Apple trees require a certain number of chilling hours, that is, cool periods of temperatures between 35°F to 45°F (2°C to 7°C) which causes them to go dormant, in order to bear fruit and be strong trees.
Each variety has its own chilling requirements, some are low chill like Granny Smith, requiring only 500 to 600 chilling hours. Others like Empire are high chill and require up to 800 chilling hours.
Apples that mature in climates that have some cold nights down in the 40's (F) just before and during harvest time will be better colored, have better keeping quality and have higher soluble solids than apples that mature in hot weather.
Note: The chilling requirement in apple trees is located in the bud scales and does not relate to the roots. Roots have no chilling requirement.