Breeding Vegetables Adapted to High Temperatures: A Case Study with Broccoli

Broccoli (Brassica oleracea L. Italica Group) has emerged in the last few decades as one of the most economically important vegetables grown in the United States.  Evidence indicates that head development of most broccoli varieties is arrested by temperatures exceeding 30 C, temperatures that are nearly optimal for vegetative growth.  This general observation tends to limit U.S. broccoli production to climates lacking high temperature fluctuations with the vast majority of the crop produced in California.  However, increasing interest in growing broccoli in eastern environments has stimulated efforts to produce broccoli under conditions that are hotter and less optimal for quality head production.  For about 15 years the USDA-ARS broccoli breeding and genetics project at the U.S. Vegetable Laboratory (USVL) in Charleston, South Carolina has conducted a breeding program to select broccoli adapted to relatively high temperatures of the southeastern U.S.  The program was initiated in the early 1990’s using a few commercial hybrids and other noncommercial broccolis obtained from other sources.  All breeding and selection trials have been conducted from May through August in Charleston with head production occurring when temperatures typically exceed 30 C during daytime hours and remain high during the night.  The program has employed a conventional pedigree breeding approach in which different inbreds are crossed to one another to form hybrids that are subsequently selfed to create segregating populations.  Through several rounds of selection and recombination, lines increasingly adapted to summer conditions of the Southeast have been identified.  The resulting broccoli lines perform very similar regardless of the temperature conditions they are grown in, making them different than most broccoli bred for adaptation to western environments.  The USVL broccoli might be appropriately described as “heat resistant” or “heat insensitive”.  Future improvement of broccoli for high temperature adaptation must recognize the role that vernalization plays in broccoli, the likelihood that hybrid vigor will be important at high temperatures, and that the trait must be considered complex.  Current efforts are underway to determine heritability of adaptation to the high temperature selection environment and to identify important genetic factors that condition the trait.