The answer is to add a 0 hours control.
In every experiment, the control group is necessary. That way, effects of all variables except the independent one will be minimized. The independent variable is variable that is not affected in the experiment. In this example, the amount of light. The dependent variable is called dependent because it depends on the independent variables. In this example, the dependent variable is the average volume of oxygen (photosynthesis).
But to minimize effects on variables, it is necessary to include the 0 hours group, so it can be compared to the other groups.
