|MadSci Network: Botany|
You are correct that angiosperms require light for protochlorophyllide reductase. Non-flowering plants, cyanobacteria, and gymnosperms have a light independent protochlorophyllide reduction. While it may be possible that there may be angiosperms that are exceptions and have a light independent protochlorophyllide reduction, it is probably unlikely. It seems more likely that lemon peel transmitted enough light for chlorophyll formation. It may have happened long before you put it in your refrigerator. Green fruits and vegetables can remain green for many weeks to a few months in a refrigerator so the chlorophyll in your lemon seedling could have formed before you bought the lemon. If you wanted to test if a lemon can synthesize chlorophyll in the dark, you could place an established lemon plant in the dark for several days and see if new leaves that develop are green or nongreen. Reference Evolution and Ultrafast Spectroscopic Studies of Protochlorophyllide (Oxido) Reductase, An Enzyme of Chlorophyll Biosynthesis
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