Changing Ratios of Phototransformable Protochlorophyll and Protochlorophyllide of Bean Seedlings Developing in the Dark

Protochlorophyll (Pchl) and protochlorophyllide (Pchlide) are at comparable levels in 2-day-old (young) etiolated bean leaves (Phaseolus vulgaris L. var. Red Kidney). During subsequent development in the dark, both pigments increase, but the rate of Pchlide increase is greater than that of Pchl, leading to the commonly observed predominance of Pchlide beyond 7 days (old leaves). Both protopigments are phototransformable to their respective chlorophyll(ide) photoproducts throughout dark development. The rate of protopigment regeneration in young leaves after illumination is rapid and displays no lag, whereas this process in old leaves begins slowly and achieves only about one-fifth the rate of younger leaves. The rate of chlorophyllide esterification is also faster in the younger tissue. Since the proplastid-related properties of young bean leaves are quite similar to those of Euglena, young leaves and Euglena may represent an evolutionarily primitive case compared with older bean leaves which contain etioplasts. Since Euglena and young beans green perfectly well when exposed to light, the extensive modifications associated with prolonged dark growth do not seem to be obligatory for plastid development. The properties of older beans are viewed as being the consequence of prolonged etiolation which may provide a faster rate of plastid development and appearance of photosynthesis as the plant nears the limits of its stored reserves.