For the experiment, use a population of Fucus zygotes. For each of several groups, allow them to orient to gravity, but for each group of zygotes apply directional light from different directions, including ones in parallel with gravity and others from the opposite direction. If light is more important than gravity, rhizoid orientation will always follow light, not gravity, no matter the direction of the former in relation to that of the latter.
Work Step by Step
The key here is to design the experiment so that sometimes light and gravity will oppose one another--whichever is more important for rhizoid orientation will determine that orientation.