22
Richard H. Shimabukuro at a/.
metabolism in plants, but rather to illustrate selected degradation reactions known
to occur in plants. The biological significance of these degradation reactions will
be discussed primarily with respect to their importance in plants.
2.2. PESTICIDE METABOLISM IN PLANTS
Plants metabolize pesticides through a series of intermediates ultimately to
insoluble residues (Baldwin, 1977; Frear et ai., 1972a). The complete course
of metabolism in a plant is not understood for any pesticide in use today. The
chemical nature and the quantity of intermediate metabolites and insoluble residues in a plant are influenced by the site of absorption of the pesticide, translocation, and time of residence in the plant (Baldwin, 1977; Shimabukuro and
Walsh, 1979). Several important differences between plants and animals that are
relevant to the metabolism of pesticides in plants have been established recently.
2.2. 1. Absorption
Regardless of how a pesticide is applied to a plant, it must ultimately
penetrate the cells where phytotoxic and biotransformation reactions occur. In
animals, oral administration or dosing by injection minimizes exposure of the
pesticides to external microbial and environmental factors. Root or foliar treatment is common in plants. Long-term treatments of plants by soil or leaf application of pesticides increase the probability that plants may absorb a mixture
of degradation products formed by microbial and/or photochemical activity
(Crosby, 1973). These problems may be circumvented to a large extent by shortterm treatments with excised leaves, roots, leaf disks, and isolated mesophyll
cells (Shimabukuro and Walsh, 1979).
The absorption of pesticides by the gastrointestinal tract of animals poses
similar barriers to all ingested pesticides. The root tips and leaf surfaces are the
major sites of penetration into plants (Bukovac, 1976; Hay, 1976). These two
sites present distinctly different barriers to penetration by the same pesticides.
The cuticle, a thin, lipoidal membrane that covers the entire above-ground parts
of a plant, is the primary barrier to penetration by foliar application. In roots,
the primary barrier appears to be the endodermis, an internal root structure that
must be traversed by an active mechanism before transport to the shoots can
occur.
2.2.2. Translocation and Distribution
Absorbed pesticides are distributed throughout the animal body in a welldefined circulatory system. In plants, translocation and distribution occur in two
Richard H. Shimabukuro at a/.
metabolism in plants, but rather to illustrate selected degradation reactions known
to occur in plants. The biological significance of these degradation reactions will
be discussed primarily with respect to their importance in plants.
2.2. PESTICIDE METABOLISM IN PLANTS
Plants metabolize pesticides through a series of intermediates ultimately to
insoluble residues (Baldwin, 1977; Frear et ai., 1972a). The complete course
of metabolism in a plant is not understood for any pesticide in use today. The
chemical nature and the quantity of intermediate metabolites and insoluble residues in a plant are influenced by the site of absorption of the pesticide, translocation, and time of residence in the plant (Baldwin, 1977; Shimabukuro and
Walsh, 1979). Several important differences between plants and animals that are
relevant to the metabolism of pesticides in plants have been established recently.
2.2. 1. Absorption
Regardless of how a pesticide is applied to a plant, it must ultimately
penetrate the cells where phytotoxic and biotransformation reactions occur. In
animals, oral administration or dosing by injection minimizes exposure of the
pesticides to external microbial and environmental factors. Root or foliar treatment is common in plants. Long-term treatments of plants by soil or leaf application of pesticides increase the probability that plants may absorb a mixture
of degradation products formed by microbial and/or photochemical activity
(Crosby, 1973). These problems may be circumvented to a large extent by shortterm treatments with excised leaves, roots, leaf disks, and isolated mesophyll
cells (Shimabukuro and Walsh, 1979).
The absorption of pesticides by the gastrointestinal tract of animals poses
similar barriers to all ingested pesticides. The root tips and leaf surfaces are the
major sites of penetration into plants (Bukovac, 1976; Hay, 1976). These two
sites present distinctly different barriers to penetration by the same pesticides.
The cuticle, a thin, lipoidal membrane that covers the entire above-ground parts
of a plant, is the primary barrier to penetration by foliar application. In roots,
the primary barrier appears to be the endodermis, an internal root structure that
must be traversed by an active mechanism before transport to the shoots can
occur.
2.2.2. Translocation and Distribution
Absorbed pesticides are distributed throughout the animal body in a welldefined circulatory system. In plants, translocation and distribution occur in two