Scientific American says "Future harvests of wheat, soybeans and corn could drop by 22 to 49 percent, mostly due to water stress" - U.S. Crop Harvests Could Suffer with Climate Change.
According to their estimates, corn and soybean plants can lose 5 percent of their harvest for every single day that is recorded above 30 C. Such crop losses could have huge repercussions for domestic food security and — given that the United States is one of the largest crop exporters in the world — affect prices in the international market.
There are multiple ways that higher temperatures could affect crop growth, and most of them come down to water stress, said Joshua Elliott, a research scientist with the University of Chicago and a co-author of the study.
Evaporation rates shoot up on hotter days, reducing the amount of moisture in the soil that's available to the plants. Moreover, plants tend to open their stomata — small pores on their leaves — to transpire water when temperatures increase, creating an additional source of stress. Certain studies have also suggested that high temperatures during a plant's flowering period could actually lead to a “sterilization” effect.