(First post on StackExchange, hello world!)
The project that I'm currently working on involves a Binary Planet system, both with life, although I'm only focusing on one. This planet has a climate similar to Earth, aside from the temperature regions due to no seasons. (which I've already kept in mind for plant life) Also since it's a binary planet, half of the world would have light reflected by the sister planet at night, while half would be extremely dark, possibly even pitch black.
Would a day length of 80 hours be long enough to see a noticeable effect on plant life? Since plants get energy from photosynthesis, I would assume that longer nights could hinder them somewhat, but I'm not sure if that would be sufficiently different enough to change how said plants would function.
From another question I found, someone said plants would need larger sugar-stores to make up for the longer period of no sunlight, but aside from that most questions are geared towards several week/ month long days rather than slightly longer ones.