There have been some extremely high temperatures the last few weeks in Kansas. While this is not unusual for this time of year, the high temperatures have caught some of the wheat during early to mid-grain fill. Wheat in northwest, north central, and west central Kansas will likely be most affected by the heat stress this year 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 matched more sensitive phases of the grain filling period there. In southeast and south central Kansas, the wheat is mostly mature and already being harvested, and should not suffer the consequences of heat stress.
What effect will these temperatures have on the wheat crop?
The effects of hot temperatures on wheat grain development and yield depend 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 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 114 hours of temperatures above 82 F in the June 1 – 15 period (Figure 1), and 87 hours in the June 9-15 period, hours in which grain yield was likely not increased.
At about 93 degrees 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 2), 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 2). 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. We observed as much as 40 hours of temperatures above 93 F in the June 1 – 15 period (Figure 1), and most of those hours were in the June 9-15 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.
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 Figure 3, for the wheat variety Iba. In Figure 3, 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.
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. Soils, either topsoil or subsoil, in the majority of the wheat-growing region in Kansas are moist this season (Figure 4).
Heat 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 unusually shallow due to dry conditions. During this growing season, wheat in some areas (especially western Kansas) was damaged by the April 29 – May 1 snow event and had kinked stems. This wheat is now more vulnerable to heat stress and have started to abort tillers (Figure 3). 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.
Romulo Lollato, Wheat and Forages Specialist
Mary Knapp, Weather Data Library