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I seem to remember learning the pecise reason why plants both respond to sunlight and to gravity during my Freshman year Biology class in high school. I remember that it is due to a single plant hormone that stunts the elongation of cells. I think it begins with an 'a'.

Aren't the mechanics of this well-known?

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Auxin, I did a science project on this in 5th or 6th grade. My father helped me rig up a centrifuge-like apparatus that would spin seeds at what was supposed to simulate greater than earth gravity. I remember doing research (reading) about auxins and coming to the conclusion that higher gravity would stunt plant height.

And what happened to the seeds on your science project? :)

For the sunlight part: sunlight inhibits growth on the sun-facing side of the plant, and so the flower faces towards the sun.

Except this works for buried seeds and seeds germinated in the dark.

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