Photodynamic Herbicides - Chlorophyll Biosynthesis and Technological Applications - page 384

17.10 Discrepancy Between the Effects of ALA
With and Without TDPH Modulators on
Greenhouse-Grown Plants and Field-Grown Plants
The ultimate interest in any herbicide resides in its eventual use to eliminate
undesirable weeds under field conditions. Yet as reported in Tables 17.4 and
17.5 , a serious discrepancy existed between the effects of ALA with and without
TDPH modulators on greenhouse-grown plants and field-grown plants.
Due to climatic conditions in Illinois, it was not possible to investigate the
molecular basis of this greenhouse-field discrepancy year round. In order to cir-
cumvent this problem, it was decided to develop a greenhouse model plant system
that simulates the field effects of ALA treatments, and then use this model to
determine whether the greenhouse-field discrepancy is due to poor ALA penetra-
tion, poor ALA metabolism, or both. From tetrapyrrole accumulation profiles, it
was decided that older greenhouse-grown plants were a good model for young field-
grown plants that had already developed a thick cuticle.
The experiments described below were aimed at understanding the molecular
basis of this discrepancy (Kulur 1996 ).
17.10.1 Tetrapyrrole and ALA Accumulation and
Photodynamic Damage in Morningglory Seedlings
of Various Ages, Using Whole Leaves for Analysis
To determine whether age-dependency of ALA-dependent photodynamic damage
was caused by slow metabolism, poor ALA translocation, or both, treated and
control morningglory leaves were analyzed separately for tetrapyrrole and ALA
content (Kulur 1996 ). Tetrapyrrole accumulation was used as a marker of metabolic
activity by the treated tissue. ALA content was considered as a crude putative
marker of applied ALA availability, which in turn may be related to ALA penetra-
tion to inner tissues where conversion of ALA to tetrapyrroles takes place.
Ten, fifteen and twenty-day old morningglory seedlings thinned down to one primary
leaf per seedling, were sprayed with solvent alone (control) and solvent + 2 lbs per acre
ALA (treatment). After spraying, the plants (six plants per treatment) were placed in
black foam rubber buckets, which were covered with aluminum foil and placed in
darkness overnight. The following day, two leaves were excised from each container
and were weighed. Since it was assumed that during overnight dark-incubation, all ALA
might have entered the leaf tissue, one unwashed leaf was used for tetrapyrrole accumu-
lation and the other for ALA analysis. The remaining four seedlings were used for
assessment of photodynamic injury after placing them in the growth room under
211 Wm 2 of metal halide light. 1,000 Wmetal halide lamps provided light intensity.
The amount of ALA detected in ALA-treated seedlings was significantly higher
in treated leaves of all ages in comparison to controls. However the amounts of