Photodynamic Herbicides - Chlorophyll Biosynthesis and Technological Applications - page 392

17.11 Effects of Two Different Treatments on
the Availability of Metabolically Active ALA
and Concomitant Photodynamic Damage
in Morningglory
As a result of the previous experiments, it was proposed that in older greenhouse-
grown morningglory seedlings which simulate the response of younger field-grown
seedlings, the reduction in photodynamic damage in comparison to younger,
greenhouse-grown seedlings maybe caused by one of two factors: (a) Slow translo-
cation of exogenous ALA to inner tissues, coupled with slow conversion of ALA to
tetrapyrroles or (b) slow translocation of exogenous ALA to inner tissues coupled
with fast conversion of ALA to tetrapyrroles. As discussed in above, distinction
between these two possibilities is essential for effective ALA field formulations.
In an effort to determine whether the slow translocation of exogenous ALA to inner
tissues is accompanied by sluggish or active tetrapyrrole metabolism, two sets of
experiments were designed.
In a first set of experiments, thioflavin T, a desiccant (Rebeiz et al. 1994 ) was
used jointly with ALA in an effort to improve ALA translocation to inner tissues,
since at low concentrations, thioflavin T is supposed to create holes in the cuticle
and facilitate herbicide penetration.
It was conjectured that, if ALA penetration was improved by that treatment and
if tetrapyrrole metabolism was highly active in older leaves, then improved ALA
accumulation and photodynamic damage should be observed.
In a second set of experiments, an attempt was made to incubate older leaves
under conditions of unlimited ALA supply. It was conjectured that if active ALA
conversion to tetrapyrroles were limiting in older leaves, then high levels of
tetrapyrrole accumulation and photodynamic death would be observed.
17.11.1 Response of Various Age Groups of Morningglory
Seedlings to ALA Treatments With and Without
Thioflavin T
As was observed in previous experiments, the amount of ALA detected in
washed leaf sections in ALA-treated seedlings was slightly, but significantly
higher in treated leaves of all ages in comparison to controls (Table 17.7 ,
Fig. 17.7b, c ). ALA-treated plants accumulated significantly higher amounts of
tetrapyrroles than untreated ones. Most of the accumulated tetrapyrroles consisted
of Pchlide a . Also, the accumulation of Pchlide a was age-dependent and the
amount of tetrapyrroles accumulated by 10 and 15-day old plants was higher than
in 20-day old plants.