Frost risk occurs virtually every year across southern and eastern agricultural regions. Actual occurrence of frost is determined by location and landscape factors as well as climate. The sequence of weather events that typically generate damaging frosts is composed of the passage of a weak cold front, followed by cold southerly winds and the establishment of a ridge of high pressure. This results in cool daytime temperatures, light winds and clear skies overnight.
Clear night skies are important to the development of frost events because they allow thermal radiation from the soil surface to escape freely to space. The rate of cooling and final temperature of the plant canopy is determined in part by the balance between thermal radiation emitted to space and radiation absorbed from the soil. A darker, heavy soil with some moisture will transmit more energy to the canopy than a lighter, low density soil. This is why frosts tend to be more frequent on sandier soils, assuming there are no other landscape influences.
Local topography is also important, as cold air tends to run down slopes and drainage lines, and will pool in flats and basins. Barriers such as tree or fence lines can impede flow and allow cold air to accumulate higher in the landscape.
The severity of the frost and hence the extent of the subsequent damage is therefore variable across the landscape. Topographic and managment practices aimed at mitigating the severity of frost concentrate on influencing the heat balance or drainage of cold air. While these practices may help for lighter frosts, severe frost events of long duration are likely to overwhelm such measures.
At what temperature does frost occur in a cereal crop?
This can depend on crop type, stage of development and if the canopy is wet. It also depends on how low the temperature gets and for how long. Frost is a three-stage response, with damage increasing for each stage.
Cold damage: occurs when plants are exposed to temperature less than 5°C down to -2°C. If this occurs during pollen development (Z39 – 45) it can cause spikelet damage.
Desiccation damage: occurs when ice forms on the outside of the leaves at temperatures from 0°C to -2°C. Moisture is drawn from the leaves leaving them dry and brittle, subsequently dying at the tips.
Freezing damage: usually occurs at temperatures below -2°C when there is rapid ice nucleation and ice crystals form within the tissue. The ice crystals physically rupture cell walls and membranes within the cells causing physical damage. Damage can be seen once thawed as dark green water-soaked areas. Ten days after a frost event bleached leaves, stems, heads and reproductive tissue might be evident.
Why does rain make it worse?
A canopy that is wet from a light shower of rain is often more prone to frost damage. This is because rain contains ice nucleators predominately bacteria and dust. These predominately Pseudomonas sp. bacteria are present in the WA cropping landscape on our decaying stubbles and in the light showers of rain which often fall before a frost (Bekuma et al. 2021; Biddulph et al. 2021). With these very active ice nucleators, water and plant tissue can freeze at plant temperatures around -4°C, well above what they would normally freeze at, (~<-8°C) causing significant yield loss. Without these ice nucleating agents, a plant may remain supercooled with no damage to the reproductive and vegetative parts. Even frost-sensitive plants grown under a glasshouse environment can supercool to between -8°C and -10°C with mild injury (Lindow 1983).
Past frost and stubble management trials have found an increase in frost severity, duration and damage with stubble retention in the WA farming system (Jenkinson et al. 2014; Smith et al. 2017 and Biddulph et al. 2019). The current recommendation is to manage and reduce stubble loads so at seeding they are at around the yield potential of the environment. For example in a 2 t/ha environment reduce the stubble loads at seeding to ~2t/ha.
Preliminary results from more recent work show rainfall before frost events and retained stubble appeared to increase frost damage due to greater activity of biological ice nucleation activity. This causes freezing damage to occur earlier in the night resulting in more terminal damage. In-field thermography after head emergence indicates stubble retention causes ice to form first on the stubble and crop residue in the interrow, before moving to the older senesced leaves and up the plant to the head. Simulated rainfall containing Pseudomonas ice nucleating proteins applied before frost events increases, but a wet crop by itself did not increase frost damage. Spring rainfall has biological ice nucleation activity from Pseudomonas sp, with the highest activity in light rainfall events <10mm which fall in the late afternoon/ early evening. For more information on this please see Bekuma et al (2021) and Biddulph et al (2021). Research is ongoing to determine if we can manage and reduce frost damage by managing the ice nucleating bacterial populations in season.
When is the crop most susceptible?
Cereal crops are most susceptible to frost damage during and after flowering but are also susceptible from stem elongation throughout grain filling.Wheat susceptibility to frost
Pulses and canola are particularly susceptible during pod filling where affected pods have absent, mushy or shrivelled and distorted seeds.
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