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William J. Wiebold
University of Missouri
Plant Science & Technology
(573) 882-0621

Effects of Hot Temperatures on Missouri Grain Crops

William J. Wiebold
University of Missouri
(573) 882-0621

July 15, 2022

minute read

Weather forecasting organizations may differ somewhat, but this coming week will likely have several days for which maximum temperature approaches or exceeds triple digits. It’s been that type of summer. These high temperatures have several direct effects on plants as well as indirectly through aggravating drought stress.

It is important to remember that plants react to temperature differently than humans. Concepts like heat index or feel like temperature do not relate to plants. But there are effects from high humidity on plants that I’ll explain later in article.

Leaf temperature is more important to plants than air temperature. Leaves function as solar collectors, that is, they are designed to absorb light energy. They do this in order to build sugars and produce other products necessary for life (and yield). However, very little of the light energy is actually used to do this work (photosynthesis). Light energy not used for photosynthesis causes leaf temperature to rise.

Plants dissipate heat through water evaporation from cell surfaces, convection, and conduction. Changing liquid water to water vapor requires substantial energy and this energy loss causes the cooling effect. Conduction means that the warm leaf surface gives energy to the air touching it if the air temperature is less than the leaf temperature. Convection means that cooler air is moved closer to the leaf surface and displaces warmer air. These three methods of heat dissipation are very much interrelated. Without them, leaf temperature would quickly rise to the point where plants could not survive.

Temperature is an important parameter as we try to understand the effects of stress on yield potential. The chemical reactions necessary for the lives of plants are controlled by enzymes (proteins). The rates of these chemical reactions increase with temperature. For example, plant growth and weight gain are greater at 80° than at 50°. These enzymes have three-dimensional shapes and can warp (change shape) at high temperatures.

An extreme example of temperature affecting protein is the frying of an egg. Heat causes the egg protein to change its shape and become solid.

six egg frying

Figure 1 Temperature increases cause changes to the proteins in eggs and result in eggs becoming more solid. Although less dramatic, changes in temperatures can also affect plant proteins. Image courtesy of - zatvornik via Shutterstock.com

The effect of temperature on plant enzymes isn't nearly that dramatic, but warm leaf temperatures affect the shapes of plant enzymes. When the shapes of the enzymes change, they no longer work as well. In other words, the reaction rate decreases. That is why 86° is often given as the optimum temperature for corn and soybean growth. Although the optimum is fairly flat for about 10°, temperatures above the optimum slow many of the important reactions including those involved in photosynthesis. This reduces yield potential.

During the day it is not uncommon for leaf temperature to be higher than air temperature, especially on bright sunny days with little wind. With good moisture supply, evaporation will be fast enough to keep leaf temperatures fairly close to air temperature. However, with limited moisture, water evaporation may not be fast enough to cool the leaf and leaf temperature will rise. If air temperature is hot, say above 95°, conduction and convection are not effective at dissipating heat. The weather parameters predicted for next week will cause leaf temperatures to rise well above optimum for plant growth and yield production.

close up of corn leaf curling due to stress

Figure 2 Corn leaves can roll up tightly, which reduces the amount of sunlight hitting leaf surfaces.

Leaf temperature is a function of absorbing light for photosynthesis. Our crop plants have several mechanisms to reduce the amount of sunlight energy hitting their surfaces if the plants are stress by temperature and/or drought. Leaves of grass plants, such as corn, roll into a cylinder (Figure 2). This reduces leaf surface area for light absorption and tilts leaves upward so it’s closer to parallel to incoming light. Broad-leafed plants, such as soybean, do not roll but orient their flat leaves so that they are parallel with the incoming sunlight. If stress continues, they flip leaves so that their lighter colored bottom surfaces face upward reflecting more light (Figure 3). Although we see these responses during water stress, it is actually an attempt to reduce sunlight absorption, and thus, leaf temperature. The reason to reduce leaf temperature is to reduce water evaporation. The two factors are interrelated.

This direct effect from high temperature on crop yields are probably small in most years. But when temperatures top 95° as they have too often this summer, corn and soybean yields may be reduced even in those few areas of Missouri where precipitation has been close to adequate.

soybean leaves flipped over to reduce stress

Figure 3 Soybean leaves can be flipped so that the bottom of the leaf faces upward. The lighter color of the bottom surface reflects more sunglight.

Unfortunately, high temperatures have other effects on plants. One almost hidden effect from increased temperature is the differential effects it has on photosynthesis and respiration. Photosynthesis is “income” for the plant world and respiration is an “expense”. The difference between the two, net photosynthesis, is “net income”. Within reason, high amounts of net photosynthesis often translate into high yield.

Some respiration is essential, just as some expenses are essential. Respiration oxidizes (“burns”) sugars to produce energy that is needed for many of the life processes. However, some respiration is wasteful because it burns away or oxidizes sugars that could have been stored in seeds as yield. Hot temperatures stimulate respiration more than photosynthesis and reduce the plant’s net income. This is particularly true during night when there is no photosynthesis. Warm night temperatures can decrease yield without showing any visible effects on the plants.

As stated earlier, heat index has no meaning for plants. High humidity during the day can actually be beneficial to plants because water evaporation is reduced and this reduces water stress. But high humidity at night also slows the rate by which temperatures cool It is not uncommon for temperature to remain above 80° during summer nights if humidity is high (dew point above 70 degrees). So, although plants do not “feel” a high heat index, they are affected by the slow temperature decline during nights of high humidity through increased respiration.

It is difficult to separate the effects of high temperature from the effects of water stress. Often these two stresses occur together and magnify the effects from each other. But high temperatures can reduce yield even if plants exhibit no symptoms of water stress.

Header image of drought-stressed corn field taken by Dr. Wiebold near Jefferson City, Missouri.

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REVISED: July 15, 2022