Primary symptoms

Low temperature: Early growth slow and leaves may turn purple. Symptoms disappear as the plant grows.

Frost damage: Uppermost leaves dried off. Spikes may have no grains or bands of no grains.

Summary

The most important effect of temperature is that it affects the rate at which plants develop through their stages and produce their leaves, tillers and other components. Everything goes progressively faster as temperature rises between a base and an optimum temperature. See minimum heat sums for development phases, and similarly development slows at lower temperatures. Calculating the size of these effects is discussed later. Good farm management can do more to counter the negative effects of high temperature than it can with low temperature and particularly frosts.

Low temperature

Development slows at lower temperature. But when temperatures are low enough for frosts, severe damage is done to young tissues (picture above). Vegetative shoots can be killed below -5° C. The consequences to yield of two or more successive frosts from spike emergence (Z5.1) through anthesis into early grain filling (Z7.1) can be severe. Official Stephenson Screen temperatures of 1.5°C measured at 1.5 metres above the ground are cold enough for effects. This temperature is equivalent to 0°C on the crop surface. A single night frost during this period may not be overwhelming because only the new tissue that has been exposed to the air during the last few days is killed. This is seen as banding of dead and live spikelets on the spikes. All tissues become more frost resistant after exposure to the air.

Is frost or low temperature a problem?

• Look for vegetative plants with dead shoots. Temperatures have been well below -5° C. This problem will apply only at higher latitudes and altitudes and in areas with extreme temperature changes between seasons and between day and night. A lighter coloured stripe across expanding leaves is a symptom of less severe frosts. This will generally disappear as the leaves age. However, plant photosynthesis is reduced and growth may stop for 1-2 days after such frosts.
• During spike emergence to grain filling, look for a band or bands around the spike. Are the spikelets empty? Is the banding in a similar position on many spikes? Each small band is caused by one frost.
• Look for spikes that are dark coloured, even black, without grains. This can be a consequence of sequential severe low temperatures during anthesis or early grain filling. At high altitudes in the subtropics (e.g. Nepal) sterility in spring wheat can result from 3 or more consecutive nights of non-frosting temperatures lower than 5°C between stages Z4.9 and Z5.9.
• Are there one or two completely dead spikes on most plants but other spikes are normal?
• Are there florets gaping open with shrivelled anthers? This may be frost damage but can be confused with boron deficit.

Can you do anything about frost?

• Planting date: The only economic way to deal with frosts in wheat is to ensure that the whole crop is not at sensitive stages when frosts are most likely to occur. Plant earlier or use later varieties that reach the most frost-sensitive stages (around anthesis) when the likelihood of frost is very low. Frosts to -4° C at earlier vegetative stages are not critical to yield.
• Variety length: Plant varieties in which heading is not synchronous amongst shoots. Having spikes at different stages and different heights means that not all parts will be damaged during a single frost.
• Variety type: Plant varieties derived from winter x spring crosses that have greater cold tolerance (Subedi et al 1998).