When would green plants carry out photosynthesis-only during at night, continuously, the day, only or never?
The process of photosynthesis takes place continuously. The Light reaction when sunlight is available, and the dark reaction all of the time.
The process of Photosynthesis is composed of two distinct processes. The Light Reaction which takes place when light energy is available, and the Dark Reaction (Calvin Cycle) which takes place continuously.
Here is a simplified explanation of the two processes.
The Light Reaction takes place in the Grana of the Chloroplast when packets of UV light called photons are absorbed by the chlorophyll pigments. This energy is used to split water molecules into Hydrogen atoms that pick up electrons
The Dark Reaction (aka Calvin Cycle) takes place in the Stroma of the Chloroplast. This reaction takes Carbon Dioxide