Photodynamic Herbicides - Chlorophyll Biosynthesis and Technological Applications - page 351

Fig. 17.1 Room temperature absorption spectrum of (a) acetone extract of etiolated control and
etiolated ALA-treated seedlings and (b) green control and green ALA-treated seedlings. Cucum-
ber seeds were grown for 5 days either in darkness (etiolated seedlings) or under a 14 h light/10 h
dark photoperiod (green seedlings) at 28 C. Etiolated and green control seedlings were sprayed in
darkness under a green safelight with water: 0.1 % Tween 80 (99:1 v/v), adjusted to PH 3.5.
Treated etiolated and green seedlings were sprayed in darkness with 20 mM ALA dissolved in
water: 0.1 % Tween 80 (99:1 v/v) and similarly adjusted to PH 3.5. After spraying, the seedlings
were wrapped in aluminum foil to induce maximum penetration of the spray and were incubated in
darkness for a period of 17 h to allow for the biosynthesis and accumulation of tetrapyrroles. At the
end of dark incubation 3 g of tissue was homogenized under subdued laboratory light (about
10-foot candles) in 15 mL of acetone: 0.1 N NH 4 OH pH (90:10 v/v). The homogenate was
centrifuged at 18,000 rpm for 10 min at 1 C to separate the acetone extract from cell debris.
The acetonic supernatant containing the pigments was decanted and used for spectrophotometric
analysis (Reproduced from Rebeiz 1991 )

In 1982 the prevailing conventional wisdom stated that this was impossible.
It was believed that green plants that had accumulated all the Chl required for
photosynthesis did not need to synthesize more Chl and had lost their Chl-making
capabilities (Perkins and Roberts 1960 ; Virgin 1961 ; Wickliff and Aronoff 1963 ).
This dogma was reinforced by observations indicating that when etiolated plants
were treated with ALA and analyzed by absorption spectrophotometry, substantial
protochlorophyllide (Pchlide) accumulation was observed in the treated plants, over
and beyond control plants that were treated with solvent only (Fig. 17.1a ).
δ

-Aminolevulinic acid (ALA) is the precursor of all tetrapyrroles in living cells,
and etiolated plants are notorious for their vigorous Chl biosynthetic capabilities in
the light and their propensity for forming Pchlide when incubated with ALA in