By using heavy oxygen (oxygen-18), researchers showed that the label only ended up in carbohydrates if the label was put in the water which plants received-- not when it was added to carbon dioxide. This shows that the oxygens in the products of photosynthesis come from water.
Work Step by Step
This is an example of how unusual isotopes of natural elements (radioactive or heavy isotopes, usually) because they are different from most of the natural form of the element, can be used to trace the process of biochemical reactions. It is important to see, here, that this work proves the important role of water as a reactant in photosynthesis.