Direct Heat Stress Effects on Corn
Kernel Set is Critical
Corn yield reduction from heat stress can be associated with reductions in both source and sink capacity. Impact on yield depends on the growth stage of the corn at the time stress occurs. The most critical period for corn yield determination is the roughly 4- to 5-week window bracketing silking when kernel number is set. Approximately 85% of total grain yield is related to the total number of kernels produced per acre (Otegui et al., 1995). Any stress during this time that reduces the number of kernels a plant is able to set will negatively impact yield. Even if the stress is temporary and the plant recovers, the damage to yield will be done.
Heat stress during this timeframe can reduce yield in a couple of ways: by inhibiting successful pollination and by reducing net photosynthesis, which can lead to an increase in kernel abortion. Both mechanisms can reduce the number of kernels on the ear. Heat stress can continue to impact yield through grain fill by reducing kernel weight, much like any other form of stress that inhibits photosynthetic carbon assimilation. Stalk quality can also be impacted if the stress forces the plant to increase its reliance on remobilized carbohydrates to complete grain fill.
Heat Stress Effects on Pollination
Temperatures above 90 °F (32 °C) have the potential to negatively impact pollination. Prolonged exposure to temperatures above 90 °F (32 °C) has been shown to dramatically reduce pollen germination (Herrero and Johnson, 1980). Temperatures above 95 °F (35 °C) depresses pollen production and can desiccate exposed silks, especially when accompanied by low relative humidity (Hoegemeyer, 2011). High temperatures and low humidity can similarly desiccate pollen grains once they are released from the anthers. Temperatures over 100 °F (38 °C) can kill pollen (Nielsen, 2020).
Corn tassel branches showing anthers extruded.
Peak pollen shed usually occurs in mid-morning. A second period of pollen can occur in late afternoon or evening as temperatures cool.
Under cool, cloudy conditions, pollen shed may continue throughout most of the day.
However, research suggests that yield loss due to heat stress effects on pollination is relatively rare in North America (Lobell et al., 2013). Daily maximum temperatures in the Corn Belt commonly reach the mid or upper 90s but pollination is usually not severely affected. Pollen shed typically occurs during early to mid-morning hours before temperatures climb to potentially harmful levels. The daily high temperature would likely need to reach well above 100 °F for temperatures to reach dangerous levels during mid-morning when most pollen shed occurs. For example, July 25, 2012 was the hottest day of a notoriously hot summer in central Iowa. Maximum temperature in Des Moines hit 106 °F (41 °C) at 5:00 pm, but temperatures between 9:00 and 10:00 am were only 90-95 °F (30-35 °C), just barely reaching the threshold for pollen and silk desiccation (Figure 2). Furthermore, pollination occurs over a period of several days, providing multiple opportunities for viable pollen to reach exposed silks.
Figure 2. Temperature over the course of the day on July 25, 2012 showing timing of peak pollination and maximum daily temperature.
Leaf Temperature vs. Air Temperature
Temperature effects on crop physiology are often characterized based on ambient air temperature; however, the temperature that photosynthesizing cells inside corn leaves actually experience can differ somewhat from that of the surrounding air. Leaves often have a lower temperature than the air around them because the evaporation of water transpired through the leaves cools them. The drier the air, the cooler the leaf of a well-watered plant will be compared to the surrounding air.
This cooling effect is illustrated by comparing the surface temperatures of living leaf tissue vs. dead leaf tissue shown in Figure 3. The temperatures of a live and dead leaf adjacent to each other in the upper canopy of a corn field differed by more than 7 °F. The temperature of the living leaf was 94.4 °F (34 °C), a few degrees above the ambient air temperature of 91 °F (33 °C), while the dead leaf was well-above ambient temperature at 102 °F (39 °C).
Temperature can also vary depending on the level of sun exposure and the position of the leaf relative to the angle of the incoming sunlight. Figure 4 shows a partially shaded leaf in the corn canopy, with the shading from other leaves creating a banded appearance in the infrared imagery. The temperature of a shaded portion of the leaf was 87.3 °F (31 °C), a few degrees below air temperature, while a portion of the leaf a few inches away exposed to direct sunlight was over 7 °F hotter. Shaded and exposed areas will shift over the course of the day, so a given spot on a leaf may experience a range of different temperatures even if the surrounding air temperature is relatively constant.
Figure 3. A live leaf and a dead leaf in the upper canopy. The surface temperature of the live leaf is 94 °F (34 °C), while the temperature of the dead leaf is 102 °F (39 °C).
Figure 4. Leaf temperature differences due to partial shading in the canopy. A shaded portion of the leaf is 87 °F (31 °C), while a sunlit portion only a few inches away is nearly 95 °F (35 °C).
Heat Stress Effects on Photosynthesis
Heat stress can also impact corn yield through reduced net photosynthesis. Decreased net photosynthesis can cause large reductions in yield if it occurs during the critical period for kernel number determination. When stress occurs during this interval, the corn plant typically starts to abort kernels at thetip of the ear and moves toward the base of the ear until it reaches a point that the remaining viable kernels can be sustained by the plant.
Temperature dependent biological reactions, such as photosynthesis and respiration, generally have an optimum temperature (Topt) for operation (Figure 5). Photosynthesis and respiration are slow at cooler temperatures, increase as the temperature increases, and decline and eventually cease when the temperature gets too high. The optimum temperature for respiration is greater than that for photosynthesis. Net photosynthesis is a measure of carbon assimilated through photosynthesis (sugar produced) minus carbon expended through respiration (sugar consumed). Net photosynthesis has a Topt lower than that of gross photosynthesis due to the offsetting effect of the higher respiration rate (Figure 5).
Figure 5. Generalized model of temperature effects on rates of gross photosynthesis, respiration, and net photosynthesis. Net photosynthesis in corn is optimized at 86 °F. (Figure adapted from Hopkins, 1999).
Plant species with the C4 photosynthetic pathway such as corn generally have a higher optimum temperature for photosynthesis than C3 plants. In C3 plants, net photosynthesis is reduced at higher temperatures due to an increase in photorespiration caused by higher oxygenase activity of ribulose-1,5-bisphosphate carboxylase-oxygenase, (rubisco), an enzyme involved in the first major step of photosynthetic carbon fixation. As temperature increases, the ratio of dissolved O2/CO2 and the specificity of rubisco for O2 increase, favoring oxygenase activity. C4 plants possess a mechanism to eliminate this inefficiency by locally increasing the concentration of CO2 available to rubisco enzymes and, as such, are not constrained by temperature in the same way.
Reduced net photosynthesis in corn under heat stress has also been shown to be associated with rubisco activity, but it is due to the inactivation of the enzyme at high temperatures. A daytime temperature of 86 °F (30 °C) is ideal for corn growth (Miedema et al., 1987). At temperatures above this level, net photosynthesis declines due to the loss of rubisco activation (Crafts-Brandner and Salvucci, 2002).
The degree to which net photosynthesis is reduced at high temperatures can depend on how quickly temperature increases. The faster the increase, the greater the reduction in photosynthesis. Crafts-Brandner and Salvucci (2002) found that a rapid increase to 113 °F (45 °C) reduced net photosynthesis by 95%, but a gradual increase to the same level reduced it by only 50%.
The level of solar radiation has also been shown to play a role in heat stress effects on corn by influencing the optimum temperature for net photosynthesis. Under light-limited conditions, the optimum temperature shifts lower due to the fact that respiration continues to increase with higher temperatures, whereas gross photosynthesis does not increase due to light limitation (Rainguez, 1979).
Figure 6. Key temperature thresholds for heat stress effects on corn pollination and growth.
1Crafts-Brandner and Salvucci (2002), 2Nielsen (2020), 3Hoegemeyer (2011), 4Waqas et al. (2021), 5Miedema et al. (1987).