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ALA and Dpy accumulated large amounts of Proto, Pchlide, and small amounts of
Mpe, (b) Since the early 1970s, Rebeiz and coworkers have been routinely using
ALA plus Dpy treatments to force the accumulation of massive amounts of Pchlide,
and Mpe by etiolated plant tissues (Rebeiz et al. 1970 ), (c) With the use of
analytical fluorescence techniques developed by Rebeiz et al. (Rebeiz 2002 ),
Bazzaz and Rebeiz ( 1978 ) had demonstrated that the chlorophyll biosynthetic
pathway was still very active in green mature tissues. It. was therefore conjectured
that in order to achieve the massive accumulation of tetrapyrroles by green mature
tissues, for possible photodynamic herbicidal purposes, such tissues should be
treated with combinations of ALA and Dpy (Rebeiz et al. 1984a , b ).
17.6.3.2 The Search for Other Classes of Compounds Capable
of Modulating the Chlorophyll Biosynthesis Pathway
The successful synergistic use of ALA plus Dpy for TDPH purposes (Rebeiz
et al. 1984a , b ), prompted a search for additional compounds that may affect the
chlorophyll biosynthetic pathway. To this end, we undertook a search of the
literature for chemicals and biochemicals that have been known to affect, in a
general way, chlorophyll and Pchlide biosynthesis. That search identified a total of
14 chemicals that had been implicated in one way or another in chlorophyll, Pchlide
and Mpe formation. One of those compound, 8-hydroxyquinoline was selected for
its effects on bacteriochlorophyll (Bchl) biosynthesis in Rhodapseudomonas
spheroides. Four compounds (1,10-phenanthroline, 2-pyridyl aldoxime, 2-pyridyl
aldehyde and picolinic acid) had been described by Duggan and Gassman ( 1974 )
for their effects on the chlorophyll biosynthetic pathway in etiolated seedlings of
red kidney bean, corn and cucumber.
Two other compounds, namely 1,7-phenanthroline and phenanthridine were
chosen because of their effect on the chlorophyll biosynthetic pathway in
Chlamydomonas reinhardtii (Bednarick and Hoober 1985 ). These compounds are
all derived from pyridine. Detailed mode of action studies of these compounds on
cucumber seedlings revealed interesting TDPH modulating properties (Rebeiz
et al. 1988b ). For example, it was noted that the presence of a nitrogen (N) atom
in a benzene-like 6-membered ring structure or a phenanthrene-like structure was
essential for TDPH activity. Indeed, benzene and phenanthrene, which are identical
in chemical structure to pyridine and phenanthroline respectively but lack N atoms
in their rings, are not active as TDPH modulators.
17.6.3.3 Selection of Other Pyridine-Based TDPH Modulators
Encouraged by these preliminary results the search continued for additional pyri-
dine derivatives that may have TDPH modulating potential. The following
compounds (Rebeiz et al. 1991 ), were chosen: nicotinic acid because it is a
geometrical
isomer of picolinic acid; nicotinamide because of its structural
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