Biology Reference
In-Depth Information
Chapter 17
Photodynamic Herbicides
It is the habit of mediocre minds to condemn all that is
beyond their grasp (La Rochefoucaud)
17.1 Prologue
In 1982, after having researched the chemistry and biochemistry of the greening process
for 18 years, it was felt that enough was known about this important biological
phenomenon to translate it into biotechnological developments. In looking for a handle
on the problem we opted for the development of photodynamic herbicides. That
decision was prompted by two considerations: (a) the size and importance of the
herbicide industry, and (b) the interesting photosensitizing properties of tetrapyrroles.
Indeed, in 1974 it was reported that etiolated cucumber cotyledons incubated with ALA
for 16 h in darkness accumulated, as expected, exogenous tetrapyrroles. It was noticed
that upon exposure of the etiolated cotyledons to light in order to study the lag phase of
Chl Biosynthesis (Rebeiz
1967
), the etiolated tissue underwent visible damage that was
attributed to the accumulated tetrapyrroles (Castelfranco et al.
1974
). However, at the
time it was not known whether green tissues incubated with ALA would accumulate
exogenous tetrapyrroles, since it was not known how active the Chl biosynthetic
pathway was in green tissues. This information was needed in order to develop a
tetrapyrrole-dependent photosensitizing herbicidal technology for green plants.
17.2 Chlorophyll Biosynthesis Is Indeed
Very Active in Green Tissues
The key to the success of the paper chemistry photodynamic herbicide tetrapyrrole
hypothesis mentioned above resided therefore in determining whether green plants
could be induced to accumulate enough tetrapyrroles to trigger a damaging photo-
sensitization reaction in the green plant tissues.
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