XVI International Botanical Congess

High temperature injury to broccoli, an unevenness in the heads, is due to widely arying bud size. One wk at 36°C caused injury only when begun while inflorescence primordia were forming and the meristem was less than 1 mm wide. The pattern of injury is consistent with high temperature reversing the phase switch from inflorescence to flower. Broccoli is defined by a uniform phase switch that it is not generally considered temperature sensitive, as it is in cauliflower. High temperature may reverse what has been considered a constitutive induction. Morphological changes allow non-destructive identification of the phase switch occurs. Screening by exposure to high temperature during the phase switch injured in sensitive lines severely, and revealed considerable genetic variation in sensitivity among varieties.