In Angiosperms, the reduction of protochlorophyllide (Pchlide) to chlorophyllide 
(Chlide), a penultimate reaction of chlorophyll biosynthesis, is catalyzed by a 
photoenzyme Pchlide oxidoreductase (POR) and completely inhibited in darkness. 
This reaction plays also a regulatory role in plant morphogenesis. In the case 
of dark-grown Angiosperms, Pchlide is accumulated, mainly in the form of 
complexes with NADPH and POR but also as an unbound pigment. Etioplasts that 
develop in the place of chloroplasts in the dark contain a highly organized 
lipid structure termed prolamellar body (PLB), which is the main site of 
accumulation of the ternary Pchlide:POR:NADPH complexes. An illumination 
triggers the photoreduction of Pchlide molecules which are bound to the ternary 
complexes. This is followed by a set of biochemical reactions and structural 
changes leading to Chl synthesis that can be monitored with fluorescence 
techniques. This chapter describes the application of low-temperature 
fluorescence spectroscopy and fluorescence lifetime measurements for monitoring 
the Pchlide to Chlide conversion in isolated prolamellar bodies. These 
techniques enable the analysis of heterogeneity of accumulated pigments: Pchlide 
and Chlide that reflect the different organization of pigment-protein complexes.