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

A ccum ulation o f protochlorophyllide (Pchlide) was determ ined after incubation o f etiolated seedlings o f garden cress (Lepidium sativum L.) w ith exogenous 5-am inolevulinate for 24 h. Y oung seedlings (start o f incubation 0 1 2 h after im bibition) accum ulated only m oderate am ounts (< 0 .2 nmol per seedling) o f Pchlide. M axim um capacity o f Pchlide accum ulation (ca. 14 nmol per seedling) was reached at 3 0 -3 6 h after im bibition. A t 6 0 -7 2 h after im bibi­ tion, the capacity o f Pchlide form ation from exogenous A LA was only 0 .2 -0 .3 nm ol per seed­ ling. U ptake o f ALA via roots was high in young and old seedlings. The carotenoid content and pattern did no t change by ALA incubation . Precursors o f Pchlide were also accum ulated by A LA treatm ent. Predom inant p roducts were p ro toporphyrin and m agnesium p ro to p o r­ phyrin (M gP) in 30 h old seedlings bu t m agnesium pro toporphyrin m onom ethylester (M gPM e) in 72 h old seedlings. The previously described interaction o f Pchlide precursors with cab gene expression cannot be correlated with a single porphyrin . A possible correlation with the ratio M gP M e: M gP is discussed.