Temperatures at or below 50 degrees will hinder phosphorous absorption, phosphorous is a very important primary nutrient especially during flowering as you probably know. Plants generally turn a dark purple as a result. Providing more water soluble phosphorous may help.
It would help even more to warm the roots. I don't know what kind of room you're growing in, or the materials or other general details of your set up but here are some things you should consider:
If your planting containers are just sitting on a concrete or otherwise cold ground/floor, raise them off of the floor by some means.
When watering use warm water if this is possible, and if your setup involves a reservoir you could use a fish-tank heater to keep the circulating water warm.
The tops of your plants are at 82 degrees, so you must have an HID in there emitting heat and this heat could be utilized. It is obviously warming your space to somewhat a significant extent if it is 3 lonely degrees outside and yet your minimum temp in there is something below 50. The problem is that your light is way up high above the plants, and heat rises and so escapes and dissipates quickly in the cold. You need to divert as much of that heat as is possible downwards to the floor and roots, allowing it to rise up from there instead.
If you have an air-cooled reflector (with one or more flanges to attach ducting) then there would be a simple solution. You would just need such a reflector, some ducting, clamps and an in-line duct fan. Even without an air cooled reflector you might be able to think up a way to do this, but an AC hood makes it a lot easier.
The idea is that you're pulling hot air from the bulb (or wherever you can) and the ducting would go from there and nearly touch the floor. The inline fan is attached to the floor-end of the ducting, and blows the air downward towards the floor/planting containers.
A 4-inch inline duct fan would work fine with the corresponding 4-inch ducting, just be sure you're pulling the air from the reflector. If done correctly the heat should circulate much better. It wouldn't be so hot up near the light or so cold down on the floor.
