We reached out to Alberta’s Provincial Oilseed Specialist Murray for an answer.
Murray Hartman’s response:
The relationship of soil temperatures to air temperatures has been fairly well studied. The soil in temperate climates warms up in spring and early summer (soil is generally cooler than daytime highs), and then cools in late summer and fall. The temperature changes in soil lags behind air. The canopy closure of crops also slows the soil warming process. There have been studies on the impact of warm to hot soil on germination (emergence heat blast), crop growth rates and disease, root/ shoot ratios, and on nutrient uptake. The plant roots do sense temperature and make adjustments through hormones to change architecture and uptake rates.
The greenhouse and growth chamber research on high temperatures causing flower blast in canola does have the limitation of small pots that would warm up the rooting media quicker than the situation in a field. So this is one reason results from greenhouse experiments cannot be precisely extrapolated to field conditions. The may be an indirect response of plants sensing high root zone temperature to the overall hormonal control of abortion, and this would be hard to separate from air temperature in trials without pots designed to have separate temperature control from the air. Obviously this has not been well studied.
In the field in western Canada, soil temperatures do not become abnormally warm unless they are also dry because water evaporation cools the soil and wet / moist soil takes more heat to warm up. So a situation where the soil becomes dry and hot is probably more of a drought or dry stress, which would affect flower abortion through hormonal balances.
So in brief, I think flower blast is mainly caused by high air temperatures during a short heat spell, but high soil temperature may be a factor together with dry soil in hot drought conditions.
Source : Albertacanola