Effects of recent high temperatures on wheat
There have been some extremely high temperatures the last few weeks in Kansas and the forecast also suggests heat in the near future. While not unusual for this time of year, the high temperatures have caught some of the wheat during early to mid-grain fill, especially due to the delayed cycle caused by below-normal temperatures during most of the growing season.
Wheat growth stages around Kansas
This year temperatures seemed to have gone directly from winter to summer. Until April 30th, temperatures were below-normal and the crop was 2 to 4 weeks behind in development. However, temperatures in May were above-normal and crop development sped up, virtually catching up to normal in parts of Kansas or about 5-7 days behind in other parts of the state. As of May 28, the crop ranged from the dough stages of grain development in southeast Kansas to heading in northwest Kansas (Figure 1).
Figure 1. Estimated wheat growth stage in Kansas based on a simple growing degree day model and observations of the wheat crop in multiple locations. Map prepared by Erick DeWolf, K-State Research and Extension.
Areas most likely affected by heat stress
The effect of heat stress on wheat yield depends on high temperatures achieved, duration of maximum temperatures, soil moisture conditions (plants can cool themselves more easily when soils are moist), and stage of development of the wheat crop. Based on our observations and estimations on crop development and number of hours spent above 28 degrees C (82.4 degrees F) (Figure 2), we estimate that wheat in central and north central Kansas will likely be most affected by the heat stress observed during May 23-28 (Figure 3) since much of the crop in those regions was behind in development (mid-berry to late milk stage) and it is likely the hot temperatures coincided with more sensitive phases of the grain filling period in those areas.
Figure 2. Upper map: Total hours with air temperature greater than 28 degrees C (82.4 degrees F); Lower map: Hours of air temperature greater than 33 degrees C (91.4 degrees F). Source: Weather Data Library/Kansas Mesonet.
Figure 3. Risk of heat stress to wheat in Kansas based on wheat growth stage and duration of temperatures above 82 and 93 degrees F. Map created by Erick DeWolf, K-State Research and Extension.
What effect will these temperatures have on the wheat crop?
Wheat begins to suffer when temperatures get above about 82 degrees F. 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. We observed as much as 60 hours of temperatures above 82 degrees F in the May 23-28 period, hours in which grain yield was likely not increased.
At around 93 degrees F key enzymes begin to break down and stop functioning. As a result, the plant does not accumulate sugar (starch) in the grain during heat stress (Figure 4), which results in decreased wheat yield. Protein accumulation, on the other hand, seems to be unaffected by high temperatures and occurs normally under heat stress conditions (Figure 4). Due to a decrease in sugar accumulation coupled with normal rates of protein accumulation, wheat under heat stress tends to have a greater percent protein content (higher concentration). We observed as much as 22 hours of temperatures above 93 degrees F in the May 23-28 period, hours in which grain yield may be reduced. 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.
Figure 4. Effect of day and night temperatures (day/night) in two temperature scenarios (mild temperatures in the blue box and hot temperatures in the red boxes) on starch (upper panel) and protein (lower panel) accumulation on the wheat kernel. Source: Dupont, F.M., Hurkman, W.J., Vensel, W.H., Tanaka, C., Kothari, K.M., Chung, O.K., and Altenbach, S.B. 2006. Protein accumulation and composition in wheat grains: effects of mineral nutrients and high temperature. European Journal of Agronomy 25(2):96-107.
Heat stress is often worsened by drought stress. A good example of the effects of heat and drought stresses on wheat leaf persistence is shown in Figures 5 and 6. In Figure 5, leaves on wheat in the dryland system senesced prematurely (upper panels) as compared to leaves the irrigated system in which wheat maintained healthy leaves for at least an additional two weeks. In Figure 5, we see the upper portion of the heads turning white/chaff colored due to heat stress, while the bottom canopy (lower stems) are turning color as well, more likely due to drought stress.
Figure 5. Canopy cover on two nearby fields planted on the same day to the wheat variety Iba near Perkins, Okla. Upper panels reflect early leaf senescence observed in the dryland system as a consequence of both heat and drought stress, while lower panels reflect the longer period of time with healthy leaves observed in the irrigated system. Photos by Romulo Lollato, K-State Research and Extension.
Figure 6. Wheat plants showing signs of both heat and water deficit stresses. Awns and upper heads turning white/chaff colored due to heat stress, while lower stems are turning straw-colored due to water stress. Photos taken at Manhattan, KS, on May 24, 2018. Photos by Romulo Lollato, K-State Research and Extension.
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 unusually shallow due to dry conditions. During the 2018 growing season, wheat in most of the state had extremely shallow root system until late March/early April due to the dry topsoil. This wheat is now more vulnerable to heat stress and might start to abort tillers. 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.
Figure 7. White heads as a result of premature death caused by period of extreme. Photo by Romulo Lollato, K-State Research and Extension.
Stay tuned for more information on the possible causes for white heads in wheat in our next eUpdate issue on June 1.
Romulo Lollato, Wheat and Forages Specialist