Any temperature above the absolute 54.5-F would be enough to keep the tree growing. Pummelos, such as the variety Valentine, require high heat requirements to mature. The heat units from temperatures such a 60 to 70 would take the fruit a very very long time to mature, and at those temperatures the fruit would produce very little soluble solids (sugars), and if it ever turned a mature coloration, the fruit would probably be very bitter (sour).