For: Avium Technical Team.
With the collaboration of Hugo Navarro A. PUC Agronomy Graduate.
Frost is a meteorological phenomenon where the ambient temperature drops to the freezing point of water, that is, below 0° Celsius, due to radiative cooling from the ground to the atmosphere, which generates a thermal inversion that reaches from 5 to 20 meters high. This type of frost is known as “radiative or white frost” due to its characteristic color due to the frost they produce, being the most common. On the other hand, when the thermal inversion is caused by the descent of a mass of cold air coming from high latitudes, it is called “convective, polar or black frost”, due to the absence of frost in its path, although much more damaging than the previous one. , since the water does not form frost or ice, it cannot release heat in said process of change from liquid to solid state, bringing disastrous consequences for the agricultural sector in general, damaging crops, especially when they happen unexpectedly and in areas where they do not normally occur.
Damage due to low temperatures and/or freezing can occur in all plants, but the mechanisms and type of damage vary considerably. This is how, in general, the species or the different cultivars present various damages due to frost at the same temperature and in the same phenological state.
Frost damage occurs when ice forms inside the tissue of plants, causing damage to their cells. The formation of ice crystals within the protoplasm of plant cells is direct frost damage (intracellular frost), while indirect damage is the formation of ice inside the plant, but outside the cells (extracellular frost). , being what really damages the plants is the formation of ice and not the cold temperatures.
Photo 1. Visual appearance of floral primordia in cherry buds, before (A) and after (B) exposure to a frost event. (Source: Valenzuela et al. 2020)
The freezing of tissues and their degree of damage find their explanation in the study of the process of cooling and freezing of the water they contain, specifically, in the speed of freezing and thawing. A rapid cooling has an irreversible effect and generates serious damage when it occurs since ice forms quickly in the meatus (space between cells) and inside the cells, and these are unable to adapt. The cell membrane and the constituents of the protoplasm break down, which generates cell death and, therefore, a blackening of these. On the other hand, in slow cooling the cells can gradually adapt to the changes, but as the extracellular water freezes, intracellular water is released to compensate for the osmotic deficit that occurs and its content is concentrated. Although tissues can survive and recover, cells can die due to both denaturation processes and mechanical effects (Urbina, 2007).
The adaptation of plants to cold temperatures before a frost is called “hardening” and it is probably related to a higher degree of solute content in the plant tissue or to a decrease in the concentration of bacteria active in the nucleation of ice. (INA) in cold periods, or indeed it can be a combination of both components. Accumulation of sugars or sugar-alcohols would lower the freezing temperature of tissues and increase supercooling in many deciduous or evergreen trees at low air temperatures.
The level of assimilates in plant tissues is an active process in which the degree of response to frost or “hardening” would be subject to the previous conditions before the frost periods.
During the summer season in the postharvest of the cherry tree, the concept of avoiding abiotic stress with respect to high temperatures with the use of different strategies to reduce this condition, and with this to be able to improve the physiological processes of accumulation of reserves in the different organs, is taken as a concept. of the plant; however, indirectly work is being done so that the reproductive structures can form a kind of “tolerance” or technically called a “hardening” at low temperatures given the better conformation of the buds in the cultivar.
When it comes to cherry plantations, a frost can cause the same damage as any other species, however, the consequence of freezing will depend on the phenological state of the plant when the frost occurs.
For several years in Chile it has been agreed that frosts were more serious the more advanced the phenological state of the fruit tree was, however, in recent times that perception has been changing. Even when the flowers are damaged above after a frost, it can be more risky when this occurs with a swollen bud or exposed cluster. That is why it is important to know the different frost control systems that can mitigate any damage to the cherry tree plant, and thus prevent the productive potential of the orchard from diminishing.
Photo 2. Swollen bud state in cherry tree.
There are passive and active methods to control frost. As passive methods we can find, the choice of the place of the plantation, coverage of the trees, an adequate pruning, the irrigation and adequate management of the humidity, among other actions. On the other hand, the active methods are characterized by the application of energy or thermal inversion of air to neutralize the heat loss and the most used are heaters, fans, helicopters and sprinklers.
Conventional technologies are frequently used by farmers in the country, and especially by cherry producers, since being a very profitable product, the fruit harvest is ensured and stable production is generated over the years. Fans are an example of this, and their function is to mix the hot air from above with the cooler air near the surface. The extent of its protection will depend on the strength of the thermal inversion, but in general terms a conventional fan protects between 4 to 6 planted hectares (FAO, 2010). It should be noted that the use of fans with heaters generates a plus in control, especially in events with lower temperatures.
Another innovative option is the Tow and Blow propeller, which, unlike traditional propellers, is much more efficient in terms of energy consumption, does not require installation, can be moved and reaches areas where other systems do not reach, has speed and adjustable angle and is inaudible at 300 meters (Zimex, 2017).
However, there are new tools for frost damage control in cherry orchards. Within these we can find the polymer based on polyvinyl alcohol (PVA), which has been used to inhibit the crystallization of ice in laboratory conditions, and that in tests under field and laboratory conditions, and with additions of surfactants and emulsifiers, managed to be effective in reducing frost damage to Prunus avium flower buds between 40 and 100%, without causing negative effects on the quality of the fruit, which generates promising results in the control of this phenomenon (Valenzuela et. to 2020)
In the economic field, the acquisition of frost control technologies will not always be the best option for the producer, since this decision depends on the variety and the area where the plantation is located. Therefore, in plantations in the south of the country where there is a greater amount of frost in critical periods for the plant, it will be justified, especially in early or mid-season varieties, whereas in areas where frost is very scarce, acquiring these technologies they are not recommended since the investment would not be justified with the economic returns (El Mercurio, 2015).
A study carried out by the professor of the University of Talca, Patricio González (2020), concluded that between the years 2001 and 2019 there were 19 average frosts per year. The question is, when should I start controlling frost in my cherry orchard?: Carlos Tapia, Technical Director of Avium SpA, suggests starting frost control prior to the phenological stage of the swollen bud. As previously detailed, damage has been seen and analyzed from early in phenology and many agree that the stage most sensitive to damage is between the swollen bud and the exposed floret, and even more complex than in more advanced stages such as the white bud at flowering.
Plant nutritional management also plays a considerable role in plant susceptibility to frost conditions. An unbalanced fertilization of nitrogen or other nutrients could have unwanted effects in this type of situation. Thus, trees that are not properly fertilized, this is in relation to their state of charge and vigor, tend to lose their leaves earlier in autumn, and they could also have earlier flowering in spring, which would increase the risk of frosts as well as by phenological conditions and by the state of the reproductive organs inadequately formed by an unbalanced nutrition of the cultivar.
Thus, as previously mentioned, resistance to frost damage is greater when photosynthates accumulate in the most sensitive tissues of plants (Proebsting, 1978). In addition to a good nutritional state, a correct sanitary state would favor acclimatization and resistance to freezing.
The literature describes that in deciduous trees such as cherry trees, it is advisable to carry out late pruning in those areas where the winter temperature is very close to zero, probably in some areas of southern Chile, since it would allow the entry of pathogenic microorganisms. through the cuts.
The delay of flowering in deciduous trees through spraying is a technique studied a few decades ago, which demonstrated that it is possible to delay flowering by 15 days in cherry trees (Anderson et al., 1973), when irrigation was applied to the trees whenever the air temperature exceeded 6.2 °C between the interruption of rest and bursting. There are studies in Chile with chemical products that can delay flowering such as Retard Cherry®, with consistent successful results for this objective.
“With the aim of reducing risks from spring frosts, in recent years studies have been conducted to delay flowering in cherry trees in late areas through biostimulant products, which have delayed flowering between 7 and 12 days depending on the variety. and that in the event of spring frosts in the most susceptible phenological stages in the cherry tree (swollen bud), notorious differences were observed when analyzing post-frost damaged primordia and consequently a considerable effect on productions mainly due to avoiding frosts in phenological states. less susceptible in the cherry tree”, added Carlos Tapia.
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THE MERCURY (2020). Frost control increasingly necessary. Recovered from:https://www.elmercurio.com/Campo/Noticias/Noticias/2020/04/30/Control-de-heladas-cada-vez-mas-necesario.aspx?disp=1
FAO (2010). Frost protection: Fundamentals, practice and economics. Volume 1. Retrieved from:http://www.fao.org/3/y7223s/y7223s.pdf
INIA (2008). Frost control in agriculture (I part). INIA inland. Recovered from:http://biblioteca.inia.cl/medios/biblioteca/ta/NR34738.pdf
RENÉ, M. & VILLAGRÁN, A. (2014). Frost protection methods. Recovered from:http://www.unifrut.com.mx/foros/2014/(P.%20F.,%202014)%20M%C3%A9todos%20de%20protecci%C3%B3n%20contra%20las%20heladas.pdf
URBINA, V. (2007). Frost damage to fruit trees. Symptoms and evaluation. University of Lleida. Recovered from:https://repositori.udl.cat/bitstream/handle/10459.1/47655/2007_Urbina_Da%F1os%20por%20heladas%20en%20frutales.pdf?sequence=3
VALENZUELA, M., BASTÍAS, R., RODRIGUEZ, S., SABANDO, C. (2020). PVA polymer as a tool to prevent frost damage in flower buds of the cherry tree Prunus avium L. Recovered from:https://revistasacademicas.udec.cl/index.php/chjaas/article/view/2205/2579ZIMEX (2017). Controlled frosts with intelligent technology. Recovered from:https://www.zimex.cl/heladas-controladas-con-tecnologia-inteligente/