Yes and no. The Solenoid is there to stop the flow of Co2 at night. If you just simply turn off your Reactor (assuming it has a powerhead or other stirring motor) then you are going to still be wasting your Co2 because it will just bubble away. You could just simply turn your regulator off every night by hand, but I bet that would get old quick.
If I understand it correctly, plants won't use the Co2 at night with out light. They will use O2 instead. I haven't set up reactor yet because I am actually waiting on two solenoid valves. Once I have them I will hook them between the regulator and the bubble counter to tell how much is actually getting to my reactor. I have a powerhead churning up the Co2 bubbles inside the reactor. At night I have the main lights go out, and plan to hook reactor and the solenoid to the same power strip as the lights. If I get more time I will hook up a T fitting and the other Solenoid valve so that the reactor will mix in Air instead of Co2. The valve for air will be hooked to the same strip as my Moon lights, wich come on when the lights to out.
If I remember it correctly from Biology class:
Photosynthesis needs light, H20 and CO2 to make glucose
Respiration takes glucose and O2 to break it down into energy
Hence, CO2 while the lights are on, O2 when they go off.