The AOG (accelerated optimal growth) concept for rapidly producing
deciduous forest tree seedlings used continuous fluorescent light with
plants grown in greenhouses. Here's one of the AOG articles available
online:

Wood, B.W. and Hanover, J.W. 1981. Accelerating the growth of black
walnut seedlings. Tree Planters' Notes 32(2):35-38.
http://rngr.net/Publications/tpn/32/32_2_35_39.pdf/file

The light level AOG used was low, about 2.5% to 5% of full summer
sunlight. It was meant to extend the photoperiod, not to increase
photosynthesis.

David R. Hershey