You can use this as a generality: Most tropical plants are seeing impaired photosynthesizing ability as temperatures fall much below 60*, and by the time they hit 50* you can probably consider their ability to carry on photosynthesis is pretty well nil. From that perspective, you can consider your houseplants are in decline when, and as long as they are, exposed to temperatures below 55*. That, because plants using more energy than they are producing = decline.
Chill injury is funny. It can occur at temperatures as high as 50*, if the temperature drops fast enough - say from 80-55* over the course of a few hours, or during exposure to sudden chill (carrying an unprotected plant to the car when it's 25*). If the temperature drops slowly, perhaps dropping from 70* to 35* over a period of a week, the plant might be able to resist that type of cold with no apparent (to the eye) injury, though we already know about the photosynthesizing thing .....
Chill injury usually manifests itself in symptoms that look much like freeze injury. As phenol compounds leak from cells affected by cold, tissues most often turn black or dark brown - sometimes wet lesions of slimy tissue appear.
Al
