It is funny that everyone refers to Helson et al. when dismissing special plant bulbs.

Original paper clearly states some interesting differences in plant (tomatoes to be exact) phenotype when grown under different bulbs. They compared standard daylight fluorescent, Gro-Lux, and supplemented both with incandescent bulbs.

Their original statement was whether observed small increase in growth was justifiable due to higher prices of plant bulbs and not that all the bulbs are same.

Regards.

Vladimir.