Biology Reference
In-Depth Information
9.3.4 Contribution of t-LW-Pchlide a (PORA)
and t-SW-Pchlide a (PORB) to Photoperiodic Greening
Under natural photoperiodic greening conditions, Pchlide
a
accumulates during the
dark cycles of the photoperiod and contributes to Chl
a
biosynthesis and accumula-
tion at the onset of light (i.e. at dawn) (Cohen et al.
1977
). Furthermore, Pchlide
a
is
always present in green tissues during the light phases of the photoperiod (Abd-El-
Mageed et al.
1977
; Carey et al.
1985
; Cohen et al.
1977
). During photoperiodic
greening Pchlide
a
(E650 F655), also known as t-LW-Pchlide
a
H [(E450 F657)],
and its apoprotein, PORA, are transient species that peak during the 7th dark cycle
and become undetectable by the 11th dark cycle (Cohen and Rebeiz
1978
). On the
other hand SW Pchl(ide)
a
H species and their apoproteins, PORB and/or PORC
are present throughout the photoperiodic greening process (Cohen and Rebeiz
1978
). In other words although t-LW-Pchlide
a
H (E450 F657) contributes to
prolamellar body reformation and Chl
a
formation during the first few dark cycles,
it is SW t-Pchlides
a
Hs and PORB/PORC that prevail during the light cycles of
photoperiodic greening, and contribute the bulk of Chl
a
accumulation under
normal field conditions (Cohen and Rebeiz
1978
).
The significance of the accumulation patterns of t-SW Pchl(ide)
a
Hs, and
PORB/C during photoperiodic greening, to the Chl
a
biosynthetic process, rests
upon the direct photoconvertibility of t-SW Pchlide
a
Hs to Chlide
a
without prior
conversion to t-LW-Pchl(ide)
a
(E450 F657) and subsequent photoreduction by
PORA. This was reported to be the case by Cohen and Rebeiz (
1978
). However, the
photoconversion of t-SW-Pchlide
a
H, was slower than that of t-LW-Pchlide
a
H
(E450 F657) which is catalyzed by PORA.
In conclusion, since etiolation and prolamellar body formation are not abnormal
phenomena, but are part of the natural succession of the dark (night) and light
(daylight) cycles during photoperiodic greening (Cohen and Rebeiz
1978
; Rebeiz
and Rebeiz
1986
), it is very plausible for PORA and PORB/C, to play definite but
distinct roles during Chl
a
biosynthesis in darkness and in the light (see below).
9.4 Heterogeneity of the Photoconversion
of the Pchlide
a
Chromophore to Chlide
a
9.4.1 Photoconversion of DV Pchlide a to DV Chlorophyllide
a in DDV-LDV-LDDV Plants via Routes 1 and 8
The biosynthesis of DV Chlide
a
from DV Pchlide
a
was first reported in isolated
cucumber cotyledons induced to accumulate massive amounts of DV Pchlide
a
(Duggan and Rebeiz
1982a
). In Fig.
9.2
, the biosynthesis of DV Chlide
a
from DV
Pchlide
a
in DDV-LDV-LDDV plants is depicted to occur in two different thylakoid
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