The results of both scientists led to the conclusion that light is necessary for plants to produce energy and release oxygen. Ingenhousz discovered that light is needed to produce oxygen, and it is said that he discovered photosynthesis because of that, and Priestley discovered that a plant produces the substance in air that is required for burning (oxygen).
It is interesting to note that with the discoveries of another scientist, Van Helmont, the three main components of photosynthesis were known, as Van Helmont realized that water was needed for the plant to gain mass (and water + carbon dioxide are used to produce energy + oxygen).