Photodynamic Herbicides - Chlorophyll Biosynthesis and Technological Applications - page 363

tetrapyrrole accumulation to take place. After dark incubation, the plant tissues
were analyzed for tetrapyrrole accumulation before exposure to light and determi-
nation of photodynamic damage. Upon exposure to light, tissues that had
accumulated tetrapyrroles in darkness exhibited rapid photodynamic damage
within the first hour of illumination. The classification of a modulator as an
enhancer, inducer or inhibitor was then determined from the pattern of tetrapyrrole
accumulation in the presence and absence of ALA and modulator (Rebeiz
et al. 1987 , 1988b ).
Based on their mechanism of action TDPH modulators were classified into four
distinct groups (Rebeiz et al. 1987 , 1988b ): (a) enhancers of ALA conversion to DV
Pchlide, which enhanced the conversion of exogenous ALA to DV Pchlide,
(b) enhancers of ALA conversion to MV Pchlide, which enhanced the conversion
of exogenous ALA to MV Pchlide, (c) inducers of tetrapyrrole accumulation, which
induced the plant tissues to form large amounts of endogenous ALA and enhanced
the conversion of this endogenous ALA, as well as any exogenously supplied ALA,
to tetrapyrroles; and (d) inhibitors of MV Pchlide accumulation, which appear to
block the detoxification of DV tetrapyrroles by inhibiting their conversion to MV
tetrapyrroles. Of all the aforementioned modulators, only inducers of tetrapyrrole
accumulation were capable of exhibiting tetrapyrrole accumulation in the absence
of added ALA, since they forced the plant tissue to form high levels of endogenous
ALA, which were then converted to tetrapyrroles. The three other classes of
modulators did not lead to significant levels of tetrapyrrole accumulation in the
absence of added ALA. However in all cases, the use of ALA together with a
modulator resulted in enhanced tetrapyrrole accumulation and photodynamic dam-
age over and beyond the levels caused by ALA alone. A more detailed description
of the four classes of modulators and of their criteria of classification can be found
in Rebeiz et al. 1987 and 1988b .
17.6.2 Response of Various Greening Groups
of Plants to TDPH Modulators
It is extremely desirable to be able to predict the mode of action of a modulator from
its chemical structure. The advantages of such an undertaking become obvious when
one takes into account: (a) the amount of time and effort involved in determining
experimentally the mode of action of a modulator in a particular plant species, (b) that
79 highly potent modulators, belonging to the four modulator classes have so far been
discovered and can be investigated, and (c) that at this rate many more additional
modulators may be discovered (see below). As a consequence we have undertaken a
research effort aimed at probing: (a) whether different plant species belonging to the
same greening group react in a similar manner toward a particular modulator,
(b) whether a particular plant species would react the same way toward a group of
modulators that belong to the same chemical category, and (c) whether a particular