Frost and plant physiology: Q&A with Glenn McDonald
How does frost damage plants?
When the temperature drops below freezing and there is moisture on the surface of plants, ice will form. Ice crystals on the plant’s surface grow through cracks in its outer layers or through the pores (stomata) in the leaves and stems, which causes ice to form inside the plant. Once water starts to freeze within plant tissue, it moves rapidly through the plant as an ‘ice front’ as long as there is continuous water and no internal barriers.
There are two ways that frost damages plants – by causing dehydration of the cells and by ice formation within the cells. When water surrounding the cells freezes, it draws water out from the cells causing them to dehydrate. This may not always damage plants under mild frosts because cells can withstand a certain degree of water loss. But with continued growth of ice within the plant tissue, the cell can be dehydrated beyond repair, causing severe plant damage. Later, when the ice melts, the cells will rehydrate, if the rate of thawing is slow, the plant tissue may be able to recover where little damage occurs, but if thawing occurs rapidly, damage can be greater. The second way that plants are injured during frost is when ice forms within the cells. This damages the membranes and internal structures of the cell causing irreparable damage.
What is acclimation and how does it protect plants?
Cold acclimation, or hardening, is an increase in a plant’s tolerance to freezing after exposure to low temperatures. Cold acclimation is associated with increases in water soluble carbohydrates and some proteins in cells which can help reduce dehydration of the cells during the frost or help protect the internal structures of the cells at low temperatures.
Exposure of a plant to low temperatures – below 10 degrees but above freezing – before a frost can reduce frost damage. For example, when plants have been grown in a glasshouse and then exposed to frost, only a mild frost may be required to kill the plant compared to the same plant grown outside, which is more acclimatised to the conditions. If acclimated plants are exposed to warm temperatures they can lose their acclimation.
The mechanisms behind acclimation are not fully understood, but one is that the concentration of water soluble carbohydrates in the cells increases so less water is drawn out during a frost, reducing the risk of severe dehydration.
Did plant water soluble carbohydrate concentrations play a part in the 2014 frosts?
In 2014, a mild start to the season meant that there had been fewer cold days prior to the August frosts and so acclimation was weaker. In addition, high moisture levels and increased nitrogen application meant that the plants were growing well, with the plant sugars being used for growth, which left fewer water soluble carbohydrates in the cells to provide protection.
A combination of early sowing and the mild seasonal conditions leading up to the frost meant that many crops were at a relatively advanced stage of development when the frost occurred, which may have also exacerbated the damage.
Is there anything growers can do to improve a plant’s frost tolerance?
Potassium is a strong factor in maintaining cell water content in plants, which can potentially influence tolerance to frost. It has been shown that plants deficient in potassium are less resistant to frost. Soils that are deficient in potassium could benefit from increasing K levels. However it is unlikely that there will be a benefit of extra potassium applied to plants that are not potassium deficient.
When plants have been frosted, do the sugars relocate to other parts of the plant, such as other tillers? Can plants reshoot?
New tillers can grow from dormant buds after frost damage, depending on the location and severity of the damage. These compensatory tillers will have delayed maturity, but where soil moisture reserves are high, they may be able to contribute to grain yield.
What effect does nitrogen applications have on frost risk?
There is some evidence that crops grown with high N suffer greater loss from frost. High nitrogen can reduce the water soluble carbohydrate levels in plant cells, which can contribute to greater sensitivity to frost, but this does not mean that growers should reduce N levels to minimise frost risk.
While high N may reduce frost tolerance, there are too many other benefits of N applications to mean that low N should be used a way of reducing the risk of frost damage. A plant that is growing well because of high N may recover better after a frost, or the increased canopy may provide some insulation from low temperatures – it’s a complex picture.
Does retained stubble alter frost risk?
In theory, stubble may prevent heat being released from the soil, resulting in a cooler canopy which is more susceptible to frost risk, but there are significant benefits to stubble retention. One benefit is increased water holding capacity, which can increase soil temperature and reduce frost risk. Previous GRDC studies in South Australia have shown negligible effect of stubble retention on frost risk. This work is on-going within the GRDC’s National Frost Initiative.
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Glenn McDonald speaks about frost and plant physiology, watch the video below.
GRDC Project Code UA00136, CSP00143