High temperature arrest of inflorescence development in broccoli (Brassica oleracea var. italica L.)

atures near optimal for vegetative growth. This lack of heat tolerance restricts broccoli production to maritime High temperature causes unevenly-sized flower buds climates lacking extreme temperatures. The alteration in on broccoli inflorescences. This deformity limits prothe inflorescence structure has been described primarily duction of broccoli to areas where summer temperin terms of the marketability of the commercial product atures rarely exceed 30 °C. The stage of development (Heather et al., 1992). sensitive to heat was determined by exposing plants Inflorescence development in broccoli involves prolifof ‘Galaxy’ broccoli at varying developmental stages eration of the meristem to make a highly ramified inflorto 35 °C day temperature for 1 week, and subsequently escence. Developmental arrest occurs during floral bud analysing the head structure. During the high temperdevelopment and internode elongation (Carr and Irish, ature exposure, the development of certain flower 1997). In contrast, cauliflower has greater meristem prolifbuds was arrested. There was no corresponding ceseration and is arrested earlier, after the first florallysation of bud initiation at the apex. No injury resulted determined gene expression but before any bud developif heat was applied before the reproductive induction, ment (Carr and Irish, 1997). Exposure to high tempernor was there injury to differentiated flower buds. ature during a time window several weeks before harvest Meristems were affected only if heat was applied maturity disrupts broccoli flower development (Heather during inflorescence production or the floral initiation et al., 1992), giving parts of the inflorescence the superfiprocess. Shorter heat exposures produced little injury, cial resemblance to cauliflower. This paper describes the and longer exposures were lethal. The plant’s developdevelopment of the broccoli inflorescence, the disruption ment at this sensitive period still appeared vegetative of that development that is brought on by excessive externally, but the youngest leaves had just begun to temperatures and the stage of development that is affected reorientate as a consequence of the reduced stem by heat. elongation rate. The meristem was less than 1 mm wide, and floral primordia were just forming, still subtended by leaf primordia. The injury was fully Materials and methods expressed by the time the head was first exposed (approximately 5–10 mm wide), though it became more Plant raising apparent as the head matured. The buds that were Seeds of cv. Galaxy were sown into a peat-vermiculite growing delayed in development by the high temperature medium in ‘Speedling No. 100A’ modular trays, containing 930 cells m−2, each having a volume of 23 ml. Trays were placed in developed into normal flowers. a glasshouse at day/night temperatures of 21/15 °C. Seedlings were fertilized 5 times weekly with liquid fertilizer containing