uptake of chemicals into plants
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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 - PowerPoint PPT PresentationTRANSCRIPT
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
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
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?