Uptake and translocation of non-ionised
pollutants by plants
Richard H. Bromilow
Transfer of organic pollutants from soil to plants
Reading University, September 25/26, 2007
Pesticides – applied directly to the environment
Industrial pollutants – escape directly or indirectly
Environmental behaviour of organic
compounds influenced by physicochemical
properties
• sorption to soil
• movement through soil
• bioaccumulation in organisms
• uptake and movement in plants
• atmospheric transport
Metabolism and breakdown of organic pollutants determines
their availability for long-term processes
Physicochemical properties of
pesticides and organic pollutants
The most important properties are:-
•
Lipophilicity - assessed using the 1-octanol/water
partition coefficient, Kow (expressed as log Kow or log P)
•
Water solubility - strongly correlated with lipophilicity
•
Vapour pressure – can be important for lipophilic
pesticides and pollutants in soil
•
Acid/base strength - the pKa is the pH at which a
functional group is 50% ionised (eg -COOH, -NH2)
Insecticides
1-Octanol/water partition coefficients (Kow) of classes
of non-ionised pesticides and pollutants
Dinitroanilines
Diphenyl ethers
Thiocarbamates
Triazines (-ones)
Phenylureas/uracils
pollutants
Industrial
Fungicides Herbicides
Carbamates
Organochlorines
Organophosphates
Pyrethroids
Acylanilines
Dicarboximides
Sterol-biosynthesis inhibitors
Chorinated solvents
Nitrophenols
Phthalates
Polyaromatic hydrocarbons
Polychlorinated biphenyls
Polychlorinated dioxins/
benzofurans
-2
polar
0
2
4
Log Kow
6
8
lipophilic
10
Pathways of compound movement in soil are
determined by the Henry Constant:-
Henry constant =
concentration in air
concentration in water
(calculated from the vapour pressure and water solubility)
Pathways of movement of organic compounds
through soil as determined by Henry’s constant
102
101
1
10-1
10-2
Henry’s
constant
(dimensionless)
10-3
10-4
10-5
10-6
F12 (CCl2F2)
ethyl bromide
carbon tetrachloride
trichloroethylene
dichloromethane
tetrachlorobiphenyl
TCDD
DDT
trifluralin
pentachlorophenol
dioctyl phthalate
dibutyl phthalate
dieldrin
chlorpropham
Movement by
diffusion in air
Diffusion both in
air and water
carbofuran
10-7
10-8
monuron
simazine
bromacil
10-9
10-10
hexazinone
Movement by
diffusion only
in the water phase
Uptake and transport in barley of non-ionised
[14C]compounds applied via nutrient solution
polar
lipophilic
intermediate
Long-distance transport of solutes in plants
•
Xylem vessels - non-living tubes that carry water and
nutrients from roots to shoots
•
Phloem vessels - living tube-like cells without
vacuoles that carry sugars and amino acids from leaf
sources to sinks such as new growth
Cross-section of root showing the arrangement
of cells and vascular tissues
Root Concentration Factor (RCF)
=
concentration in root
concentration in nutrient solution
Relationship between the lipophilicity of non-ionised chemicals
and their uptake by barley roots from nutrient solution
Root Concentration Factor
100
Mean uptake over 24 & 48 h
Carbamoyloximes
80
Phenylureas
log (RCF-0.82) = 0.77log Kow - 1.52
60
40
20
0
-1
0
1
2
Log Kow
3
4
5
Uptake of non-ionised pesticides by plant
roots - conclusions
•
Uptake is an equilibrium process that is rapidly
attained
•
Uptake occurs by both equilibration into the aqueous
phase of roots and, more importantly for lipophilic
compounds, by partitioning into the plant solids (eg
lignin)
•
The concentration factor is independent of uptake
time, pesticide concentration and the solution pH
Transpiration Stream Concentration
Factor (TSCF)
=
=
concentration in xylem sap
concentration in nutrient solution
amount in plant shoot
vol. water transpired x conc. in nutr. solution
Transpiration Stream Concentration Factor
Relationship between the lipophilicity of non-ionised chemicals
and their translocation to barley shoots via root uptake from
nutrient solution
TSCF = 0.784exp -[(log Kow - 1.78)2 /2.44]
1
Mean over 24 & 48 h
Carbamoyloximes
0.8
Phenylureas
0.6
Equation
0.4
0.2
()
0
-1
0
1
2
Log Kow
3
4
5
Translocation of non-ionised pesticides
from roots to shoots - conclusions
•
Translocation is an equilibrium process, rapidly
attained and limited by the Casparian Strip
•
Movement across the membranes is optimal at log
Kow 1.8, and less for more polar or more lipophilic
compounds
•
Translocation is a passive process (TSCF < 1.0)
Conclusions
•
Uptake and translocation into plants from soil water are
controlled by the physicochemical properties of the
compound
•
But difficult to model due to uncertainties in the
distribution of the compound in soil, the distribution of
roots and the source of water
•
Vapour transport, important for the more lipophilic
compounds both in soil and above soil, is difficult to
quantify
•
Metabolism in the plant reduces accumulation