What phase of photosynthesis does the increase in pressure demonstrate?

This demonstrates the splitting of water into two hydrogen ions, electrons and an oxygen atom which occurs during the light dependent phase of photosynthesis. The oxygen atom immediately combines with a second oxygen atom to produce oxygen gas.

Was there any difference in the pressure change when the Elodea was exposed to light and dark? Why?

The higher the exposure to light the more oxygen bubbles produced by the Elodea. Light excites electrons from P680. These electrons are passed to a primary electron acceptor and then down an electron transport chain.The electrons removed from P680 are replaced when a water splitting enzyme extracts electrons from water and passes them to P680. The removal of electrons splits water into two hydrogen ions and an oxygen atom.The oxygen atom immediately combines with a second oxygen atom to produce oxygen gas. In summary, the greater the light intensity, the faster electrons are excited from P680 and the rate at which they must be replaced increases. As a result, water is split at a faster rate and more oxygen gas is released.

The excitation of electrons from P680 is dependent on light energy. If electrons are not removed from P680 they do not have to be replaced by the splitting of water. As a result, the production of oxygen gas resulting from the splitting of water would not occur in the dark.