Frost hardiness of flower buds of 16 apricot cultivars during dormancy

One of the most important risk factors in apricot growing is the frequent occurrence of frost during dormancy and flowering periods (Szymajda et al., 2013; Szalay et al., 2019; Yang et al., 2021). Frost damage occurs mainly during the blooming period, which is between middle of March and end of April in Hungary, when temperatures drop below freezing point at night for a few days in the main production areas. Although flower buds may survive a short period of freezing temperatures, the longer they are exposed and the lower the temperature, the greater the risk of lethal damage. In recent decades, winters have become milder, with higher average temperatures, but with sudden temperature changes (Bakucs et al., 2020). There are also examples of frost damage to the flower buds in January and February due to the accelerated dehardening before the blooming. In addition, a slight shift towards earlier blooming is expected by mid-century (Korsakova et al., 2023). Therefore, frost hardiness is an important factor, both during the dormancy and the blooming periods, and a selection aspect in the breeding of apricots (Krška and Zdenek, 2016; Sun et al., 2018) and other temperate zone fruit species (Butac et al., 2013; Ikase, 2015) as well.

Frost hardiness can be assessed through direct and indirect methods. Indirect methods, such as electrical conductivity measurements, starch and sugar content analysis (Yun et al., 2014), and differential thermal analysis (DTA) (Kaya et al., 2018) are only capable of estimating frost hardiness. The two direct methods for assessing frost damage are on-site evaluations and artificial freezing tests. In artificial freezing tests, the flower buds or other plant tissues/organs are examined directly after the freezing treatment (Salazar-Gutierrez et al., 2016; Jha et al., 2019). The discolouration of flower bud tissues caused by freezing injury can be observed visually (Lenz et al., 2013; Moghadam et al., 2015), which is considered reliable and is widely used (Kishimoto et al., 2014; Salazar-Gutiérrez et al., 2014).

Field observations have already shown that there can be significant differences in the frost hardiness of flower buds between different apricot genotypes during the dormancy and blooming periods (Szalay et al., 2021). However, the accuracy of the results is limited due to the variability of environmental factors. As a more objective method, artificial freezing tests are more appropriate to provide us with the data on the frost hardiness capabilities of different apricot cultivars (Gusta et al., 2003). The essence of this method is exposing plant parts to artificial cold and evaluating the damage incurred. By subjecting the plant parts to varying experimental temperatures, frost damage can be analysed using a sigmoid curve, which determines the frost resistance of the specific organ of the plant at that given time. To characterise frost resistance, the LT₅₀ value, which corresponds to the 50% value of the sigmoid curve, is typically used (Lappi and Luoranen, 2018).

Flower bud development can be divided into paradormancy, endodormancy and ecodormancy phases (Melke, 2015; Fadón et al., 2020). Environmental factors have a significant impact on the development of flower bud and subsequent blooming periods (Viti et al., 2010), which also play a crucial role in determining the frost tolerance of flower buds (Szalay et al., 2022).

In temperate regions, spring frosts are a significant cause of yield loss, and subsequent economic losses for agriculture (Snyder and Melo-Abreu, 2005). To prevent or reduce the damage caused by spring, frosts multiple methods are in practice. However, heaters, air-mixers, fogging and irrigation are less desirable because they have high installation and operational expenses. Delaying the blooming using various chemicals (ethephon, antifreeze proteins, abscisic acid, etc.) may be effective, but excessive or untimely use can cause bud and flower drop or reduced fruit set (Mertoglu and Evrenosoglu, 2017). However, selecting and breeding of late-blooming and cold-tolerant cultivars appear to be the most efficient and sustainable long-term solution (Khadivi-Khub and Khalili, 2017; Prudencio et al., 2018).

In this study, over a period of 3 years between 2019 and 2023, we conducted regular artificial freezing tests to assess the frost hardiness of the flower buds of 16 apricot cultivars. The aim of this study is to provide growers with valuable information regarding cultivars and site selection, as well as supporting their decision-making process regarding the necessity of implementing a high-cost frost protection method under specific circumstances. Through determining the frost hardiness profiles of the selected cultivars, we aimed to contribute to the advancement of the cultivation industry.

Monthly observations of frost damage percentage on flower buds were carried out for each cultivar following artificial freezing tests. The LT₅₀ values were calculated via spline regression method, as shown in Figure 2.

Figure 2.

Figure 3 - Figure 5 display the frost hardiness profiles of the 16 studied cultivars during the dormancy period for each year of the experiment, based on LT₅₀ values of their flower buds. The first stage of the profile is the hardening, which begins before leaf fall, in response to shorter days and decreasing temperature, and continues until midwinter, during which time frost resistance of the flower buds gradually increases. The second stage of the profile is dehardening. During this stage, the frost resistance of the flower buds is gradually decreases, until blooming occurs. The turning point between these two stages typically falls in December or January, depending on the winter climatic conditions. The LT₅₀ values of the flower buds are observed through artificial freezing tests.

Figure 3.

Figure 4.

Figure 5.

During the dormancy period of 2019/2020, on 15 October, the LT₅₀ values of the flower buds of the examined cultivars ranged from −10.6°C (‘Petra’, ‘Tsunami’) to −14.6°C (‘Harlayne’). Subsequently, from late December to early January, the frost hardiness of the flower buds gradually increased. The LT₅₀ values corresponding to the most frost-hardened flower buds were measured in December (12 cultivars) or January (4 cultivars), indicating a value range between −17.5°C (‘Primaya’) and −23.7°C (‘Harlayne’). During the latter half of the winter season, there was a gradual reduction in the frost resistance of flower buds. On 15 February, the latest sampling date, the LT₅₀ values ranged from −9°C (‘Petra’) to −17.6°C (‘Harlayne’).

During the 2021/2022 dormancy period, LT₅₀ values of the flower buds of the examined cultivars were measured on 15 October ranging from −7.5°C (‘Sweet Red’, ‘Primaya’) to −10.5°C (‘Harlayne’). In December, the flower buds reached their most hardened state with only ‘Bhart’ and ‘Bergarouge’ showing an exception. During this time period, LT₅₀ values were between −16.5°C (‘Sweet Red’, ‘Primaya’) and −19.6°C (‘Rózsakajszi C.1406’). On 15 February, measured LT₅₀ values fell between −8.9°C (‘Primaya’) and −12°C (‘Rózsakajszi C.1406’).

Similarly, in the dormancy period of 2022/2023, on 15 October, LT₅₀ values of the flower buds of the examined cultivars ranged from -10°C (‘Sweet Red’) to −13.3°C (‘Rózsakajszi C.1406’). The LT₅₀ values for the highest frost hardiness of the flower buds gauged for all cultivars in mid-December. The recorded values ranged from −16.6°C (‘Aurora’) to −20.2°C (‘Rózsakajszi C.1406’, ‘Harlayne’). The last date of measurement was 15 February, when the LT₅₀ values ranged from −4.2°C (‘Sweet Red’) to −10.6°C (‘Rózsakajszi C.1406’).

The daily minimum and maximum temperatures (°C) were recorded from the beginning of September to the end of April in each year. The measured values are shown in Figure 6.

Figure 6.

Cultivars were classified according to their frost hardiness ability using the results from artificial freezing tests performed over 3 years. Hierarchical cluster analysis, employing Wards method, was used for the classification, for the 3 years separately and together. The dendrograms of the cluster analysis are illustrated in Figure 7. Four distinct groups (G1–G4) are identifiable on each dendrogram. G1 corresponds to apricot cultivars that are extremely sensitive to frost, while G2 pertains to those that are slightly susceptible to it. Cultivars with moderate frost resistance fall under G3, and G4 comprises cultivars that display the highest resistance to frost. The classification of the 16 apricot cultivars in terms of their frost hardiness ability is presented in Table 1.

Figure 7.

Temperature may be the most important environmental factor affecting the hardening/dehardening process and the frost hardiness of the overwintering organs (Quinones et al., 2020). This is due to its role in cold acclimation/deacclimation and meeting the chilling requirements for the release of endodormancy (Bartolini et al., 2020). However, frost hardiness can be impacted by various factors, such as cultivars (Dumanoglu et al., 2019), rootstocks (Lichev and Papachatzis, 2006), health condition of the trees, irrigation and other technological methods (Bálo et al., 2005). Frost hardiness is acquired through cold acclimation in response to low non-freezing temperatures (Chinnusamy et al., 2007). This complex process involves changes in gene expression, membrane modification (Orvar et al., 2000), accumulation of soluble sugars (Ma et al., 2009), amino acids and antioxidants, and a decrease in water content. However, deacclimation can occur due to rising temperatures in autumn and early winter, resulting in a loss of freezing tolerance (Vyse et al., 2019). Related to these metabolic processes, several environmental factors, such as temperature fluctuations, fertilization and nutrient content in soil, precipitation and irrigation, crop load, and sunlight exposure can modify the frost hardiness of a cultivar during a certain dormancy period. Therefore, the frost tolerance of individual cultivars varies from year to year due to this complex background. Several years of testing are required to determine the frost tolerance characteristics of a particular cultivar.

Although frost damage is a common issue in apricot cultivation in many places, there is limited literature on the frost resistance of overwintering organs of apricot cultivars. Studies have reported continuous variation in frost resistance during dormancy and significant differences between cultivars (Gorina and Korzin, 2016; Szalay et al., 2016). Most studies on the topic of frost hardiness of apricot flower buds focus on the periods of budburst and flowering (Gunes, 2006; Szalay et al., 2019; Nesheva and Bozhkova, 2021). However, there is a significant risk of frost damage at the end of dehardening, between the end of endodormancy and the start of flowering (Julian et al., 2007), when the flower organs are the most sensitive to frost damage. Our results confirm this statement. During January and February of 2022 and 2023, there were several nights when the minimum temperature recorded in the field closely approached the LT₅₀ values obtained from artificial freezing tests (Figure 6B and Figure 6C).

In this study, we examined the frost hardiness of flower buds in 16 apricot cultivars during three dormancy periods. We focused solely on the genetic abilities of the cultivars, excluding other factors. The experiment was conducted at a single site, with consistent soil and the weather conditions, and all cultivars were grafted onto the same rootstocks. All trees were cultivated using the same integrated technology, which included maintenance pruning, fertilization, and crop protection to maintain their good health. Fruit thinning was only applied in rare cases of overbearing, although most years experienced serious frost damage during blooming. A frost hardiness has been outlined for each tree, based on LT₅₀ values provided by artificial freezing tests.

Recent experiments on the cell death points (CDPs) of apricot flower organs have shown that the receptacle is the most sensitive organ, while the pistil is the least sensitive one (Kaya and Kose, 2019). The organs of the pistil were found to have a significantly higher content of minerals and amino acids as compared to the organs of the receptacle. Furthermore, a negative correlation was found between the N, K, Mg and glutamate contents of the flower organs and their CDP value, suggesting a potential positive effect on frost tolerance (Kaya et al., 2021). Upon close examination of the entire flower bud, it was observed that the ice formation initially occurred at the base of the bud axis and then ice spread throughout all of the flower organs (Kuprian et al., 2016). During the evaluation of frost damage after the artificial freezing tests, it was observed that in most cases, only the receptacle was damaged by the frost. However, there were also instances where only the pistil was damaged as shown in Figure 1D, which is consistent with the findings of other researchers (Meng et al., 2007).

The frost tolerance of the overwintering organs gradually increased during the hardening period and reached its maximum in late December or early January. During the dehardening period, the frost tolerance of the organs decreased gradually. The variation in frost hardiness among cultivars was less considerable in October, averaging a difference of 3.43°C between the least and most frost-tolerant cultivars over 3 years. This difference slightly increased each month with 3.97°C, 4.43°C and 5.07°C in November, December and January, respectively. The highest difference was observed in February with a variation of 6.03°C.

The LT₅₀ values of the flower buds of the studied cultivars varied in each year, indicating the significant role of winter temperature in hardening and dehardening and thus the development of frost hardiness ability of the cultivars. When comparing the LT₅₀ values for each cultivar separately in each month across the 3 years, the differences ranged from 2.8°C (‘Petra’) to 5.1°C (‘Bergarouge’) in October and from 2.3°C (‘Kurezia’) to 6.3°C (‘Harogem’) in November, while from 0.6°C (‘Primaya’) to 5.8°C (‘Bergarouge’) in December and from 3.0°C (‘Petra’) to 8.1°C (‘Bergarouge’) in January, finally from 1.9°C (‘Petra’) to 7.2°C (‘Harlayne’, ‘Goldrich’) in February. Due to the mild winter weather, the cultivars did not achieve their genetically possible level of frost hardiness, which is a common occurrence in other fruit species (Szalay et al., 2017; Vitasse et al., 2018).

Several authors have reported that apricot flower buds can survive extremely low temperatures even below −25°C. However, this requires a harsh winter without significant fluctuations in temperature, which is suitable for the strong hardening of flower buds (Istrate et al., 2013; Szymajda et al., 2013; Korzin et al., 2021). In our experiment, the lowest LT₅₀ temperature recorded was −23.7°C for the ‘Harlayne’ cultivar, which is known for its high frost resistance, in December 2019. During the experiment, one of the most frost sensitive cultivars, ‘Primaya’ recorded the highest LT₅₀ temperature of -17.5°C. This was the harshest winter of the 3-year experiment, resulting in a notable difference (–6.2°C) in the frost hardiness between cultivars. In the other 2 years, the winters were milder, resulting in smaller differences between the LT₅₀ values of the most frost-sensitive and the most frost-tolerant cultivars (–3.1°C and −3.6°C). Similarly, during the early hardening period in October and the late dehardening period in February, we observed smaller differences in LT₅₀ values during mild winters.

Based on the hierarchical clustering classification performed separately for each year, 8 out of 16 cultivars consistently belonged to the same group every year. These cultivars include ‘Tsunami’, ‘Sweet Red’ and ‘Primaya’, which were found to be very frost sensitive (G1); ‘Bhart’ and ‘Goldrich’, which were slightly frost sensitive (G2); ‘Harogem’, which was slightly frost resistant (G3); and ‘Rózsakajszi C.1406’ with ‘Harlayne’, which were very frost resistant (G4) every year. The frost hardiness ability of the cultivar varied from year to year: ‘Petra’, ‘Aurora’ and ‘Pinkcot’ were either very frost sensitive (G1) or slightly frost sensitive (G2); ‘Magyar kajszi C.235’, ‘Hargrand’, ‘Farbaly’ and ‘Bergarouge’ were either slightly frost sensitive (G2) or slightly frost resistant (G3); and ‘Kurezia’ were either slightly frost resistant (G3) or very frost resistant (G4).

However, the classification done for the 3 years combined has resulted in some changes to the groups: ‘Tsunami’, ‘Sweet Red’ and ‘Primaya’ are still highly susceptible to frost (G1), while ‘Petra’, ‘Aurora’ and ‘Pinkcot’ were classified as moderately susceptible to frost (G2). ‘Magyar kajszi C.235’, ‘Hargrand’, ‘Farbaly’, ‘Bergarouge’, ‘Goldrich’, ‘Bhart’ and ‘Harogem’ were classified as slightly frost-resistant (G3), while ‘Kurezia’, ‘Rózsakajszi C.1406’ and ‘Harlayne’ were classified as very frost-resistant (G4). These cultivars received similar frost hardiness evaluations based on measurements. ‘Harlayne’ was found to be frost hardy in Lednice, Czech Republic (Krška, 2018), ‘Goldrich’ exhibited some resistance to early spring frosts in the region of Plovdiv, Bulgaria (Nesheva and Bozhkova, 2021). ‘Aurora’ was found to be very frost sensitive, while ‘Bergarouge’ had good frost resistance in Belgrade, Serbia (Milatović et al., 2013). For other cultivars such as ‘Aurora’, ‘Farbaly’, ‘Hargrand’, ‘Harogem’ and ‘Tsunami’, estimation on frost hardiness can be based on measurements of blooming period or yield period (Yao, 2011; Milatović et al., 2018; Maglakelidze et al., 2021; Glišić et al., 2023).

Considering the impact of climate change and the increasing yield loss caused by frost damage, frost hardiness is a crucial factor in apricot breeding as well as for other fruit species in temperate zones. Hungary is well-positioned for apricot cultivation, but ecological conditions pose limitations, with winter and spring frosts being among the most significant risks. When planning an orchard, it is essential to consider and harmonise the frost hardiness of the selected cultivars with growing site conditions. Therefore, it is crucial to have adequate information on the frost hardiness of different apricot cultivars, which should be included in cultivar descriptions.

The cultivars ‘Magyar kajszi C.235’, ‘Hargrand’, ‘Farbaly’, ‘Bergarouge’, ‘Goldrich’, ‘Bhart’ and ‘Harogem’ were found to have slight frost resistance, while ‘Kurezia’, ‘Rózsakajszi C.1406’ and ‘Harlayne’ demonstrated very high frost resistance. This makes them a more suitable choice for commercial plantations and breeding programmes.