Coping with Cold and Late Frosts (Impacts and Preventive Solutions)
Understanding the impact of cold on plants and the preventive solutions to implement.
Frost
To protect itself, the plant expels water from its cells to the intercellular spaces and increases the concentration of water in salts and sugars. When this is no longer sufficient, the cells freeze, biological functions stop, and the cells burst. The plant or the organ in question dies.
Risk factors
In plant biology, there is a minimum temperature (called the "critical" temperature) that must be reached for a crop to suffer injuries, and in extreme cases, death. This temperature varies depending on the plants and depends on many factors such as:
- Species and variety;
- The physiological or vegetative stage;
- The vigor of the plant; Soil condition and the nature of the vegetation cover;
- The intensity and duration of frost: the critical temperatures ("freezing threshold") necessary for damage to occur vary depending on how long they remain below the threshold. For example, the buds of trees can be damaged by a temperature of -2 °C persisting for more than 24 hours, but can survive if exposed to a temperature of -6 °C for less than 2 hours. This explains why the critical temperature of a radiation frost lasting only a few hours early in the morning can be lower than that of an advection frost which can last throughout the day.
- The presence of clouds and/or wind during the frost;
- The climatic conditions during the thawing phase.
- The phenological stage: The more advanced the phenological stage of the plant (close to maturity), the greater the risk.
- The health of the plant: A healthy plant often resists frost better.
- The cultural interventions that precede.
Types of frost
A frost day is when the temperature does not exceed 0°C and a frost when it does. Frost can be deadly if temperatures cause damage to plants.
A distinction is made between advection frost and radiation frost depending on the atmospheric conditions causing it:
- Advection frost is caused by the passage of a cold air mass coming from another region, when winds are relatively strong.
- Radiation frost (or white frost) occurs only locally and on clear, calm nights.
Cultural interventions
If the cultural itinerary causes the plant to concentrate water in its cells, a frost episode following this cultural intervention will cause more damage.
This is the case with:
- The application of a fertilizer based on nitrates in particular. Their high local concentration requires the plant, to denitrify the nitrate molecule, to absorb a large amount of water.
- An application of phytosanitary products which causes a detoxification phenomenon in the plant that induces the use of a greater amount of water in its cells, just as we might drink a lot "to cleanse" our body.
Effects of cold
Dehydrating effect of cold
Cold temperatures in the soil induce a decrease in water availability for the plant and can lead to dehydration of it. Instead of water moving from the soil to the roots, water moves from the roots to the soil by osmosis. In this way, the plant can no longer absorb mineral elements, causing induced deficiencies.
To combat this phenomenon, some plants adapt by increasing the concentration of their sap in sugar or minerals.
Mycorrhizae no longer function
Mycorrhizae do not like cold very much and for once do not seem to play an important role in the plant's fight against cold. Arctic plants are not mycorrhizal. When temperatures drop, they no longer participate in plant nutrition. A study conducted in Norway comparing plants growing at 8°C or 15°C with or without mycorrhizae shows that mycorrhizal plants have little advantage at 8°C compared to non-mycorrhizal plants.
Hormonal activity
The plant cannot move to fight stress in its environment. Hormones are its main adaptation mechanism allowing it to react to the attack of a biotic stressor or cold. The plant's symbiosis with bacteria in the rhizosphere increases hormone secretion. Studies on green bean exposing the plant to temperatures between -2 and -16°C show that the green bean is less damaged by frost if inoculated with rhizobium than without inoculum.
Before frost, improving plant resistance
According to the crop
The plant's ability to resist frost is partly genetically determined and a corn will never be as frost resistant as a rapeseed.
Similarly, the stage of the crop is important for sensitivity due to hormonal reasons and sap concentration.
The first action is obviously to adapt the choice of crops, genetics, and precocity to circumvent the problem. But this action is already used by most farmers and is not sufficient.
Sensitivity depending on sap
The less water in a plant, the higher the concentration of mineral salts and sugars in the plant cells. This is exactly what happens on 70% of the Earth's surface because unlike fresh water, which freezes at 0°C, seawater must drop below this temperature depending on salt concentration. Water with a salt concentration of 10% freezes at -7°C.
- Sap will freeze at a lower temperature if there is an increase in carbohydrate content and concentration of certain ions such as potassium, chlorine, sodium, or magnesium (which can lower the freezing temperature by up to 3 degrees). The goal here is to provide the plant with electrolytes by foliar spraying before frost, supplying potassium, magnesium, or large amounts of trace elements (copper, boron, or cobalt).
- High levels of trace elements, especially manganese, boron, zinc, copper, iron, and cobalt, could allow better plant resistance to stronger frosts, and this in the days or hours following application. Sprays of marine algae also have this effect. The betaines (amino acids) contained in marine algae in particular would play this role in plant frost resistance. An additional foliar urea spray could lower the hygroscopic frost point by a few degrees. A foliar application of missing nutrients and sugar in anticipation of a frost episode can therefore be carried out while ensuring not to favor mineral inputs the plant does not need. In this context, knowledge of the plant's nutritional status via a sap analysis helps limit risk.
- Similarly, preventive work to improve plant health and optimize photosynthesis increases the sugar content of the plant and lowers the frost limit temperature by a few degrees.
Thus, plant resistance to frost is also influenced by plant nutrition (the concentration of mineral elements in the sap).
After frost, curative solutions
In post-frost and to boost a plant that has undergone stress, a mixture of amino acids, molasses, and algae or plant macerates provides the stressed plant with the means to recover from the frost period endured. Amino acids are known for their high nutritional efficiency and interesting properties as bio-stimulants. They increase the assimilation of nutrients and especially trace elements, promote the development of microorganisms, and stimulate resistance to abiotic stresses such as frost.
The following mixture, for example, can be applied after a frost episode to promote crop restart:
- 2 L/ha of amino acids;
- 1 L/ha of algae;
- 100 g/ha of manganese (i.e., 1 L/ha in liquid or 0.5 kg/ha in powder);
- 30 g/ha of citric acid;
- 0.5 L/ha of molasses (prefer cane molasses as it has a higher proportion of reducing sugars).
Sources
Cold, late frosts what impacts for the plant and what preventive solutions, AgroLeague
