Photodynamic Herbicides - Chlorophyll Biosynthesis and Technological Applications - page 382

17.8 Translocation of TDPH in Intact Plant Seedlings
The translocation of TDPH in intact plant seedlings was investigated by Chung and
Rebeiz (unpublished).
17.8.1 Acropetal (Upward) Translocation
Acropetal (upward) translocation, presumably via the xylem, was monitored by
following the dark-accumulation of tetrapyrroles in the vegetative parts of the
cucumber seedlings upon watering with solutions of ALA and modulators. Trans-
location of ALA and modulator from the roots to the cotyledons of cucumber
seedlings appeared to be very extensive as evidenced by the massive accumulation
of tetrapyrroles in the cotyledons (Rebeiz et al. 1991 ). It is very unlikely that
the build up of tetrapyrroles in the cotyledons was caused by translocation of
protoporphyrinogen from the roots.
It is also worth noting that in contrast to excised or intact roots, the cotyledons
accumulated mainly protochlorophyllide instead of protoporphyrin. Furthermore, it
was suspected that the acropetal translocation of ALA plus modulator resulted in
the accumulation of significant amounts of tetrapyrroles in the cucumber
hypocotyls. This was suggested by the rapid desiccation and death of the
hypocotyls, which preceded that of the cotyledons.
17.8.2 Basipetal (Downward) Translocation
Basipetal (downward) translocation, presumably via the phloem, was monitored by
following the accumulation of tetrapyrroles in the hypocotyl and roots when the
cotyledons of cucumber seedlings were sprayed with ALA and modulators. Tetra-
pyrrole accumulation, mainly Pchlide, was observed in the cotyledons and in the
hypocotyls. None was observed in the roots. This is turn suggested that the basipetal
translocation of TDPH was of limited range and did not proceed past the hypocotyl
in hypogenous seedlings (Smith and Rebeiz 1979 ).
Finally, it should be pointed out that all the above work was carried out on
seedlings growing in vermiculite. When similar experiments were carried out on
seedlings grown in soil, less dramatic results were observed, suggesting that the
effect of the soil environment on the availability of ALA and modulators to the root
system is significant (Rebeiz et al. 1991 ).