Photodynamic Herbicides - Chlorophyll Biosynthesis and Technological Applications - page 362

17.5.2 Dependence of the Differential Photodynamic
Herbicidal Susceptibility of Plant Species
Upon Greening Group Affiliation of Plant
On the basis of the photodynamic herbicidal response of cucumber a DDV/LDV
plant species and soybean a DMV/LDV species (see Chap. 12 ) , toward various
ALA and modulator combinations (Abd-El-Mageed et al. 1997 ; Rebeiz et al. 1987 ,
1988b ) the following working hypothesis was proposed: (a) that plants poised in the
DV tetrapyrrole biosynthetic state are more photodynamically susceptible to the
accumulation of MV tetrapyrroles than to the accumulation of DV tetrapyrroles;
and (b) that plants poised in the MV tetrapyrrole biosynthetic state are more
susceptible to the accumulation of DV tetrapyrroles than to MV tetrapyrroles.
This phenomenon rested on the hypothesis that plant species poised in the
MV-greening pattern could not cope as well with a massive influx of DV
tetrapyrroles. Likewise, plants poised in the DV greening pattern could not cope
as effectively with a massive influx of MV tetrapyrroles. Being unable to rapidly
metabolize the wrong tetrapyrroles, In the light the latter would linger around and
photosensitize the destruction of the host plant before being eventually degraded by
light. With the discovery of the Dark-Light (DL) subgroups in 1997 (Abd-El-
Mageed et al. 1997 ), this hypothesis which was proposed in 1990 (Mayasich
et al. 1990 ) did not withstand the rigors of further testing.
17.6 Modulation of TDPH Activity
The dependence of TDPH susceptibility upon the nature and amount of
accumulated tetrapyrroles suggested that it may be possible to chemically modify
the activity of TDPH by appropriate selections of modulators. It was conjectured
that this may be achieved with the use of chemicals that may modulate the Chl
a biosynthetic pathway by forcing ALA-treated plants to accumulate substantial
amounts of various types of tetrapyrroles. An initial search led to the identification
of 14 chemicals that acted in concert with ALA and that exhibited a definite
modulating propensity toward the Chl a biosynthetic pathway. These chemicals
were therefore designated as TDPH modulators. They were classified into four
groups depending on their effects on the Chl a biosynthetic pathway.
17.6.1 The Four Classes of Modulators
In order to determine whether a compound acted as an ALA-based photodynamic
herbicide modulator (TDPH), the chemical was usually sprayed on a plant with and
without ALA, and the treated plant was kept in darkness for several hours for