2018 ASHS Annual Conference

Expansion of broccoli production in the Eastern US requires adapted varieties. They have not been available because commercial breeders found adaptation too hard to breed for. The eastern environment is notable for greater variation in temperature and water, with warm and humid nights during the growing season. Traits for adaptation include temperature sensitivity of growth and reproductive development. Rapid, uniform but not excessive vegetative growth despite transient excesses of temperature and water, coordinated flower bud development through elongation and arrest at a small size, ability to shed rainwater, and resistance to purple sepals from sunburn or cold.

Some of the traits have been difficult to select for because heritability is low. There are two possibilities for the low heritability: the variation is largely due to environment, or traits are expressed only in specific environments where selection was not done. If the latter is true, breeding by selection has good promise. In both cases the main effect of genotype would be relatively low, but only in the latter case would genotype-by-environment interaction be large (as long as some of the environments produced genotype-dependent variation).

Breeding lines that showed promise an initial screen replaced in replicated trial consisting of 20 environments (five planting dates at each of four locations—Maine, New York, North and South Carolina). Four control entries were consistent in each of four years, with 8 to 13 entries changing annually as new hybrids were produced by breeders and whose promise was identified in screening. The traits were evaluated on a referenced scale with the raters at each site trained to use the scale in the same way.

A particularly difficult trait to improve has been temperature sensitive delay of floral development. This defect occurs when insufficient low temperature exposure temporarily arrests development of reproductive meristems and floral primordia, and causes the flower buds to vary in size at harvest maturity. We scored that trait as “bead uniformity”. In the four years, genotype explained 11, 7, 3 and 21% of the total variance. Genotype by environment accounted for 18, 20, 24, and 18.5% of the total variance. The greater contribution of the interaction indicates that the trait can be improved by selecting in the right environment. The comparable numbers for High Dome were lower, with G = 6, 8, 7, 17% and GxE = 15, 12, 13, 28%.