Effects of high heat on wheat

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There were some extremely high temperatures last week in Kansas. While this is not unusual for this time of year, the high temperatures  caught some of the wheat during early to mid grain fill. What effect will these temperatures have on the wheat crop?

click image to zoomFigure 1. Number of hours with temperatures above 82 degrees on June 11 (map on left) and June 12 (map on right) of 2013. For example, the darkest red area had between 21 and 24 hours above 82 degrees on those dates. Source: Pennsylvania State Climate Office and the Penn State Center for Environment, courtesy of Erick DeWolf, K-State Extension Plant Pathology. The answer depends on the stage of development of the wheat, the moisture condition of the soil, and how long the extreme heat lasts.

Wheat begins to suffer when temperatures get above 82 degrees. At these temperatures, photosynthesis slows and stops but the rate of respiration continues to increase. Basically, the plants begin to use more sugars than they can produce by photosynthesis. At 95 degrees key enzymes begin to break down and stop functioning.

A period of high heat will also destroy membranes of chloroplasts and chlorophyll molecules. Once destroyed, these compounds will not be replaced. This will result in permanent browning of the leaves.

Still, grain fill can usually recover from short periods of heat stress if conditions are otherwise favorable. We might expect the impact of the heat stress will be worse in with prolonged periods of temperature above 82 degrees especially in areas where there is little or no soil moisture. Plants can cool themselves more easily when soils are moist than under dry soil conditions. 

click image to zoomJim Shroyer, K-State Research and ExtensionFigure 2. White heads as a result of premature death caused by period of extreme heat, together with drought stress. Damage may be minimal in fields where the wheat is at the dough stages of development. But where the wheat is still in the milk stages of kernel development, the wheat may experience reductions in test weight and poor grain fill. 

Another common effect of both extreme heat and drought is premature death of the heads. This can happen to heat-stressed wheat in which the root systems were unsually shallow due to dry conditions. In this situation, the extreme heat can cause enough additional stress that the entire head simply dies. When this happens, the heads will turn white – almost overnight in some cases.

Aside from the heat, the drought this year has resulted in very short flag leaves in many cases, and reduced the overall photosynthetic potential of the wheat. All of this will have an effect on grain fill, yields, and test weight.

Wheat in western Kansas will likely be most affected by the heat stress this year since temperatures have been hotter in that region and there is less soil moisture available to buffer the effects.

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