Answer:
All plants and algae need light to photosynthesise. Plants compete for light by growing quickly to reach it and often shade other plants with their leaves.
there are 3 limiting factors in photosythesis
1 light
2 CO2
3 temperature
so when one of these is removed from a plant the photosynthesis rate stops or decreases then stops leading to a plant to obviously stop growing. so when light is taken away, and there is shade, the limiting factor is therefore light and so photosythesis stops.
hope that makes sense
:))