Chilling stress can be measured in various plants' processes, such as germination, growth, photosynthesis, fruit-set, yield, and fruit quality. As is the case for heat stress, the primary transducer of chilling stress is at the cellular and organelle level, with the resulting repercussion at higher levels of plant organization. Tolerance to chilling temperatures at germination and emergence is probably the most investigated topic in chilling stress and tolerance. Freezing stress, in its widest meaning, involves complex stresses and "strains" that occur when plants are subjected to subzero temperatures. Tolerance to freezing stress embraces terms such as "winter hardiness", "winter survival", "frost resistance", etc. The multiplicity of such terms is symptomatic of the array of stresses and plant responses involved when plants are subjected to subzero temperatures. Plant evolution is apparently responsible for the fact that most plants that are adapted to survive freezing enter the cold period in a relatively dormant state, with a minimum amount of active tissues.