Here’s how high temperatures alone increase water stress in corn…
The 2012 growing season featured a generational drought. That fall, combines that normally couldn’t make a full round shelling corn in a half-mile field made six rounds before dumping. However, it wasn’t dry weather alone that doomed the corn crop. Abnormally high temperatures cooked it too.
High temperatures and water stress go hand in hand, explains Mark Jeschke, agronomy manager with Pioneer, based in Johnston, Iowa. Some of those relationships are obvious, but others are not as well understood.
Jeschke points out five factors that tie high temperatures and water stress together in corn:
1. Transpiration. First, a definition: Transpiration is the rate at which water is released from plant leaves into the environment through evaporation. It’s an invisible process that can vary widely depending on many factors, including weather variables, soil type and soil conditions.
“Higher temperatures cause transpiration rate in corn to increase,” Jeschke says. “That places a greater demand on soil water supply. It can accelerate the onset of drought stress.”
2. Water demand. What many people didn’t realize in 2012 was how much higher temperatures bump up water demand within corn plants. Couple that with soils with reduced water-holding capacity, like those with gravel at 3-foot depths or less, and the results were disastrous.
“Raising temperatures from 80 degrees F to 95 degrees F causes water demand to double,” Jeschke says. Think of it this way: If a block of corn needs a semitanker of water to get through the day at 80 degrees, it needs two tankers at 95 degrees.
3. Leaf stomates. Here is another definition. Leaf stomata or stomates are tiny openings on the surface of leaves that allow gas exchanges between outside air and the leaf’s internal air system.
“Corn plants respond to water stress by closing their stomates,” Jeschke explains. “That helps preserve water, but also reduces carbon dioxide intake. Carbon dioxide is needed in photosynthesis.”
Closing stomates is a natural reaction by corn plants, like rolling of leaves. When leaves roll, the goal is to conserve moisture within the plant.
4. Pollination and photosynthesis. Reduced pollination and photosynthesis are direct impacts of high temperatures on corn, Jeschke says. However, the greater impact occurs through the interaction of heat and water stress.
Interactions can include, but are not limited to, delayed silking. It can be so delayed that pollen is gone by the time silks emerge, resulting in barren cobs or ears with limited numbers of kernels.
5. Vapor pressure deficit. Higher temperatures in a cornfield create a vapor pressure deficit between the saturated interior of the leaf and the ambient air.
VPD is a measurement that combines relative humidity and temperature into a single variable to describe the evaporative potential in the atmosphere. Air space inside the plant is fully saturated with water at 100% relative humidity, Jeschke explains. Water vapor moves from higher concentration to lower concentration. So, if relative humidity outside is less than 100%, VPD pulls water out of the plant.
The greater the VPD, the faster water is pulled out and evaporates. Temperature is the big driver, Jeschke says. VPD increases exponentially as temperature rises, even if relative humidity stays steady.
How corn reacts to high heat
You see corn leaves roll. But what else happens “behind the scenes”? Jeschke says corn plants mostly react to VPD, which captures the evaporative potential of the atmosphere.
Here are changes that happen in cornfields on high-heat days that you may not see right away:
Stomates on leaves close. This preserves water inside plants until it cools off and evaporative demand is lower.
Photosynthesis slows down. Less carbon dioxide enters plants. Rate of carbon fixation during photosynthesis slows down during this part of the day.
Growth is reduced. Scientific experiments have proven that when daily highs reach the mid-90s, there is reduced photosynthesis and slower growth, both due to high VPD.
Activity peaks in morning. On high VPD days, photosynthesis and stomatal activity peak during late morning, then drop off in the afternoon as temps and VPD climb.
Irrigation doesn’t eliminate issues. Even in irrigated plots, photosynthesis drops off in the afternoon, although not as much as in dryland plots. This explains why farmers often report that in super-hot summers, even irrigated corn doesn’t yield as well as expected. Jeschke notes that damage from extreme heat can be partially offset by adding water, but not eliminated.
Higher temps hurt more. One researcher compared reducing precipitation for a month by 20% vs. increasing temperature by 2 degrees C, or 3.6 degrees F, and found that the temperature increase impacted water stress more than having less water.
In fact, total rainfall for the season only weakly correlates with yield, Jeschke says. It’s another indicator that water demand matters as much or more than water supply.
Source: https://www.farmprogress.com/corn/5-things-that-happen-when-corn-gets-too-hot