In the soft red winter wheat growing regions of the eastern U.S., mid-winter warm temperatures accelerated wheat development, exposing wheat that transitioned to reproductive stages to cold temperatures during the late spring. Wheat plants can be damaged by frost (low temperatures cause damage but do not result in fully frozen tissue), freezing of the plant tissue, or a combination of both. The magnitude of freeze damage depends on: 1) temperature 2) duration of temperature 3 ) wheat growth stage.
Freeze Damage at Feekes 6 Growth Stage
Prior to Feekes 6 growth stage, the growing point of wheat is below the soil surface, protected from freezing temperatures. However, at Feekes 6 growth stage, the first node appears and pushes the growing point (developing spike) above the soil surface, and this developing spike can be damaged by low temperatures. In our research, 5%, 10%, 25%, and 50% reduction in wheat grain yield occurred at 20.1, 17.9, 14.8, and 11.9ºF, respectively, when the plants were exposed for 15 minutes (Table 1).
At Feekes 6 growth stage, damage from freezing temperatures will cause discoloration of the leaf tissue, with leaf tips or edges exhibiting symptoms first. Initially, leaf tips and margins may appear dark purple (variety-dependent) or water-soaked (Figure 1). After a few days to a week of active growth, leaf and stem tissue may turn yellow or brown. Discoloration can be caused by temperatures <39°F with visible symptoms of damage appearing when temperatures are >40°F for several days (Figure 2). However, discoloration does not necessarily indicate a reduction in grain yield. Grain yield reductions can be attributed to death of leaf tissues and stems, resulting in the formation of tertiary (regenerative) tillers from surviving plant crowns (Wu et al., 2014) (Figure 3). These tertiary tillers may produce seed, but often do not fully mature, resulting in small, lightweight kernels (Rawson, 1971). Overall grain yield is reduced in these situations as primary and secondary tillers account for the majority of grain yield (Rawson & Evans, 1971).
At Feekes 6 growth stage, the developing spike within the wheat stem can exhibit visual symptoms of damage due to freeze injury (Figure 4). Damaged spikes can be observed by carefully cutting the wheat stem lengthwise to expose the developing spike at the first node. Damaged spikes will appear discolored and shriveled. A healthy, developing spike should be rigid and whitish-green.
|Feekes growth stage||5% reduction in yield||10% reduction in yield||25% reduction in yield||50% reduction in yield|
Freeze Damage at Feekes 8 Growth Stage
At Feekes 8 growth stage (flag leaf visible, but still rolled up), plants are larger and have a lower solute concentration in cells, leading to the reduction in cold tolerance compared with wheat at Feekes 6 growth stage (Zech & Pauli, 1960). At Feekes 8 growth stage, the emerged flag leaf accounts for nearly 75% of photosynthates during grain fill (Evans & Rawson, 1970). A damaged flag leaf has reduced ability to accumulate photosynthates during grain fill, producing less dense, lightweight kernels (Frederiks, Christopher, Sutherland, & Borrell, 2015). In our research, wheat grain yield was reduced by 5%, 10%, 25%, and 50% at 30.7, 27.6, 24.6, and 21.9ºF, respectively, when exposed to freezing temperatures for 15 minutes (Table 1).
At Feekes 8 growth stage, damage from freeze may include yellowing or browning of the flag leaf and the flag leaf may appear twisted or in a spiral (Figure 5). As the plant continues to grow, the wheat spike may get stuck in the leaf sheath, causing a crooked appearance at heading (Figure 6). This phenology can also be associated with spikes that emerge quickly due to warm temperatures. Phenoxy herbicides, such as dicamba, may also result in similar damage to the flag leaf and spike (Friesen, Baenziger, & Keys, 1964).
Freeze Damage at Feekes 10.5.1 Growth Stage
At Feekes 10.5.1 (beginning flowering), symptoms of freeze damage include spikelets and awns appearing white or bleached (Figure 7). At anthesis, flower anthers are very susceptible to low temperatures. Sterility, embryo death, and complete loss of the spike has been found to occur at 30°F (Marcellos & Single, 1984; Cromey, Wright, & Boddington, 1998). Freeze damage to exposed anthers reduces fertilization and the number of seeds per head (Fuller et al., 2007). In our research, wheat grain yield was reduced 5%, 10%, 25%, and 50% at 28.7, 27.8, 26.3, and 24.1°F, respectively, when plants were exposed to freezing temperatures for 15 minutes (Table 1).
At Feekes 10.5.2, flowering is halfway complete, so flowers already pollinated were less affected by cold temperature treatments. Fertilized and developing embryos can withstand freezing temperatures better than reproductive structures prior to pollination; however, both may be injured by freezing temperatures (Livingston and Swinbank, 1950).
After a freeze event, wait approximately two weeks after active growing conditions resume to check for visual signs of freeze injury. Make sure to examine several areas of the field as landscape features influence the micro-climates within fields. For example, east-facing slopes are cooler and drier than west-facing slopes (Hanna, Harlan, & Lewis, 1982). Small differences in temperature can cause large differences in damage and grain yield (Table 1). If cold temperatures occur during heading (Feekes 10.1 to 10.5) or flowering (Feekes 10.5.1 to 10.5.3), check for grain development at least two weeks later. Examine spikelets from the top, middle, and bottom portion of the wheat head as flowering is asynchronous. It is possible only a portion of the head is damaged, depending on which part of the head was flowering at the time of freeze.
Individual spikelets should be dissected to examine if there is a healthy flower or developing kernel (grain). A healthy flower should have a stigma (female portion) that looks white and feathery when it is receptive (Figure 9). Anthers (male portion) may also be inside the spikelet or may have already extruded from the spikelet. A healthy, developing kernel is shown in Figure 10.
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