Quantifying the Impact of Low-temperature Exposure on Cauliflower Head Initiation

Planning a harvest schedule for some cauliflower cultivars 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. The objective of this study was to determine the phenological stage of cauliflower at which the juvenile phase is completed and the crop begins to be sensitive to low temperatures. According to information available in the literature, a temperature of 10 °C seems to be the optimum temperature for this vernalisation process. In order to determine the end of the juvenile phase, 84 cauliflower seedlings (cultivar Casper) were produced in greenhouse and exposed to a temperature of 10 °C during 14 days in growth chambers beginning at the 3, 5, 7, 9, 11, and 13 leaf stages. In the greenhouse, the temperature was 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. Compared to the control treatment, exposure at the 3 leaf stage delayed head initiation and exposures at 5 to 13 leaf stages promoted head initiation. Based on these results, a second experiment was established to determine the minimum exposition time required for promoting head initiation. Again, 84 cauliflower seedlings were produced in greenhouse and exposed at the 9 leaf stage to a temperature of 10 °C during 2, 4, 6, 8, 10, and 12 days in growth chambers. The results suggest that an exposure of 4 days is sufficient to promote head initiation at 10 °C. An exposure of 2 days also accelerates the head initiation but the effect is smaller than the other exposures. 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 planting and harvest.