Fig. 2. The vernalization responses of the Morgan–Mercer–Flodin function (Eq. [2]) and the three-stage linear functions (linear function) used by Xue (2000), based on Wang and Engel (1998), for two winter wheat cultivars, Arapahoe and Karl 92. Observed data are from a controlled growth chamber experiment and represent the fraction of full vernalization: 0 = unvernalized plants and 1 = fully vernalized plants. VD, effective vernalization days.