I'm picturing a world with a permanent, dark, overcast sky, but I'm wondering how dark it can be before photosynthesis is no longer an option.
It looks like there are a good number of plants that have good shade tolerance, but I can't find any real information on the bottom limit. There also appear to be, a few plants that don't need to photosynthesize at all.
The closest analog I can think of is the oceanic zones, photic and aphotic.
Photic being the top layer of a body of water where most, if not all, of your photosynthesis is happening. Aphotic being too deep for enough light to penetrate.
Apparently the photic zone:
extends from the surface down to a depth where light intensity falls to one percent of that at the surface
So everything else being equal, temp, atmosphere, etc. could a thriving planetary ecosystem exist at say slightly above 1% normal daylight?
Failing near total darkness, how dark can it be?