A wood conditioner is normally used on a wood that has hard and soft grains like pine or douglas fir. The idea is to seal the soft part of the wood so when you apply stain the color will be more uniform. Poplar doesn't have that characteristic. The texture of the wood is pretty uniform in texture. The biggest problem with poplar is the natural wood color can run from white to green to black. When I finish something with poplar with mixed colors like that I normally have two different sprayers with two different colors of stain. I use a darker stain on the white woods and a lighter stain on the green wood and when wiped off together it is usually near the same color. Sometimes I have to come back with a toner to make further adjustments. If the piece has the black streaks you either have to bleach it with two part wood bleach or live with it.