Heat Stress

To keep pace with increasing temperature, the crop requires more of everything (nutrients, water, sunlight) each day. So to avoid significant yield losses as temperature rises, farm management must become increasingly precise. With water and supply of nutrients optimised, high yields can still be achieved at high temperatures.

During grain filling, development accelerates faster than growth as temperature rises. Consequently, even if management is optimum yield can decline by up to 4% for every 1° C rise in mean temperature at higher temperatures (Stapper and Fischer 1990) because the grain filling period becomes very short.

High temperature damage is commonly associated with water stress, so water management is critical. As long as plants can transpire freely they cope with high temperature. Field crops provided with sufficient water can withstand air temperatures of 40°C. But if water is limiting, 40° C will kill leaves. The reason for this is that water-stressed plants attempt to conserve water by closing their stomata. As a consequence evaporative cooling diminishes and, without that cooling, leaf temperatures might approach 50°C. At 50°C, plant processes break down. Seedlings in very hot dry soils can readily reach those critical temperatures.

Is high temperature a problem?

  • During the seedling stages look for poor emergence or for seedlings with dried or dead leaves. Emerging seedlings can rapidly become desiccated if soil temperatures reach 40°C or greater. If it is hot and there is bright sunshine, solar radiation will heat a dry soil up to 50°C, particularly if the soil is dark coloured. Peacock et al (1994) found a 30% reduction in seedling emergence and survival as soil temperature at 5 cm increased from 37 to 45°C.
  • Is anthesis and grain filling occurring during the hottest part of the year? Is the grain filling period very short?
  • Are grains shriveled? Have there been frequent hot desiccating winds during grain filling.

What you can do about high temperatures

  • Mulch: Use a mulch to protect seedlings. This keeps soil temperature down during the day by insulating it from incoming solar radiation and conserving water. Mulch also reduces soil cooling at night.
  • Reduce soil moisture loss: Sow as soon as possible after seedbed preparation so that water losses from the freshly turned soil are minimised. Planting into moisture can then be achieved with shallower seeding ensuring plants emerge more rapidly. Seedbeds will also be cooler.
  • Avoid moisture stress: If sprinkler irrigation is available, reduce high soil temperature during seedling emergence by irrigating at that time. Irrigate during the evening. Minimise the effects of high temperature at other times by ensuring that the crop is not water stressed. Evaporative cooling by the crop via transpiration can reduce crop temperature below that of air temperature by more than 5°C in conditions of low humidity (some claim 10-15° C).
  • Optimum sowing date: Select the optimum sowing time, avoiding high temperature during anthesis and grain filling. High temperature at that time shortens the season and reduces yield. Avoid timing grain filling for periods when there are frequent hot and strong desiccating winds. Then the crop can not transpire fast enough to keep cool.
  • Variety: Choose a variety that makes optimal use of the available season; one that allows planting at a time that the field is free, but avoids high temperature during anthesis and grain filling (see optimum sowing time).