Well, it relates by how much water the plant sucks up and how much water is oxidized to make oxygen which is release throught the plants stomatas.
Here is the equation for what happens to water during photosynthesis:
H2O--> 1/2 O2 + 2H+ + 2e-
The plant needs the H+'s from the water to make ATP and the plant needs the electrons given off by oxidizing water to go in the electron transport chain.
Oxidizing is when a molecule loses electrons. H2O lost electrons to create O2
