No. 16 Article 3/July 22, 2011

High Temperatures and Crops

The dome of heat that has settled over the Midwest this past week promises to bring high day and night temperatures, with relief currently predicted to be some time away, and perhaps modest at that. It has been some time--2005 in some parts of the state, even earlier in others--since we've had temperatures stay this high for this long.

The crop report shows that by July 17, more than 60% of the Illinois corn crop had silked. This is about the same as the 5-year average, so even with the modestly late planting, warm weather has helped the crop catch up. Growing degree-day accumulations since May 1 exceed the average by 100 or so. That's not a large excess, but it has accumulated mostly during the past month. We would rather not have GDD accumulations higher than average during July; because July is already the warmest month, above-average accumulations mean unusually high temperatures.

How high is too high for crops? Afternoon temperatures in the mid-90s are not a problem for corn and soybean plants if they have enough soil water available. In experiments, plant temperatures have been raised to 110 or higher without doing direct damage to photosynthetic capacity. The level required to damage leaves depends on the temperature the leaf has experienced before, but it generally takes temperatures above 100 in field-grown plants.

Night temperatures have also been higher than normal. Because daytime temperatures are typically at or above the 86-degree cutoff for GDD calculations, high night temperatures are the direct cause of higher-than-normal GDD accumulations. High night temperatures mean larger losses of sugars to respiration at night, and so less sugar available to fuel crop growth. We don't know precisely what percentage of the sugars in the plant might be lost at different night temperatures, but we certainly expect higher losses at higher temperatures. This is a drag on growth rates because the plant has to produce extra sugar just to keep up. When high night temperatures occur at the time of seed setting, as is the case this year, there is likely some negative effect on final seed numbers and yield.

High humidity means higher night temperatures, and so contributes to that problem. Relative humidity drops as temperatures rise in the morning, and this helps the air pull water vapor out of the leaf through stomata. While water loss is faster at lower relative humidity, air at 50% relative humidity (a high afternoon value) is a strong draw, and lowering it may not increase demand as much as factors like wind speed. High relative humidity and low wind speed slow drying of leaves in the morning and may contribute to spread of foliar diseases.

Wide-open stomata mean maximum intake of CO2 and high photosynthetic rates. The high rate of water vapor loss with wide-open stomata means high rates of water use for the crop, but as water evaporates from cell surfaces, it cools the leaf. Without such cooling, the sunlight not used in photosynthesis will rapidly heat the leaf, causing damage and a drop in photosynthetic ability. If water becomes limiting as soils dry out, the stomata start to close. This reduces the rate of water loss while also reducing photosynthetic rates.

Thus what we often describe as a crop "burning up" is really a consequence of running out of water. Soils don't suddenly run out of water for the plant, but as water is lost from leaves, it gets more and more difficult for the plant roots to take up water from the soil. When this happens, stomata start to close, and as soils get drier, leaf tissue may begin to wilt as cells lose internal pressure. In corn this can mean leaf curling, but in some cases the first visible symptom will be a silvery sheen on the upper leaf surface. Both mean that the leaf has more or less stopped working, and if the wilting lasts long enough, leaf tissue can be killed.

Soybean leaves under such stress tend to droop rather than wilt, but the end result is the same--low photosynthetic rates, plus loss of leaf area if it stays very dry. Later-planted soybean plants do not have root systems as extensive as those of corn, and at present they seem to be struggling more than corn for water in dry fields. Both crops are showing more symptoms of inadequate water in compacted areas, where the crop was planted late, and in lighter soils.

As crops cope with ongoing heat and dry weather, a persistent question is how long the crop can hold out without rain before it starts to lose yield. Cooler temperatures extend this period because they bring a drop in water use rates (and photosynthesis), thereby increasing the likelihood of rain before serious damage occurs. But statements like "we need rain by [date]" or "we're losing [X] bushels per day without rain" are mostly guesswork; we can recognize changes in leaf appearance that signal a drop in photosynthesis, but we can't be very exact. We do realize, though, that corn plants trying to set kernels and soybean plants trying to set seeds both benefit from maximum rates of photosynthesis and from the high sugar levels that result.

So it is likely that crops that undergo stress in July start to lose some yield potential fairly soon after the stress begins. Once kernels are fertilized, kernel abortion rises as cob sugar levels drop. In soybean, pods fail to form or may drop off as sugar levels drop. Soybeans retain more ability to recover pod and seed numbers than corn, mainly because the soybean flowering period lasts 3 weeks or longer, and more flowers can form at nodes if stress is relieved. If corn kernels abort, lost yield potential is generally not fully recoverable.--Emerson Nafziger

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