If you just put up a glass roof, the space underneath it is sunlit as if there weren't a roof there, as it should be. But if you put walls around that space to make the plants look green and healthy instead of the sickly grey colour sims' outdoor plants wear for most of the year, suddenly no light comes through the roof and only the areas very close to windows have any sunlight.
A glass roof is a glass roof. The light should come through that roof as if the roof weren't there, or only very slightly dimmed if it's tinted glass. Whether there are walls around the sides of the space under the roof shouldn't change how much light comes through the roof.
