How do you test water?

How can we test for the necessity of water for photosynthesis?

• It is impossible to test for the need of water using a plant directly as the plant needs water to survive and will die without. My question is therefore: "How can we test for the need of water for creation of starch by photosynthesis?"

• Answer:

  Actually U cannot test da water need in photosynthesis. Because da two rules in all experiments to test da agents of photosynthesis r, That da plant should be destarched and Da experiment should be controlled. But in the case of water u cannot control water..