Heat Stress during Flowering in Summer Brassica

Temperatures greater than 27°C, in a growth cabinet, have resulted in floral sterility and yield loss in Brassica napus L. Maximum daily temperatures often exceed this in the major canola growing regions. Our objective was to examine the effects of heat stress during flowering on yield and yield components of B. napus , B. rapa L., and B. juncea (L.) Czernj. & Cosson. Cultivars of the three Brassica species were grown in a split‐plot design in the field for 3 yr at Ottawa, Canada. Three seeding dates were used each year to obtain different levels of heat stress during flowering. The number of flowers on the main raceme and the first and second branch racemes were counted at anthesis, from 10 plants per plot. At maturity, pods from the sampled plants were counted and seed numbers and weight determined. Two rows of the remainder of each plot was harvested for seed yield. A heat stress index was developed on the basis of the accumulation of daily maximum temperatures greater than a threshold temperature. The threshold temperature during flowering, which resulted in seed yield losses, was 29.5°C for all Brassica species. High mean maximum temperature during vegetative development resulted in a reduction in flower number for all Brassica species. Seed yield decreased as heat stress during flowering increased. The reduction in seed yield was primarily due to a reduction in flower number and in the number and size of the seeds produced per flower. Plant breeders should actively select for heat stress tolerance in future canola cultivars.