The Greening Process in Cress Seedlings. III. Age-Dependent Changes in the Capacity of the Tetrapyrrole Pathway

Accumulation of protochlorophyllide (Pchlide) was determined after incubation of etiolated seedlings of garden cress (Lcpidium sativum L.) with exogenous 5-aminolevulinate for 24 h. Young seedlings (start of incubation 0-12 h after imbibition) accumulated only moderate amounts (<0.2 nmol per seedling) of Pchlide. Maximum capacity of Pchlide accumulation (ca. 14 nmol per seedling) was reached at 30-36 h after imbibition. At 60-72 h after imbibition, the capacity of Pchlide formation from exogenous ALA was only 0.2-0.3 nmol per seedling. Uptake of ALA via roots was high in young and old seedlings. The carotenoid content and pattern did not change by ALA incubation. Precursors of Pchlide were also accumulated by ALA treatment. Predominant products were protoporphyrin and magnesium protoporphyrin (MgP) in 30 h old seedlings but magnesium protoporphyrin monomethylester (MgPMe) in 72 h old seedlings. The previously described interaction of Pchlide precursors with cab gene expression cannot be correlated with a single porphyrin. A possible correlation with the ratio MgPMe: MgP is discussed.