Chloroplast Biogenesis

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Abstract
A study of greening in cucumber (Cucumis sativus L.) cotyledons grown under a light(14 h)-dark(10 h) photoperiodic regime was undertaken. The pools of protoporphyrin IX, Mg-protoporphyrin IX monoester, protochlorophyllide and protochlorophyllide ester were determined spectrofluorometrically. Chlorophyll a and b were monitored spectrophotometrically. Pigments were extracted during the 3rd h of each light period and at the end of each subsequent dark period during the 1st 7 growth cycles. Protoporphyrin IX did not accumulate during greening. Mg-protoporphyrin IX monoester and longer wavelength metalloporphyrins accumulated during the light cycles and disappeared in the dark. Their disappearance was accompanied by the accumulation of protochlorophyll. Higher levels of protochlorophyll were observed in the dark than in the light, and the greatest accumulation occurred during the 3rd and 4th dark cycles. Protochlorophyllide was present in 3- to 10-fold excess over protochlorophyllide ester; it was detectable during the period of net chlorophyll accumulation as well as afterward. Protochlorophyllide ester was observable only during the 1st 4 photoperiodic cycles, suggesting that it was a metabolic intermediate only during the early stages of chlorophyll accumulation. Between the 3rd and 4th growth cycles, a rapid increase in area and fresh weight per cotyledon began. This was accompanied by a 250-fold increase in the level of chlorophyll a + b during the 3 subsequent growth cycles. No lag period in the accumulation of chlorophyll b was observed, and at all stages of greening, the chlorophyll a/b ratio was approximately 3.