freezing temps

Okay so I am on my 2nd crop, AK-48 from Nirvana, and this is the first time I've grown in the winter. So I already noticed a problem with my room, I built it to handle the Texas heat and not the freezing cold. My flower room got to 50 degrees last night and my clone/veg room got to 63 degrees! I have space heaters in both rooms trying to heat everything up and I finally got a control on the temps at 78 degrees. My question is how cold can the plants handle till they die? My clones are looking very stressed and droopy, one of them has even started turning yellowish. I am also 3 weeks into flowering and those plants look fine, but what happens to the buds in the colder temps? Will I end up having to flower longer or does the cold stall the budding process? I'm just trying to find out the effects of cold temps on the plants. Thank you in advance for any tips and advice.

Here are some pics to give you examples with what I'm dealing with. This is my 3rd batch of clones I've done and the only thing that is different is the cold

if u can get it 78 in day ,should be ok no worries, at least nodes will stay close. biggest prob with flowering is too hot and too hum