Chilling requirement

A chilling unit in agriculture is a metric of a plant's exposure to chilling temperatures. Chilling temperatures extend from freezing point to, depending on the model, or even . One chilling unit, in the simplest models, is equal to one hour's exposure to the chilling temperature; these units are summed up for a whole season. Advanced models assign different weights to different temperature bands. Contrary to popular belief, lack of chilling hours does not usually kill a plant, nor prevent a dormant plant from eventually leafing out. However, it may add stress that can be one factor in a plant’s demise; as such, a previously injured plant may not survive long after a warm winter. Fruit trees that do not meet chilling requirements in a given season will not leaf or flower at normal times or for normal durations, and may only produce a few or no fruit. Stone fruit trees and certain other plants of temperate climate develop next year's buds in the summer. In the autumn the buds become dormant, and the switch to proper, healthy dormancy is triggered by a certain minimum exposure to chilling temperatures. Lack of such exposure results in delayed and substandard foliation, flowering and fruiting. ==Requirements==

According to Fishman, chilling in trees acts in two stages. The first is reversible: chilling helps to build up the precursor to dormancy, but the process can be easily reversed with a rise in temperature. After the level of precursor reaches a certain threshold, dormancy becomes irreversible and will not be affected by short-term warm temperature peaks. Peach cultivars in Texas range in their requirements from 100 chilling units (Florida Grande cultivar, zoned for low chill regions) to 1,000 units (Surecrop, zoned for high chill regions). Planting a low-chilling cultivar in a high-chill region risks loss of a year's harvest when an early bloom is hit by a spring frost. Rest-breaking agents (e.g. hydrogen cyanamide, trade name BudPro or Dormex), applied in spring, can partially mitigate the effects of insufficient chilling. BudPro can substitute for up to 300 hours of chilling, but an excessive spraying and timing error can easily damage the buds. Other products such as Dormex use stabilizing compounds. Chilling of orange trees has two effects. First, it increases production of carotenoids and decreases chlorophyll content of the fruit, improving their appearance and, ultimately, their market value. Second, the "quasi-dormancy" experienced by orange trees triggers concentrated flowering in spring, as opposed to more or less uniform round-the-year flowering and fruiting in warmer climates. Biennial plants like cabbage, sugar beet, celery and carrots need chilling to develop second-year flowering buds. Excessive chilling in the early stages of a sugar beet seedling, on the contrary, may trigger undesired growth of a flowering stem (bolting) in its first year. This phenomenon has been offset by breeding sugar beet cultivars with a higher minimum chilling threshold. Such cultivars can be seeded earlier than normal without the risk of bolting. ==Models==

All models require hourly recording of temperatures. ==References==