COLUMBIA, Mo. – Extreme heat affects plants differently than humans.

With triple-digit temperatures this summer, grain crop growers should understand how heat affects plants, says University of Missouri Extension agronomist Bill Wiebold.

First, human concepts such as “heat index” or “feels like” do not apply to plants, Wiebold says. People and plants feel and react differently to heat.

Leaf temperature matters more to plants than air temperature. Leaves absorb light to build sugars and other things needed for life and yield, he says. Plants use little of the available light energy. Extra energy causes leaf temperature to rise.

Changing water from liquid to vapor (evaporation) uses a sizable amount of energy and causes a cooling effect. Conduction occurs when the warm leaf surface returns energy to the air that touches it if the air temperature is lower than the leaf temperature. In convection, cooler air moves closer to the surface of the leaf and displaces warmer air.

Temperature directly affects yield potential, Wiebold says. Enzymes (proteins) control the chemical reactions needed for plants to live. The rates of these reactions increase with temperature. For example, plant growth and weight gain are greater at 80 F than at 50 F. The three-dimensional shapes of plant enzymes can warp or change at high temperatures.

An extreme example of how temperature affects protein is seen in the frying of an egg, says Wiebold. Heat causes the egg protein to change shape and solidify. Although less dramatic, high leaf temperatures affect the shape of plant enzymes, making them not work as well.

Agronomists consider 86 F the optimum temperature for corn and soybean growth. Temperatures above 86 F slow important reactions, including those involved in photosynthesis, reducing yield potential.

During the day, leaf temperatures are often higher than air temperatures, especially on bright, sunny days with little wind, says Wiebold. With good moisture supplies, evaporation happens quickly enough to keep leaf temperatures near air temperature. However, with limited moisture, water may not evaporate fast enough to cool the leaf. This causes leaf temperature to rise. Conduction and convection are not effective at driving heat away from the leaf when air temperatures are high.

Plants respond to the stress of high leaf temperatures in several ways. Leaves of grass plants such as corn roll into a cylinder to reduce the amount of leaf surface exposed to light. Leaves also tilt upward. Broad-leaved plants such as soybean do not roll. Instead, they turn their flat leaves to be parallel with incoming sunlight.

If heat stress continues, soybean and other broadleaf plants flip their leaves so that lighter-colored bottom surfaces face upward to reflect light. This reduces leaf temperature and limits exposure to sunlight. Reducing leaf temperature also reduces water evaporation.

The direct effect of high temperature on crop yields is small in most years, Wiebold says. But when temperatures top 95 F, as they often have this summer, corn and soybean yields may drop even in the few areas of Missouri where there was adequate precipitation.

One less obvious effect of high temperatures occurs in photosynthesis and respiration, he says. In the plant world, photosynthesis is “income” and respiration an “expense.” The difference, net photosynthesis, is “net income.” Within reason, high net photosynthesis high yield.

Plants need some respiration to burn sugars to make energy for many life processes. However, some respiration is wasted as it burns or oxidizes sugars that could be stored in seeds as yield. High temperatures fuel respiration more than photosynthesis and reduce the plant’s “net income.” This is especially true at night, when no photosynthesis occurs. Warm nights can lower yield without any visible effects on the plants.

The term “feels like” also has no meaning for plants. High daytime humidity can benefit plants because lower evaporation reduces water stress. High nighttime humidity slows the rate at which air temperatures fall. It is not uncommon for temperatures to remain above 80 F on summer nights if humidity is high (dew point above 70 F). So, although plants do not “feel” a high heat index, the slow rate of temperature decline during high-humidity nights shows through increased respiration.

“It is difficult to separate the effects of high heat from the effects of water stress,” says Wiebold. “Often these two stresses occur together and magnify the effects from each other. But high temperatures can reduce yield even if plants show no signs of water stress.”

Originally published July 27, 2022.