Biology Reference
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Table 4.3 Comparison of the rates of Chl(ide)
a
net synthesis by etiochloroplasts in vitro, with the
highest rates observable during greening in vivo
Δ
change after 2 h incubation
nmoles/
mole chl(ide) present
before incubation
Chlide
a
Chl
a
μ
Experiment Treatments
Chl(ide)
a
A
Etiochloroplasts were isolated from kinetin and
Gibberellic acid-pretreated cucumber
cotyledons as described in Methods and
incubated in the presence of 0.33 mM ALA
under a repetitive light dark regime that
consisted of red actinic light followed by 30 min
of darkness
2557.80
2172.96 4730.76
B
Cotyledons pretreated with kinetin and Gibberellic
acid were incubated in the presence of 0.33 mM
ALA under a repetitive light dark regime that
consisted of 2.5 ms of red actinic light followed
by 30 min of darkness
148.90
229.50
80.60
C
Cotyledons pretreated with kinetin and Gibberellic
acid were illuminated under 320
μ
Wcm
-2
of
cool white fluorescent light for 2 h
36.01
2109.88 2145.89
D
Cotyledons pretreated with water were illuminated
under 320
64.40
2327.37 2262.97
Wcm
-2
of cool white fluorescent
μ
light for 2 h
E
Etiolated cucumber seedlings were exposed to
50
35.17
2466.87 2502.06
Wcm
-2
of cool white fluorescent light for
30 min followed by 3 h of dark incubation
(15) to eliminate the lag phase in Chl(ide)
a
biosynthesis. The Cotyledons were excised with
hypocotyl hooks and were illuminated under
320
μ
Wcm
2
of white light for 2 h
μ
The Chls present in the acetone extracts, were extracted into hexane, while the Chlide remained in
the hexane-extracted acetone fraction as described in (Bazzaz and Rebeiz
1979
). The amounts of
Chl
a
and Chlide
a
were determined by spectrofluorometry as described in (Bazzaz and Rebeiz
1979
). The
change refers to the pigment content at the end of the incubation minus the pigment
content before incubation. Chl(ide) refers to the total amount of Chl + Chlide
a
Adapted from Daniell and Rebeiz (
1982b
)
Δ
The isolated etiochloroplasts approximately quadrupled their Chl(ide)
a
content
during the 2 h incubation under the forementioned light-dark regime (Daniell and
Rebeiz
1982b
). For comparison purposes, the Chl(ide)
a
net synthesis and accumu-
lation in excised cucumber cotyledons, which were greening at the highest rate
observable in nature is compared quantitatively with the in organello rate in
Table
4.3
. For such a comparison to be meaningful, we have reported the rates on
a unit Chl(ide)
a
present before incubation, which in effect normalized the biosyn-
thetic rates to the same number of plastids in both systems (Rebeiz et al.
1982
). It is
apparent from Table
4.3
, A, E that the isolated plastids accumulated Chl(ide)
a
at a
rate about twice as high as the highest rate of greening achievable In vivo.
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