Photodynamic Herbicides - Chlorophyll Biosynthesis and Technological Applications - page 383

17.9 Is a Postspray Dark Incubation Period

Needed for Effective TDPH Activity?

Since in addition to light, a certain level of tetrapyrrole accumulation in the dark is
essential for the expression of photodynamic damage, it was natural to wonder
whether a post-spray incubation period is essential for effective TDPH treatments.
Preliminary experiments reported by Rebeiz et al. ( 1984a , b ) indicated that this was
apparently the case. Further experimentation revealed, however, that the reduced
photodynamic injury observed in plants treated with ALA and Dpy, and exposed to
light without a post-spray dark incubation period, was more probably related to the
penetration of the active ingredient to target sites, than to tetrapyrrole accumulation
per se. For example, with appropriate formulation, it was possible to achieve the
destruction of broadleaf weeds in a Kentucky bluegrass lawn without a post-spray
dark incubation period (Rebeiz et al. 1988b ). These studies were extended by
Mayasich and Rebeiz to ten common weed species (Rebeiz et al. 1991 ). The weed
species were sprayed with ALA and one of four different TDPH modulators using a
solvent system developed for experimental field applications (Rebeiz et al. 1988b ).
Some treatments involved a postspray dark incubation period while others did not.
Tetrapyrrole accumulation and photodynamic injury were then evaluated. Seedlings
that were subjected to a post-spray dark incubation period accumulated large amounts
of tetrapyrroles in darkness. The steady state formation of tetrapyrroles in plants that
werenotexposedtoapost-spraydarkincubationperiodwasmonitored1hafter
spraying and exposing the plants to low light intensity.
The low light treatment was meant to allow sampling the steady state formation
of tetrapyrroles in the light without excessive tetrapyrrole destruction (Rebeiz
et al. 1991 ). In response to ALA plus modulator treatment, tetrapyrrole biosynthesis
and accumulation was observable under the low light conditions used in these
experiment Under these conditions, however, tetrapyrrole accumulation was
much lower than in darkness, probably due to photodestruction and metabolism.
Nevertheless, for all practical purposes photodynamic damage in weeds that did not
receive a post-spray dark-treatment was essentially as good as in weeds that did
(Rebeiz et al. 1991 ). Minor differences in response between the dark and light
treatments was attributed to a combination of factors, that included greening group
affiliation of the weed species, and type of accumulated tetrapyrrole in response to
the ALA plus modulator treatment. It is obvious that the pre-accumulation of
massive amounts of tetrapyrroles is not essential for effective photodynamic injury
to take place. What appears to be needed is a steady state supply of tetrapyrroles at a
rate sufficiently large to initiate and sustain damaging free radical reactions. The
same phenomenon was observed with insects treated with ALA and Dpy in the light
(Rebeiz et al. 1988a ).