eration of photoactive Pchlide occurring during Chlide liberation [29]. If the spectroscopic properties of
C670–675are well defined, its biochemical state remains to be determined precisely and it is not clear
whether C670–675corresponds to a free pigment or to a pigment-protein complex. In this case, the protein
moiety cannot be LPOR because it has been reused for the regeneration of photoactive Pchlide. Almost
nothing is known about the regeneration process. Analyses of excitation spectra have indicated that
P642–649is an intermediate [43].
The remaining part of C676–688is transformed to another spectral form of Chlide (C684–696). From the
biochemical point of view, C684–696is similar to C676–688but contains NADPH instead of NADP[42].
These events are summarized in Figure 4A, which displays the so-called Pchlide-Chlide cycle. Similar re-
sults have been obtained with Spirodela oligorrhiza, a plant that does not develop etioplasts when culti-
vated in darkness [44].B. Chl Formation in Plants Cultivated for a Long Time in the Dark
(i.e., Etiolated Leaves)During dark growth, proplastids develop to etioplasts, which are characterized by the presence of a pro-
lamellar body (PLB) and some single perforated membranes called prothylakoids (reviewed in Ref. 3).
Simultaneously with the differentiation of proplastids to etioplasts, photoactive Pchlide is accumulated
[26,45] into the PLB, where LPOR is by far the most abundant protein [46,47]. Etiolated leaves contain
the same spectral forms of Pchlide as the young leaves, i.e., P638,650–657and P628–633, but the ratio of pho-
toactive to nonphotoactive Pchlide is in favor of the photoactive form [29,48].
The first product of the photoreduction of photoactive Pchlide in etiolated leaves, C678–690, has
slightly different spectral properties than found in young leaves (see Sec. III.A). This minor difference in
the position of the absorbance and fluorescence maxima (77 K) may reflect a slightly different environ-
ment of the pigment.
The absorbance and fluorescence kinetics of the Pchlide photoreduction are monoexponential when
the process is studied on the second time scale. The rate constants of the kinetics are identical in young
and old leaves, indicating that the photoreduction mechanism is identical [45]. The formation of C678–690
is preceded by the formation of several nonfluorescent intermediates (reviewed in Ref. 49), whose chem-
ical structure remains unknown.
In etiolated leaves, only a minor part of C678–690is transformed to C670–675[29,50]. The major part
is transformed to C684–696, which is an efficient fluorescence quencher at room temperature [51].
C684–696formation, which occurs readily in the dark after the initial phototransformation step, can re-
vert to C678–690under illumination [52,53] (Figure 4B). C684–696is the photoreceptor for this transfor-CHLOROPHYLL BIOSYNTHESIS DURING PLANT GREENING 269Figure 4 The Pchlide-Chlide cycle in leaves developing (A) under natural conditions and (B) under condi-
tions similar to those found in the field.