Figure 2.

Testing the rigour in which chilling units (CUs) need to be defined to develop a thermal time model of Arabidopsis seed dormancy and germination responses to chilling. The accumulation of CUs, expressed as estimated degree hours (see §5) during a typical autumn in York. The effect of varying the upper threshold below which the temperature is defined as chilling between 8 and 20°C on the rate and timing of CU accumulation is presented. The number of CUs shown in figure 1 to break the dormancy of low dormancy accessions set at 20°C (5 days at 4°C; green line), for maximum dormancy loss (14 days at 4°C; blue line) and re-introduction of secondary dormancy (28 days at 4°C; red line) are shown, in order to estimate the date these may occur in a wild setting.