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Corn is like a giant wick, pulling moisture out of the soil. Some of that moisture escapes through the plant’s leaves and goes into the atmosphere. This is called transpiration. Water vapor also enters the air from lakes and other water surfaces, which is called evaporation. Together, the processes are known as evapotranspiration.
Mark Licht is an Extension cropping systems specialist at Iowa State University. He says the biggest factor in evapotranspiration values is air temperature. The higher the temperature, the higher plant demand for water.
"As water evaporates out of the leaves, it cools the plant down. Part of the reason that the demand is higher is because when the air temperature is higher, the air can hold more water vapor, so then that creates a very large gradient internally within the plant leaf and the atmosphere around the leaves," says Licht.
Other factors include relative humidity, and water available in the soil. A lack of soil moisture will reduce evapotranspiration rates. The corn will tell you when it’s stressed. The leaves start curling up.
"That’s showing that the plant’s not only under heat stress, but moisture stress because it’s not able to run through at a high rate of transpiration. So, as that plant shuts down the stomata, the rolling is essentially a cooling mechanism that also helps reduce the transpiration flow through the plant," he says. "When we see that type of an effect, that’s a good sign that we’re under a combination heat stress and moisture stress."
When the crops do a good job of transpiring, it can make already humid air feel even more sticky.