Re: Why is my germinated lemon seed green?

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