stomata opens

1. in day time – for gaseous exchange
2. in day time – no gaseous exchange
3. in night time only – for gaseous exchange
4. in night time – no gaseous exchange

Carbon dioxide, a key reactant in photosynthesis, is present in the atmosphere at a concentration of about 390 ppm (as of December 2011). Most plants require the stomata to be open during daytime. The problem is that the air spaces in the leaf are saturated with water vapour, which exits the leaf through the stomata (this is known astranspiration). Therefore, plants cannot gain carbon dioxide without simultaneously losing water vapour.