Photodynamic Herbicides - Chlorophyll Biosynthesis and Technological Applications - page 361

thylakoid membranes, as Chl b in healthy thylakoids, does not fluoresce but
transfers its excitation energy to Chl a . The 77 K fluorescence excitation recorded
at F685, 696 and 739 nm, were similar. They exhibited excitation maxima at
418, 443, and 476 nm and a 485 nm excitation shoulder (Amindari et al. 1995 ).
The presence of the 476 and 485 nm excitation bands indicated that the disruption
of the Chl b -carotenoid association was not as complete as in the case of DV
Pchlide a . The Soret excitation maximum at 443 nm is equivalent to that of MV
Chl a coordinated to two small ligands such as pyridine at room temperature
(Belanger and Rebeiz 1984 ). The 418 nm excitation peak probably corresponds to
the η 1 transition of Chl a .
17.5 Molecular and Plant Tissue Bases of Tetrapyrrole-
Dependent Photodynamic Herbicide Selectivity
Originally photodynamic herbicides were assumed to be non-selective in their mode
of action. Further experimentation under controlled laboratory and field conditions
indicated that various ALA and modulator combinations exhibited a significant
degree of photodynamic herbicidal selectivity. This selectivity appeared to be rooted
(a) in the different tetrapyrrole accumulating capabilities of various plant tissues,
(b) in the differential susceptibility of various greening group of plants to the
accumulation of various DV and MV tetrapyrroles, and (c) in the differential response
of various greening groups of plants to photodynamic herbicide modulators.
17.5.1 Dependence of the Differential Photodynamic
Herbicidal Susceptibility Upon the Extent of
Tetrapyrrole Accumulation by Plant Tissues
It soon became apparent that different plant tissues were not equally capable of
tetrapyrrole accumulation. Since the tetrapyrrole-dependent photodynamic herbi-
cidal (TDPH) phenomenon depended on photosensitization by accumulated
tetrapyrroles it was expected that only tissues that accumulate tetrapyrroles will
be susceptible to TDPH. This situation was encountered in green soybean seedlings
where the stems, leaves and cotyledons, exhibited different susceptibilities toward
ALA plus Dpy treatments. The leaves, which accumulated high amounts of
tetrapyrroles, were quite susceptible to photodynamic damage while the stems
(Rebeiz et al. 1984a , b ) and cotyledons (Rebeiz et al. 1988a , b ), which were very
poor tetrapyrrole accumulators, exhibited resistance to treatment.