Pesticide Metabolism in Plants
23
distinct vascular systems (Crafts and Crisp, 1971). Generally, pesticides are
translocated apoplastically in the xylem (passive transport) from roots to all
transpiring leaves. Symplastic translocation in the phloem (active transport) occurs selectively from mature leaves to centers of high metabolic activity associated with the young growing points of the shoot and root (Zimmerman and
Milburn, 1975). Therefore, the distribution of pesticides in plants may not be
uniform and may vary according to the pesticide and the site of penetration into
the plant. Redistribution of pesticides or their metabolites is often limited in
plants, and translocation does not appear to be related to the hydrophilic properties
of the pesticide or its metabolites (Hussain et ai., 1974; Shimabukuro and Walsh,
1979).
2.2.3. Metabolism
Metabolism of pesticides in plants may be slower than in animals (Casida
and Lykken, 1969). Plants do not have a specific organ as highly active in
pesticide degradation as the mammalian liver. In contrast to animals, senescence
is a hormonally controlled phenomenon in plants that may be induced, retarded,
or reversed by different environmental factors (Thiman, 1977; Woolhouse,
1974). The same organs may be in various stages of senescence on the same
plant. The leaves and roots, which are the likely centers of pesticide metabolism
in plants, range in physiological age from senescent organs to young, developing
primordia. Therefore, a pesticide may be subjected to a wide range of metabolic
or degradation activities that vary according to the site of penetration into the
plant and its translocation.
In plants, the metabolism of organic pesticides to more hydrophilic forms
by oxidation and conjugation does not result primarily in secretion of the pesticides as it does in mammals. Plants lack an excretory mechanism comparable
to the enterohepatic and renal excretory systems in mammals. Therefore, plants
metabolize pesticides to water-soluble conjugates and insoluble "terminal" residues that remain in the plant during its life (Baldwin, 1977; Kaufman et ai.,
1976) or until the organ containing the residue is detached from the plant.
Complete oxidation of organic pesticides in plants to CO2 and H20 is generally
not a significant reaction (Frear et ai., 1972a).
2.3. DEGRADA TlON REACTIONS OF PESTICIDES IN
PLANTS
Much is known about the metabolic fate of pesticides in plants (Casida and
Lykken, 1969; Kearney and Kaufman, 1975). However, our understanding of
biotransformation mechanisms based on the plant enzyme systems involved is
23
distinct vascular systems (Crafts and Crisp, 1971). Generally, pesticides are
translocated apoplastically in the xylem (passive transport) from roots to all
transpiring leaves. Symplastic translocation in the phloem (active transport) occurs selectively from mature leaves to centers of high metabolic activity associated with the young growing points of the shoot and root (Zimmerman and
Milburn, 1975). Therefore, the distribution of pesticides in plants may not be
uniform and may vary according to the pesticide and the site of penetration into
the plant. Redistribution of pesticides or their metabolites is often limited in
plants, and translocation does not appear to be related to the hydrophilic properties
of the pesticide or its metabolites (Hussain et ai., 1974; Shimabukuro and Walsh,
1979).
2.2.3. Metabolism
Metabolism of pesticides in plants may be slower than in animals (Casida
and Lykken, 1969). Plants do not have a specific organ as highly active in
pesticide degradation as the mammalian liver. In contrast to animals, senescence
is a hormonally controlled phenomenon in plants that may be induced, retarded,
or reversed by different environmental factors (Thiman, 1977; Woolhouse,
1974). The same organs may be in various stages of senescence on the same
plant. The leaves and roots, which are the likely centers of pesticide metabolism
in plants, range in physiological age from senescent organs to young, developing
primordia. Therefore, a pesticide may be subjected to a wide range of metabolic
or degradation activities that vary according to the site of penetration into the
plant and its translocation.
In plants, the metabolism of organic pesticides to more hydrophilic forms
by oxidation and conjugation does not result primarily in secretion of the pesticides as it does in mammals. Plants lack an excretory mechanism comparable
to the enterohepatic and renal excretory systems in mammals. Therefore, plants
metabolize pesticides to water-soluble conjugates and insoluble "terminal" residues that remain in the plant during its life (Baldwin, 1977; Kaufman et ai.,
1976) or until the organ containing the residue is detached from the plant.
Complete oxidation of organic pesticides in plants to CO2 and H20 is generally
not a significant reaction (Frear et ai., 1972a).
2.3. DEGRADA TlON REACTIONS OF PESTICIDES IN
PLANTS
Much is known about the metabolic fate of pesticides in plants (Casida and
Lykken, 1969; Kearney and Kaufman, 1975). However, our understanding of
biotransformation mechanisms based on the plant enzyme systems involved is