Modelling development of broccoli (Brassica oleracea L-var. italica) from transplanting to head initiation

A model is suggested to predict head initiation in broccoli. Head initiation is defined as the time when the apex diameter reaches 0.6 mm and predictions are made from observations of air temperature. The model is constructed from three years of field experiments with three cultivars and four plantings per year. No juvenile phase after transplanting could be estimated and the model consists only of a head induction phase starting at transplanting. The rate of head induction is modelled by a piecewise linear temperature response function with a base, optimum, and maximum temperature. At optimum temperature the duration of the head induction phase is shortest. The estimates for base, optimum, and maximum temperature are 9.9 degrees C, 16.1 degrees C and 22.3 degrees C, respectively. The estimates for cultivar dependent thermal time requirements are 49 day-degrees, 55 day-degrees, and 38 day-degrees for cvs Caravel, Shogun and Emperor, respectively. The three cultivars were anticipated to have the same cardinal temperatures for head induction. The model of the head induction phases can account for 68% of the variation in the observed durations from transplanting to head initiation. A model of leaf appearance is used to standardize transplant size to four visible leaves at planting. The leaf appearance rate is described by a linear relationship to number of leaves and air temperature and the model can account for 89% of the variation in time from transplanting to appearance of a certain number of leaves. The temperature response of leaf appearance rate had a base temperature of about 2 to 3 degrees C, depending on cultivar, and showed no indications of having a temperature optimum below 20 degrees C. The leaf appearance rate at 15 degrees C range from 0.2 leaves per day for plants with four visible leaves to 0.4 leaves per day for plants with 14 visible leaves.