I don't know what finally happen to this pulsed led test,
But I found this:
http://www.geocities.ws/sash_elias/files/thesis1.pdf
Many thing are interesting in his work such as saying Pulsed light never did better at any frequency than continuous light.
But It also sugest there is no difference between 12 to 120Hz in growth with similar iradiation.
Then I thought why do we use Driver then ?!
A simple bridge rectifier like
this one (chose randomly ^^ but seem way to strong for our need) should drive the led with a frequency of 100 or 120Hz that shouldn't bother the plants... Then voltage would be high, and I wouldn't want to use a transfo but increasing the number of led could do it isn't ?
Maybe we could reduce the cost of our lamps this way.