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When dark-grown leaves of Phaseolus vulgaris, Hordeum vulgare, Zea mays and Pisum sativum were irradiated for 3 sec at 2 degrees the first product of protochlorophyllide 650 conversion had an absorption maximum at 678 nm. This form was then converted in a dark reaction to chlorophyllide 684, the form generally observed and regarded as the in vivo product of the photoreaction. The dark conversion at 2 degrees was complete in 6 to 10 min in the various plants. The time course of the dark reaction was followed at 690 nm near the maximum of the difference spectrum for the conversion. There was a constant relationship between the initial amount of chlorophyllide 678 and the final amount of chlorophyllide 684. The rates of the dark reaction at 2 degrees varied 3-fold among the plants treated. The reaction was not first order. At 25 degrees the reaction followed at 690 nm was complete in 20 to 60 sec. Q(10)'s varied from 2.8 to 3.7 between 2 degrees and 25 degrees . Phytochrome absorbancy changes were shown to be too low to interfere with these measurements except in pea leaves. In a subsequent stage of greening newly regenerated protochlorophyllide went through the same sequence upon photoconversion. Chlorophyllide 678 probably corresponds to the product formed in vitro from the protochlorophyllide holochrome. The dark reaction appears to represent the first interaction between the photoconverted holochrome and other elements of the proplastid. The lack of this dark reaction could also account for the spectral properties of certain albino mutants.

A Short-lived Intermediate Form in the in Vivo Conversion of Protochlorophyllide 650 to Chlorophyllide 684