ha, its funny that you're asking this question.

Just on the science aspect...you're (as you've already stated) pitting gravitropism against phototropism. I did a similar experiment with pea plants growing in agar. ultimately gravity won, and the plants grew upwards and spread out along the surface of the agar. I can only assume they would have grown up further if the dishes had been placed on their side instead of directly upside down.

so, what happened first was that the seeds were sprouted till about an inch high. I then inverted the dish (called a magenta box, even though its clear) and blocked out the light except coming from below. so the plants had to choose between light and gravity.

what happened was that they stressed out, their growth was much delayed, they turned upwards and grew away from the light, got kind of chlorotic, and the abaxial (lower side) leaf surface was facing the light...not the greatest thing, the palisade mesophyll under the adaxial (upper) side of the leaf is most efficient for photosynthesis, lots of chlorophyll all in one spot. just look at the color difference from the top of the leaf to the bottom.

the lowest bend of their stem before they turned back upwards sprouted roots, which traveled downwards towards the light. they simply arent able to ignore the gravitropic response. since the light still got to them even though they grew upwards...it wasnt as determining as gravity.

hope this helped at all.


oh, also, obviously if your HPS is under your plants...you're gonna have to have it covered with a pane of glass, you cant have stuff falling on the light.