The oxidation of the apple surface is due to the oxygen present in air. As you have verified in the end of the experiment, the apple without lemon juice was darker. On the other hand, the apple with lemon juice didn't have so much oxidation effects. This fact is due to the presence of ascorbic acid (vitamin C) in the lemon juice. The ascorbic acid reacts with oxygen contained in air, not allowing it to attack the apple. Therefore, when you want to preserve a fruit (apple, pear, banana), you should wet the exposed surface in lemon juice. The lemon juice can be considered as a conservative.