Low-temperature Exposure at Early Foliar Stages Affects Both Cauliflower Head Initiation and Development Rate

Planning a harvest schedule for some cultivars of cauliflower is a complex task because they require exposure to temperatures below 15°C for the production of their inflorescences. However, it is not clearly understood when and how long this exposure at such low temperatures should occur during the course of their phenological development. A previous study showed that exposure at the 3 leaf stage delayed head initiation and exposures at 5 to 13 leaf stages promoted head initiation. The objective of this study was to determine the minimum required exposition time at temperatures between 2.5 and 15.0°C for promoting head initiation. In order to determine these minimum exposition times, cauliflower seedlings (cultivar ‘Casper’) were produced in greenhouse and exposed at the 9 leaf stage to temperature of 2.5, 5.0, 7.5, 10.0, 12.5, and 15.0°C during 2, 4, 6, 8, 10, and 12 days in growth chambers, and then returned to the greenhouse, in which the temperatures were set at 22°C during the day (16 hours) and at 18°C during the night (8 hours). The phenological stage of each plant was evaluated at least twice a week in the greenhouse and in growth chambers. Plant height and head diameter were measured once and twice a week, respectively. With respect to the optimum exposure time for each of the temperatures tested, the shorter periods do not appear optimal for earlier head initiation and faster head development rate of the ‘Casper’ cauliflower cultivar. Exposure times of 8 to 12 days at temperatures of 5.0, 7.5, and 10.0°C resulted in the earliest head initiations and fastest head development rates. However, regardless of the exposure temperature and duration at the 9 leaf stage for this cultivar, all treatments had a positive effect on head initiation and development rate. Such information will be integrated in a weather based dynamic phenology model in order to assist vegetable producers to improve their field planning of cauliflower transplantation and harvest.