Photodynamic Herbicides - Chlorophyll Biosynthesis and Technological Applications - page 370

Preliminary screening of the 247 putative modulators was performed under controlled
experimental conditions using greenhouse-grown cucumber seedlings (cotyledon
stage) a DDV/ LDV plant tissue and whenever possible single representative species
of the DMV/LDV and DMV/LMV greening groups. The seedlings were sprayed in
the late afternoon with 5 mM ALA plus 20 mM modulator at a rate of 40 gal/acre and
an average droplet size of 75

m . The sprayed plants were wrapped in aluminum foil
to maximize penetration of the active ingredients and were placed in darkness at 28 C
to induce tetrapyrrole accumulation. The next day the plants were unwrapped and
were exposed to light in the greenhouse. Photodynamic damage was evaluated
visually and photographically over a period of 10 days. Modulators that exhibited
photodynamic damage of 88-90 % or better were retained for further experimentation.
Seventy-nine commercially available compounds exhibited 88-100 %.
Photodynamic kill on cucumber seedlings are reported in Table 17.2 . These
TDPH modulators belonged to 13 different templates. The 13 TDPH templates and
the 79 corresponding analogues that exhibited TDPH activity are reported in
Table 17.2 . Perhaps the most dramatic spinoff of the modulator search strategy was
the discovery of 9 modulators that belonged to the nicotinic acid and nicotinamide
templates and which exhibited (88-100 %) photodynamic kill on cucumber when
used in concert with ALA. These modulators are simple vitamin derivatives and may
herald an era in herbicide design where it may be possible to design totally biode-
gradable and safe TDPH formulations made up of ALA and a vitamin derivative.

μ

(d) Discovery of Additional 5-Membered N-Heterocyclic Modulators

The successful discovery of several TDPH pyrrole analogs prompted a closer look at
various commercially available 5-membered N-heterocyclic compounds. By follow-
ing the same approach as the one described above namely: (a) development of a
commercially available database of 5-membered N-heterocyclic, compounds,
(b) computer search of the database for 5-membered N-heterocyclic TDPH analogs
of' the selected templates and (c) screening for TDPH: activity of the selected
analogs, 75 new modulators that exhibited minimal phytotoxicity on corn and 88 %
or better kill on cucumber, pigweed and johnsongrass, were discovered. They are
depicted below in Table 17.3 .

17.7 Effect of TDPH on Plant Tissues Lacking Chlorophyll

In 1988 we described the concept and phenomenology of a new insecticidal system
(Rebeiz et al. 1988a ). The system consisted of certain modulators of the porphyrin-
heme biosynthetic pathway, which when used singly or in combination with ALA,
induced the massive accumulation of Proto in treated insects.
The uncontrolled protoporphyrin biosynthesis and accumulation caused death of
the treated insects in darkness (dark death) via an unknown mechanism, and in the