Water and temperature affect many crop processes simultaneously ranging from crop growth, development, photosynthesis, respiration, water transpiration, grain fill, and nutrient cycling. The magnitude of high temperature or drought stress depends also on the stage of the crop. Another factor that should be considered is how high minimum temperatures are.
High nighttime temperature affects yield in a couple different ways. First, high nighttime temperatures result in more heat unit accumulation. Over time this results in the speed up of developmental time. What this really means is if high nighttime temperatures occur during reproductive grain fill, then the grain fill period will be shortened.
Second, high nighttime temperature are result in high respiration rates at night when photosynthesis is not active. The result is the plant sugars are used for energy rather than growth. During vegetative growth, this means less biomass production and less stem elongation. During reproductive stages this results in less plant sugars available for grain fill and subsequently fewer and/or lighter kernels.
What does this mean for 2017 growing season? At the present time, most of the high temperatures have occurred during the vegetative stages. Many soybean field planted in 30-inch rows have yet to close canopy. In general the corn crop does not have the height of past years. This past week and into the coming week higher temperatures during the night will take a toll on the number and size of corn kernels.
(Source – http://www.farms.com/news/heat-effects-on-corn-and-soybean-crops-125388.aspx)