Surging concentrations of carbon in the atmosphere, caused largely by burning fossil fuels, have produced potent changes in the way plants grow — from increasing their sugar content to depleting essential nutrients like zinc. Experts fear the degradation of Earth’s food supply will cause an epidemic of hidden hunger, in which even people who consume enough calories won’t get the nutrients they need to thrive.
There is another a fundamental conceptual flaw in these studies.
Nutrient density in grains generally decreases as kernal size increases. This is due to the ratio of starch storage versus other nutrients. Elevated CO2 levels trigger more starch formation and larger kernal size in C3 plants. This is what all these studies are detecting (if they find anything).
There is a wide variation of genetically controlled kernal size found in all C3 grains. For example healthy wheat can be anywhere from 6,000 kernals/lb to 20,000 kernals/lb. Environmental conditions can also change kernal size by more than 40% for any cultivar.
What happens when a cultivar produces extra large kernals due to environmental conditions? The stalk falls over AKA lodging. Lodging decreases overall yield (molds, shriveled kernals, harvesting equipment loss etc.). Any variety that consistent lodges is discarded by growers. So rising CO2 levels will be compensated for by farmers planting smaller kernal sized cultivars.
Then there is the “quality” issues. Larger kernals sizes are often deficient in critical traits for their end use (protein, etc). This makes these varieties the lowest grade (cheapest price) and growers move away from them rapidly.