How is the greenhouse effect related to photosynthesis?

The only relation there is between the two is carbon dioxide (CO2).

Carbon dioxide is a greenhouse gas. Too much of it in the atmosphere causes the 'greenhouse effect' whereby the earth warms.

In the photosynthesis process CO2 is captured from the atmosphere and, with the help of sunlight (and water in the case of plants) turned into organic compounds. A waste product of this process is oxygen.