Photodynamic Herbicides - Chlorophyll Biosynthesis and Technological Applications - page 364

modulator would exhibit the same effects on the chlorophyll biosynthetic pathway in
plant species belonging to different greening groups.
Determination of the mode of action of a modulator were performed on cucum-
ber cotyledons, a representative DDV-LDV tissue, on soybean primary leaves, a
representative DMV-LDV tissue, and on Johnsongrass, a representative
DMV-LMV species. Treatments consisted of spraying the seedlings with 2.5 and
5 mM solutions of ALA with or without various concentrations of modulators
ranging from 5 to 30 mM. At the end of 17 h of dark incubation the plants were
extracted with ammoniacal acetone under a green safelight and the porphyrin
metabolic pool sizes between ALA and Chl were determined from room tempera-
ture and 77 K fluorescence spectral analysis (Rebeiz 2002 ). From a comparison of
the relative pool sizes of the treated and control seedlings, the site of action of the
various modulators along the various Chl monocarboxylic biosynthetic routes was
then determined.
Preliminary results suggested that: (a) a modulator that acted in a certain way
on the chlorophyll biosynthetic pathway of one greening group of plants did not
necessarily act the same way on plant species belonging to a different greening
group, (b) different plant species belonging to the same greening group tended to
exhibit similar chlorophyll biosynthetic reactivities toward a given modulator,
and (c) modulators that belonged to the same chemical category tended to exhibit
the same chlorophyll biosynthetic mode of action toward a particular plant
species. The above results suggested that it may be possible to make certain
predictions about the mode of action of a modulator on a particular plant species
belonging to a particular greening group once the mode of action of the chemical
category to which it belongs, had been determined on that particular greening
group.
17.6.3 Discovery of Novel TDPH Modulators
Because of the central importance of modulators to the performance of TDPH,
considerable time and efforts have been devoted during the past several years, to the
discovery of novel TDPH modulators. The experimental strategy used in that
successful undertaking used two dimensional and three dimensional computer
modeling and resulted in the discovery of several hundred potent TDPH modulators
(Rebeiz et al. 1990 , 1991 , 1994 ; Reddy and Rebeiz 1994 ).
17.6.3.1 The Use of 2,2 0 -Dipyridyl as a TDPH Modulator
The first modulator that was used jointly with ALA for TDPH purposes was
2,2 0 -dipyridyl (Dpy) (Rebeiz et al. 1984a , b ). The decision to use Dpy in concert
with ALA was based on the following considerations: (a) In the early 1960s
Granick ( 1961 ) had demonstrated that etiolated barley leaves incubated with