Winter cold is perhaps the factor or variable that receives the most attention in winter, as it is directly related to the productive potential of a cherry orchard. This winter accumulation is absolutely necessary to break dormancy, to vegetate and produce fruit in optimal conditions, which is why it is important for the crop. Insufficient cold during the winter break delays the phenology of the trees, which can cause delays in flowering; it also causes a slow opening of the flowers and an asynchrony between the development of the leaves and fruits, leading to greater susceptibility to abortion or fruit drop (Lemus, 2005).
Plants generate physiological processes preparing for dormancy, being first of all mechanisms that allow them to survive the adverse conditions of winter in temperate and cold climates (beginning of Endodormancy), before the arrival of extreme temperatures (Campoy et al., 2011). Shortening of the photoperiod and temperature are the primary environmental signals that control the cessation of shoot growth, and low temperatures (the first autumn frost) mark the clear signal for plants to begin dormancy.
The cherry is a species that requires the accumulation of Cold Units (UF), which are counted through different methodologies, the most commonly used in fruit growing being the cold hours model (Bennet, 1949; Weinberger, 1950), the Utah or Richardson model (Richardson et al., 1974) and the dynamic model or cold portions (Fishman et al., 1987).
In addition, Lang et al., in 1987, proposed a new terminology for dormancy research, distinguishing it into three phases, in order: para-dormancy, endo-dormancy and eco-dormancy (Fig. 1). According to their definition, para-dormancy refers to the suppression of growth imposed on particular organs by other structures of the tree (e.g. apical dominance), due to the production and/or action of inhibitory molecules. Fadon et al. (2020) refers to this phase as “establishment of dormancy” (Fig. 1), where the plant manages to detect the shortening of days coupled with the decrease in temperatures, triggering physiological changes and expressions in certain genes in the plants. A regulation of plant hormones begins. Abscisic acid (ABA) and gibberellins (GAs) play important roles in the establishment of dormancy. ABA induces the suspension of the cell cycle and the deposition of callose in the plasmodesmata, blocking cellular communication in the shoot apical meristem. On the other hand, GAs antagonize ABA, showing a decrease during the establishment of dormancy.
During endodormancy, growth is not possible even under suitable temperature conditions, since the buds have not received sufficient exposure to cold, compared to this phase. et al., calls it “endo-latency” and explains that continuous exposure to cooling temperatures leads to the progressive restoration of sap flow, through the degradation of callose in plasmodesmata; this facilitates the transition of the buds to a state of rest, while maintaining cold tolerance. On the other hand, the phytohormone that plays a role is abscisic acid (ABA), which is crucially important in maintaining buds in dormancy. Ecodormancy refers to the subsequent period, in which tree buds only need sufficient exposure to warm temperatures to resume growth. The same authors mention that in this phase a series of physiological processes occur that prepare the plant to resume growth in spring, such as the restoration of communication and transport channels between plant cells; the callose plugs generated at the beginning of dormancy are eliminated, allowing transport through the phloem and xylem. Phytohormone levels are reorganized with the decrease in ABA, while other growth-promoting phytohormones increase their concentrations.
Figure 1. Phenological appearance of sweet cherry buds throughout winter dormancy: (a) establishment of dormancy (BBCH 93, start of leaf fall), (b) endo- and eco- dormancy (BBCH 00 for dormant vegetative bud and BBCH 50 for dormant floral bud), and (c) resumption of growth after release from eco- dormancy (BBCH 01 for start of vegetative bud swelling and BBCH 51 for inflorescence bud swelling). (Fadón, 2020).
There are a wide variety of factors that must be analyzed in order to understand the responses in the sprouting and the processes that support the flowering of the crop. It is in this sense that it is necessary to be able to obtain objective information regarding the management that is being carried out in the post-harvest stage; one of the ways is to be able to determine the level of reserves that are available, especially in the fruit centers (FC), and that in effect will be the source of immediate reserves that would be used for the initial flowering processes, with carbon reserves playing an important role at that time. As previously mentioned, the dormancy period plays a fundamental role in making the reserves available, if there were actually any, and where different mechanisms are combined.
The base index to be able to count the accumulation of cold is the degree of leaf fall and/or yellowing of these, where at least 50% of fallen/yellow leaves should be obtained on May 1, a theoretical consideration to be able to count cold. In recent studies it can be inferred that the month of May can be relevant in terms of the accumulation of these cold units (UF) with respect to June, when the latter is deficient in this item, being that the "reception" of these UF begins from the first moment of the month of May. A practical exercise is what happened last season, where for example in the area of Requínoa, O'Higgins region of Chile, where a total of 528 hours of base cold of 7.2 °C (HF) were accumulated; Assuming that the plant was “receptive” to accumulate HF (in terms of its leaf fall since mid-May), it would have accumulated 115 HF, being around 20% of the total accumulated.
Dormancy-breaking strategies should be primarily aimed at maximizing the crop's productive potential as the first objective and, consequently, the climatic zone is assigned the added value in its marketing with respect to its "prime" fruit. From this point of view, Hydrogenated Cyanamide (HC) applications should be aimed at ensuring this productive potential with the highest amount of accumulated HF according to each variety.
In many cases it is pointed out that CNH has an effect of “compensating for the lack of cold”, a term that must be clarified and pointed out that its mechanism is through the generation of a high degree of stress inside the bud that would cause the breaking of the bud (budding) as a response. Thus, CNH acts by inhibiting the enzyme catalase, which is responsible for the decomposition of hydrogen peroxide (H2EITHER2) in water and oxygen, which would detoxify the latter through a series of complex reaction sequences that would increase metabolism and yolk rupture. In effect, there is a synergy in the endogenous metabolic processes caused by exogenous conditions (environmental conditions) that merge at an opportune moment to establish these strategies.
In the context of being able to understand the importance of a correct date for CNH applications, a study was carried out in the 2021-22 season (Table 1), where CNH applications were compared on 6 different dates, using a concentration of 2% of commercial product, in Lapins cultivar, located in the Sagrada Familia area, Maule region (Table 1), recording the cold accumulation at the time of each application and subsequently measuring productive and fruit quality indices.
Table 1. Productive and quality indices in the use of CNH at different application dates in cvs. Lapins.
In terms of production and quality, it was observed that, for the Lapins variety for that particular season, the best application date was 14.07.21, obtaining better results in production and quality indices. This research showed that CNH applications on certain dates could favorably influence the productive potential of the orchard. This could be clearly shown in cv. Lapins, since early CNH application dates negatively influenced the productive potential of the indicated treatments. This may be due to the fact that the HF that could be accumulated in the last part of July is eventually underestimated and that, in effect, would also affect finding lower temperatures after applications prior to budding, this in contrast to slightly later application dates that would precisely have this characteristic.
It is important to mention that the study was carried out in a season in which the accumulation of HF had a different dynamic, and that when compared to the last season 2023-24 with 475 HF vs 715 HF with 2021-22, dormancy-breaking strategies must indeed be adapted from another perspective.
Considering that the accumulation of winter cold has dissimilar dynamics in each season, Table 2 shows records of HF accumulation, Degrees base 10 (GDA10) and production (kg/ha), in the town of Requínoa, in the O'Higgins region, for the cv. Santina. Although, in 2020-21 and 2022-24 they recorded similar HF accumulations, the difference was probably marked by the spring conditions in the GDA base 10 in the first phenological stages of the crop, and consequently the impact caused by the flight of bees (VA) on the pollination of the species. In the analysis, it is interesting to mention that in the 2022-23 season, a low accumulation of GDA was observed compared to 2023-24, but an accumulation of over 900 HF was recorded, which could be deduced from the fact that a very good accumulation of cold could compensate for a bad spring in terms of production.
Table 2. Accumulation of chilling hours, base degree days 10 and production for a Santina/Colt farm in four seasons for a field located in Rengo, O'Higgins region.
*1 GDA base 10 only until October 21st.
By carrying out a similar analysis in the town of Romeral in the Maule region (Table 3), it can also be inferred that the climatic conditions of HF and GDA10 could have an important weight in terms of productive indices in the 2020-21 vs 2023-24 seasons, which reinforces the idea that environmental conditions in spring were inadequate in the last productive season.
Table 3. Accumulation of chilling hours, base degree days 10 and production for a Santina/colt farm in four seasons for a field located in Romeral, Maule region
Previous studies have shown a positive correlation between HF accumulation during the crop's latency period, however, it is important to mention that the analysis must be addressed with other variables that are highly important and determine productivity; GDA10 conditions, bee flight, the stress index to which the crop was exposed and the time/amount of rainfall are some of the most important factors.
Consequently, and based on the studies carried out, it is necessary to analyse the strategies that should be adopted to maximise the productive potential of the species for the current season; that is, to obtain precise information regarding the environmental conditions and productive potential for each variety, adapting it to each reality, which would allow reducing the risk inherent to fruit production.
Bibliographic References
- Bennett JP 1949. Temperature and bud rest period. California Agric. 3(9): 12.
- Campoy, J., Ruiz, D., and Egea, J. 2011. Dormancy in temperate fruit trees in a global warming context: A review. Scientia Hortícolae, 130(2), 357–372.
- Fadón, E., Fernandez, E., Behn, H., and Luedeling, E. 2020. A Conceptual Framework for Winter Dormancy in Deciduous Trees. Agronomy, 10(2), 241.
- Fishman, S., Erez, A., and Couvillon, GA 1987. The temperature dependence of dormancy breaking in plants: Computer simulation of processes studied under controlled temperatures. Journal of Theoretical Biology, 126(3), 309–321.
- Lang, G., Early, J., Martin, G., and Darnell, R. 1987. Endodormancy, paradormancy, and ecodormancy – Physiological terminology and classification for dormancy research. HortScience 22, 371–377.
- Lemus, G. 2005. Cherry cultivation. INIA – Institute of Agricultural Research bulletin. no. 133. [Online]. Retrieved from: < https://biblioteca.inia.cl/handle/20.500.14001/7061>. Accessed on April 4, 2024.
- Richardson, E., Seeley, S. and Walker, D. 1974. A model for estimating the completion of rest for “Redhaven” and “Elberta” peach trees. HortScience 9(4):33 1-332.
- Weinberger, J.H., 1950. Chilling requirements of peach varieties. Proc. Am. Soc. Horticul. Sci. 56, 122–128.
