600 2.5-4.5

Corn fields 400-500

Pasture, meadows 300-400 3-6

General rule:

About 2/3rd of precipitation is transpired by plants.

1 mm = 1 L/m2

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 = CXylem/CWater

Definition TSCF

TSCF = ”Transpiration stream concentration factor”

[mg/L : mg/L]

If TSCF is high, good translocation upwards.

Two methods:

1) Regression to log KOW (Briggs et al., Dettenmaier et al.)

2) Calculation from root model

C_water

C_xylem TSCF

2.44

1.78) - K (log-exp0.784 TSCF

2OW

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

OWKTSCF

log6.211

11

Method 2: Calculation of TSCF with Root Model

RW

RW

RWW

R

W

Xy KkM

K

QQ

KC

C

C

C//

RW

RXylem K

CC 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

RRRRWR

SWSR

R CkCKM

QCK

M

Q

dt

dC

is influx to leaves and fruits

RRWL

L CKM

Q

dt

dC

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

AWWater

Air KC

C

LAAWLWAir

Water

Water

Leaves

Air

Leaves KKKC

C

C

C

C

C /

The concentration ratio between leaves and air is then

Because KAW < 1 and KLW > 1 KLA >> 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 roots from air to air

LL kCI

dt

dCeasy to solve: linear diff. eq. of the type

LLLLLA

LA

L

LR

RWL

L CkCMK

mLgAC

M

gAC

KM

Q

dt

dC

31000

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 way stomata 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 roots from air to air

kCIdt

dCeasy to solve: linear diff. eq. of the type

FFFFFA

FA

F

FR

RWF

FF CkCMK

mLgAC

M

gAC

KM

Q

dt

dC

31000

growth & degradation

Summary: "Standard Model"

LLLLLA

LA

L

depLR

RWL

L CkCMK

mLgAC

M

vAC

KM

Q

dt

dC

31000

FFFFFA

FA

F

FR

RWF

FF CkCMK

mLgAC

M

gAC

KM

Q

dt

dC

31000

RRWRWR CkMQKCMQC

dt

dC ///

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 (m2), 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

-2 0 2 4 6

1

-3

-7

0.0001

0.001

0.01

0.1

1

10

100

1000

C Leaves

log Kow

log Kaw

1

-1

-3

-5

-7

-9

partitioning air-water

Accumulation in leaves: polar, non-volatile compounds (such as pesticides, detergents, pharmaceuticals)

Uptake from soil into fruits

-2 0 2 4 6

1

-3

-7

0.0001

0.001

0.01

0.1

1

10

C Fruit

log Kow

log Kaw

1

-1

-3

-5

-7

-9

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

1 -1 -3 -5 -7 -9

-2

2

6

0.0001

0.001

0.01

0.1

1

10

C Fruits

log Kaw

log Kow

-2

0

2

4

6

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 KOW 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]

Lettuce 260 Wheat 4.8 Cabbage 1.1

Default value: 1% attached soil (wet weight)

BCF(leafy vegetables to soil) = BCF model + 0.01

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?