Initiation of early summer cauliflowers in response to temperature

Two experiments were conducted with the early summer cauliflower cvs Perfec tion 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 fiel d 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 c urd induction phases. Both the end of juvenility and curd initiation were d efined in terms of apex diameter. For cv. Perfection these were respectivel y 0.25 and 0.55 mm while with cv. Gypsy they were respectively 0.27 and 0.5 1 mm. The number of leaves produced at the end of juvenility was very diffe rent in the two experiments. When applied to the field temperatures from pl anting, the model predicted the end of juvenility early and therefore estim ates of curd initiation were inaccurate. However, when the model was starte d, with the recorded apex diameter exceeding that at the end of juvenility, it predicted the day of curd initiation well. The temperatures representin g the lower limit, optimum and upper limit of curd induction, were estimate d 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 adequa tely described by number of leaves and varies considerably with growing con ditions. 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.