The plant will continue to convert the N to sugars (carbs.) as long as the roots are continuing to grow. The roots will grow as long as the ground is not frozen... you know that already.
As I understand it, the problem you have is TIME. The colder it is the slower the roots grow. The less N will be taken into the plant and less N already in the plant will have time to be converted. The plant will continue to convert N already absorbed by the roots when the ground thaws (at a very slow rate).
So the answer is... 33 degrees. Just remember, although the roots are still growing, they are growing very slow. Therefore, the amount of N uptake is significantly reduced. So, a lot of the N is being lost to leaching or runoff before it has time to be taken into the plant when app. is applied during extended cold periods.
Someone correct me if I am wrong.
Dave