Initiation of early summer cauliflowers in response to temperature

Two experiments were conducted with the early summer cauliflower cvs Perfection and Gypsy. The first was in controlled environment cabinets with nine temperatures ranging from 6.3 to 22.8 degrees C. The second was in the field with three planting dates and plants grown with and without fleece cover. The experiment in controlled environments enabled a model to be developed which predicted changes in apex development through both the juvenile and curd induction phases. Both the end of juvenility and curd initiation were defined in terms of apex diameter. For cv. Perfection these were respectively 0.25 and 0.55 mm while with cv. Gypsy they were respectively 0.27 and 0.51 mm. The number of leaves produced at the end of juvenility was very different in the two experiments. When applied to the field temperatures from planting, the model predicted the end of juvenility early and therefore estimates of curd initiation were inaccurate. However, when the model was started, with the recorded apex diameter exceeding that at the end of juvenility, it predicted the day of curd initiation well. The temperatures representing the lower limit, optimum and upper limit of curd induction, were estimated to be 2.2, 9.4 and 24 degrees C for cv. Perfection and 2.9, 13.0 and 23.1 degrees C for cv. Gypsy. The results suggest that juvenility is not adequately described by number of leaves and varies considerably with growing conditions. They indicate that, under optimum conditions, curd induction only takes about six days and that the critical difference between crops is the duration of juvenility.