Risk of Extreme Early Frosts in Almond

Abstract

In almond, late frost is the main factor determining the viability of the culture in cold areas, and late flowering remains the key factor in addressing this issue. Climate change is causing an increase in the frequency of extreme weather events. In 2021, the cold period in Spain after the Filomena storm (11–14 January), in which temperatures reached −20 °C in some regions, was a clear expression in this regard. In many almond-growing areas, these low temperatures affected the closed flower buds of most of the cultivars, except for the extra-late and ultra-late cultivars and despite the early stage of development of the buds, leading crop failure. In this work, we show that early extreme frost only affected the extra-early, early and late almond cultivars that had overcome their endodormancy and fulfilled 40%, 31% and 23% of their heat requirements for flowering. Extra-late (with only 2% of their heat requirements covered) and ultra-late (still in endodormancy) cultivars were not damaged. These results show the importance of cultivating extra- and ultra-late almond flowering cultivars to avoid not only the late frost but also the early frost during the very early development of the flower buds.

1. Introduction

Frost is one of the greatest threats endangering the viability of Prunus species, almond (Prunus dulcis (Mill.) D.A.Webb) included. Damaging frosts, which occur in 20–30% of cultivation years, can severely affect crop viability, making crops unprofitable. Although differing levels of frost resistance between cultivars have been reported [1,2], the most important factor for frost avoidance is late flowering time [3]. Indeed, even small flowering delays when frosts can cause serious injuries (e.g., the end of winter or beginning of spring) can be crucial to ensure profitable production. Due to this, extra- and ultra-late flowering cultivars are key players in the colonization of those areas that suffer from extremely low temperatures throughout these periods [4].

At the end of the summer, almond enters into a dormant period known as endodormancy, which reaches a peak in late October or early November. Endodormancy is a protector state that allows the survival of the tree against the cold temperatures of winter [2,5]. This process is mainly governed by the flower or the vegetative bud itself, inhibiting their growth even under suitable conditions [6,7]. Endodormancy development does not depend on the leaf fall or the photosynthetic rate [8]. However, the time and depth of the endodormancy peak vary according to the cultivar [9].

Once trees reach this peak, they start accumulating chill up to a certain cultivar-dependent amount, known as the chilling requirement (CR) [2,10]. When the CR is fulfilled, trees achieve endodormancy release, passing into the ecodormancy stage [2]. The CR is therefore the most important factor to determine whether a cultivar is eligible for a particular area or not.

In order to measure chill accumulation, different models have been used, the most important being the model dealing with the number of hours during which the temperature is below 7.2 °C [11], the Richardson (also known as the Utah) model [12] and the dynamic model [7]. The Richardson model has proven to be effective for cold areas. This model shows that endodormancy release is promoted by temperatures between 1 and 12 °C and is primarily effective around 7 °C. Temperatures below 1 °C or between 12 and 16 °C are inefficient, whereas values over 16 °C counter endodormancy release [12]. Endodormancy release takes place once buds have reached the B-C state defined by Felipe (1977), in which a small green tip appears on the top of the bud (Figure 1) [13].

Once the CR has been satisfied, trees need to accumulate heat to flower. The heat accumulation is usually quantified using growing degree hours (GDH) [14]. As in the case of the CR, every cultivar has its own heat requirement (HR) after endodormancy release, which must be met in order to flower. In contrast with the CR, the HR presents smaller variations between cultivars and is not as decisive in determining flowering time as the CR [2,15].

As stated above, frost is one of the main threats that affect almond productivity in cold areas; thus, proper cultivar selection is essential to decrease risks of frost. Late flowering time diminishes the probability of frost injury [16]. In terms of breeding, it is desirable to obtain extra- and ultra-late flowering cultivars for these areas.

Many studies have pointed out that frost resistance is mainly related to flower bud development (Figure 1) [17]. Deeply endodormant flower buds are exceptionally resistant (the A state of the Felipe classification [13]), withstanding temperatures of up to −20 °C. The development of the flower bud reduces the resistance to frost, the swelled bud (the B state of the Felipe classification [13]) being less resistant than the deeply endodormant bud [18]. Likewise, flowers and young fruits are extremely susceptible to frost, suffering damage at temperatures below −1 or −2 °C depending on the exposure time. In such states, a couple of hours at these temperatures can produce serious damage and even ruin production for the year [19].

Although most known frosts occur during and after flowering time, there are still slight probabilities of frosts just after endodormancy release. Nevertheless, temperatures during this period are not usually low enough to reach the frost point and damage the tissues (−14 °C) [18,20].

The main objective of this work was to explain the different behaviors of almond cultivars during extreme frost events occurring close to endodormancy release dates in relation to their CR and flowering time.

2. Materials and Methods

The plant material consisted of five representative groups of almond cultivars, selected based on their flowering time and covering all the flowering range in almond: extra-early (represented by Desmayo Largueta), early (Marcona), late (Ferragnès), extra-late (Penta) and ultra-late (Tardona). At least 10 trees from each cultivar were observed to determine the effects of the frost.

The study was carried out in the experimental field of “Las Tiesas” in Barrax (Albacete, Spain; latitude: 39.048755° N, longitude: 2.082894° W) during the cold snap following the Filomena depression (January 2021). Hourly temperatures from early September (2020) to the end of April (2021) were recorded. The frost period took place during the night from 11 to 14 January. In this period the lowest temperature was recorded during the night of 12 January, in which temperatures went down to −20 °C.

Chill accumulation in the experimental orchard started on 30 September. It was measured using the Richardson model, which has been successfully used in the cool temperate zone to determine chill accumulation [21]. This model quantifies the amount of chill accumulated during each hour, establishing equivalence between the hourly temperature and chill accumulation, this equivalence being measured in chill units (CU). In addition, the CRs of the studied cultivar groups have been widely described using this model. The CR for the endodormancy release of each cultivar was established according to previous work on almond in this and other fields (

Table 1. Chill requirements (CU) for endodormancy release and the endodormancy release date for each cultivar group.

Cultivar Group	CU	Date
Extra-early	270	21 November
Early	426	1 December
Late	558	8 December
Extra-late	880	29 December
Ultra-late	1100	25 January

) [9]. The endodormancy release date for every cultivar group was determined as the day on which the CR was fulfilled.

Moreover, heat accumulation, from endodormancy release to flowering, was calculated using the Anderson GDH model [15], which allowed us to determine the percentage of the heat requirement met at the time of extreme frost (12 January).

3. Results

3.1. Bud Fall

Cultivar monitoring showed that flower buds from extra-early, early and late cultivars suffered great damage, despite apparently being in an early developmental state on 12 January. At the beginning of February, all the flower buds from these cultivars fell from the tree (100% damage) except for the late cultivars, which retained a few buds (95% damage). These damages were not observed in the extra- and ultra-late cultivars, where flowering and fruit setting were normal (0% damage). Vegetative buds did not suffer any damage in any cultivar.

3.2. Chill Accumulation and Endodormancy Release

Extra-early cultivars reached endodormancy release on 21 November (270 CU), early cultivars on 1 December (426 CU), late cultivars on 8 December (558 CU), extra-late cultivars on 29 December (880 CU) and ultra-late cultivars on 25 January (1100 CU) (Table 1, Figure 2). Regarding these results, it is obvious that the frost took place after the expected endodormancy release date of the extra-late cultivars, when ultra-late cultivars were still endodormant.

3.3. Heat Accumulation and Flower Bud Development

The forecasted flowering date (50% opened flowers) according to cultivars’ CRs and HRs set the full blossom date as 24 February for the extra-early cultivars (6038 GDH), 2 March for the early cultivars (6681 GDH), 16 March for the late cultivars (7466 GDH), 5 April for the extra-late cultivars (7181 GDH) and 15 April for the ultra-late cultivars (7892 GDH) (

Table 2. Heat requirements (GDH) needed to flower, percentages of heat requirements met on 12 January (frost date) and flowering date for each cultivar group.

Cultivar Group	GDH	Percentage of Heat Requirement Met on 12 January	Predicted Flowering Date
Extra-early	6038	40	24 February
Early	6681	31	2 March
Late	7466	23	16 March
Extra-late	7181	2	5 April
Ultra-late	7892	0	15 April

). By the time of the frost (12 January), the extra-early, early and late cultivars had satisfied 40, 31 and 23% of their HRs, respectively. Extra-late and ultra-late cultivars fulfilled just 2 and 0%, respectively (Figure 3).

4. Discussion

Bud Fall and Heat Accumulation

Among the many elements involved in flower development, heat accumulation after endodormancy release is the main factor for the development of flower buds towards flowering during ecodormancy [14,22]. This development produces a progressive loss of resistance against cold, making the flower buds more susceptible to damage by frost [19,23,24,25].

In recent decades, spring frosts induced by climate change have been the greatest threat for almond in cold areas [23]. In this study, we report how extreme early frost ruined the production of all the extra-early-, early- and late-flowering cultivars in a cold area of Spain, whereas extra-late and ultra-late cultivars remained intact.

Calculation of GHD values after endodormancy release showed that extra-early-, early- and late-flowering cultivars affected by the frost had already met 40, 31 and 23% of their heat requirements, respectively. On the other hand, extra-late cultivars accumulated almost no heat, while ultra-late cultivars had not even reached endodormancy release yet.

These findings agree with those of a previous study, which showed that almond flower buds rapidly lose their cold resistance once flower buds have reached the C state, resisting temperatures of up to −11 °C (Figure 1) [18]. Nevertheless, due to the extremely low temperatures during the frost of 12 January 2021 (−20 °C), only the cultivars that had not developed their flower buds were able to flower and set fruit.

The results of this study demonstrate that extreme temperatures from climate change endanger almond production more than expected. However, extra-late- and ultra-late-flowering cultivars, which are the main solution for late spring frosts due to their flowering time, may also be the solution to avoid the early frosts that threaten this crop because of their late endodormancy release [26]. Moreover, extra- and ultra-late cultivars, such as Penta, Makako and Tardona, have been shown to solve this issue, resulting in optimal flowering and production in both cold and warm years [4,16].

5. Conclusions

In this work, we aimed to study the effects of the early frost that took place after the Filomena depression on cultivars with different CRs. Among all the cultivars, extra- and ultra-late cultivars were the only ones that escaped the effects of severe early frosts when the flower buds were still undeveloped. This means that these kinds of cultivars must always be selected when designing a new plantation in cold areas with risks of early frost.

Future work should focus on detecting whether temperatures between −20 and 0 °C are still able to damage flower buds in the early–mid stages after endodormancy release, as well as on establishing equivalence between the different chill accumulation models.