Sorghum And Heat Stress

DR. BRENT BEAN

LUBBOCK, TEXAS

Sorghum is known for its low-input seed cost and drought tolerance, but its ability to tolerate high temperatures is becoming increasingly important. In any grain crop, the most sensitive time to heat stress is around the pollination and early grain set stages. 

Sorghum has two characteristics that help it tolerate high temperatures during this period. The first is that unlike some of the other grain crops, sorghum has perfect flowers, with the male and female parts located together, making them primarily self-pollinating. What this means is that pollen is produced within millimeters of the ovary that, when pollinated, eventually becomes the grain kernel. This is in contrast to corn, which has imperfect flowers, with pollen being produced in the male flowers of the tassel that must travel to the female flowers on the ear for pollination to occur. Each sorghum panicle has hundreds to thousands of these perfect flowers, each with the potential to produce a kernel, or what agronomists sometimes refer to as a berry. An average-size sorghum panicle will typically produce 2,000 sorghum kernels. Since there is practically no distance that the sorghum pollen must travel for pollination to occur, this greatly reduces the pollen exposure to high air temperatures, making it less susceptible to heat damage.

A second and important characteristic is that sorghum avoids heat during the pollination process by releasing pollen from its anthers in the early morning hours when air temperature is typically at its coolest. The pollination process is completed long before temperatures are high enough to damage pollen. In addition, each sorghum flower produces an abundance of pollen with an estimated 45 million pollen grains produced per panicle.

Although sorghum has these mechanisms for increasing its heat tolerance, yield can be reduced under extreme heat conditions. The most sensitive time for heat stress to occur is from head emergence through flowering. This period lasts approximately 15 days.

A Kansas State University growth chamber study showed grain set was reduced approximately 50 percent when heat stress was imposed for several hours with a maximum daily temperature of 97 degrees and a minimum night temperature of 79 degrees for five days during this 15-day critical growth stage. Very little loss in grain set was noted when stress was applied outside of this 15-day window. In the field, yield losses as high as 40 percent were recorded when the maximum average daily temperature was 109 degrees over a 28-day period. Of course, this temperature is extreme and would not be expected to occur under most conditions. Although generally the focus is on maximum day temperatures, high night temperatures during this sensitive time period can also lead to reduced grain set.

Recognizing heat-stress-damaged panicles can be challenging, often mistaken for insect or disease damage. In extreme cases, the entire panicle can be completely devoid of grain. This is commonly referred to as blasting. Most often, only a portion of the panicle is affected. This is because flowering begins at the top of the panicle and progresses downward over a three- to five-day period. For example, if extreme temperatures occur in the middle of this time period, then only the middle portion of the panicle may be affected. Other times, missing grain may be scattered throughout the panicle.

Plant breeders continue developing hybrids with greater heat tolerance. Until those advances reach the marketplace, adjusting planting dates to avoid extreme temperatures during this critical 15-day window remains the best management strategy.   ∆

DR. BRENT BEAN

SORGHUM CHECKOFF AGRONOMIST