From the 1995 Amer. Soc. Hortic. Sci. meeting, Montréal

Thomas Bjorkman and Karen Pearson, Department of Horticultural Sciences, NYSAES, Cornell University, Geneva, NY 14456
Production of broccoli in areas where summer temperatures exceed 30C is difficult because the head may not form properly. The high temperature causes and unevenness in the head due to widely differing sizes of buds. The sensitive stage of development was determined for the early-maturing variety Galaxy by exposing it to 1 week at 36C at varying developmental stages, and subsequently analyzing the head structure. The injury is a cessation of bud enlargement during the high temperature exposure. There is no corresponding cessation of bud initiation at the apex. The pattern of injury is consistent with susceptibility over a relatively small range of bud development: even with a 1-week exposure, only about 1/3 of the buds will be affected. The plants most developmental stage at this sensitive period still appears vegetative, but the youngest leaves are just beginning to reorient as a consequence of the reduced stem elongation rate. The meristem is less than 1 mm wide, and scanning electron micrographs show floral primordia just forming, still subtended by leaf primordia. The injury is fully expressed when the head is first exposed (approx 10 mm wide), though it becomes more apparent as the head matures. The buds that were delayed in development by the high temperature developed into fertile flowers, albeit about a week late.

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