Stomata open in light or dark cycle?

In most plants the Krebs cycle takes place during the day when the stomata are open. The stomata then close during the night time when it is cold. However the reverse is true of plants that grow in very hot climates and they close their stomata during the day because it is too hot and they would loose a bunch of water.

So when does a marijuana plant open it's stomata in light or dark cycles. For those of you going the question basically is " When is the best time to run CO2, light or dark cycle?"

Carbon Dioxide (CO2): Marijuana plants use CO2 when the light is on, so you will have to provide them with a constant fresh supply. In a room with no air circulation, plants will use the available carbon dioxide in a few hours.