Impaginato 433 Adv. Hort. Sci., 2023 37(4): 433­449 DOI: 10.36253/ahsc­14381 The habit of strawberry flowering is the key for runner propagation, where the photoperiod is the main environmental factor ­ A review I.A.H. Al­Madhagi Department of Horticulture and Its Technologies, Faculty of Agriculture, Foods and Environment, Sana’a University, Sana’a, P.O. Box 1247, Yemen. Key words: Chilling, division, flowering, Fragaria x ananassa Duch, gibberellin, photoperiod, proliferation. Abstract: Despite the advancement of tissue culture in strawberry plant propa­ gation, the degree of elite for field cultivation depends on forcing the plant to produce runners. The strawberry flower habit [everbearing (EB), seasonal berry (SB), short­day (SD), long­day (LD), and day­neutral (DN)] defines the method of encouraging the plant to generate runners, since the formation of runners is mostly influenced by genetic factors before being influenced by environmental factors. Stolon production, which occurs as a result of vying for resources under certain environmental circumstances, is the reverse of blossoming. Therefore, any stimulus that encourages stolon formation and vegetative growth limits the development of flower buds, which is necessary for elite propagation. Long photoperiod, temperature, chilling hour, or cold storage, and plant growth reg­ ulators (PGR) are cited as these variables. Temperature has a significant impact on runner development, although the long daily photoperiod (LD) remains the most crucial component in runner induction. However, when LD interacts with other factors like temperature, cold storage, and gibberellins, its efficiency is increased. Thus, based on the cultivars and the seasonal climate of the geo­ graphical location, the best approach for strawberry propagation is identified by optimising the planting date for propagation or adjusting the propagation circumstances. 1. Introduction Strawberry (Fragaria x ananassa Duch) is a commercial crop grown worldwide for its nutritional and health benefits. Strawberries are con­ sumed as fresh fruit or juice, or processed industrially into jam used in a various of desserts such as candy, milk and ice cream. According to the FAO statistics service agency (FAO, 2021), strawberry production has increased significantly over the past half­century (from 1960 to 2021). The total global production in 2021 is estimated to be around 9,175,384.43 t, with a cultivated area of about 389,665 ha. China con­ (*) Corresponding author: isam.madhagi@gmail.com i.madhagi@su.edu.ye Citation: AL­MADHAGI I.A.H., 2023 ­ The habit of straw‐ berry flowering is the key for runner propagation, where the photoperiod is the main environmental factor ‐ A review. ­ Adv. Hort. Sci., 37(4): 433­449. Copyright: © 2023 Al­Madhagi I.A.H. This is an open access, peer reviewed article published by Firenze University Press (http://www.fupress.net/index.php/ahs/) and distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Competing Interests: The authors declare no competing interests. Received for publication 12 February 2023 Accepted for publication 1 August 2023 AHS Advances in Horticultural Science Review paper https://doi.org/10.36253/ahsc-14381 http://www.fupress.net/index.php/ahs/ http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ Adv. Hort. Sci., 2023 37(4): 433­449 434 tributes for approximately 37% to global production, while the Arab nations contribute only 6%, with Egypt, Morocco, and Jordan making up the majority. The development of strawberry production in var­ ious countries depends on the selection of the most suitable cultivars for their annual climate. Vegetative propagation is the ideal technique for strawberry propagation since it retains the mother’s characteris­ tics (Li et al., 2020). Strawberry nurseries, as a result, play an important role in the expansion of strawberry cultivation within a specific geographical area. It is critical for commercial plant production to select the best factors to stimulate the plant to produce run­ ners. In this review, the vegetative growth of strawber­ ry plants will be covered as a technique for contain­ ing blossoms and encouraging the plant to generate runners. Blossoming and the development of runners are mutually exclusive. Flowers must be controlled in the practical application of runner production, either by eliminating the flowers or by altering the environ­ mental conditions. Long photoperiods (DL) and high temperature (HT) are crucial for promoting stoloniza­ tion (Smeets, 1955; Smeets and Kronenberg, 1955; Went, 1957; Leshem and Koller, 1965; Smeets, 1980), which is related to increased gibberellin production (Tafazoli and Vince­Prue, 1978). In addition, cold storage of plants promotes the production of runners (Hamano et al., 2009; Watanabe et al., 2009; Al­mad­ hagi et al., 2018). Exogenous application of growth regulators such as gibberellins, cytokinins or their combination supports the development of the run­ ners (Kender et al., 1971). Long photoperiods (LD) are the most essential factors for runnering induction in strawberry, but its effectiveness is enhanced by its interaction with other factors such as temperature, cold storage, and gibberellins. Within this context, the primary goal of this paper is to present an overview of the factors that influence strawberry runner yield. Strawberry flowering habit and cultivars division Genetics is the primary component governing strawberry proliferation. Moreover, it pinpoints the best techniques as well as the coefficients of the propagation means. Since F. ananassa Duch, the cul­ tivar of the strawberry, is a hybrid plant, the varia­ tions in strawberry cultivars may be attributed to variations in its fundamental parents, F. virginiana and F. chiloensis, each of which has a unique bloom­ ing and runnering behaviour. F. chiloensis began to bloom before F. virginiana and its majority of the leaves stayed evergreen throughout the winter. In the meantime, genotypes of F. virginiana seemed to become dormant, and their leaves became brown and withered off in the late fall and winter, outper­ forming F. chiloensis for runner production (Darrow, 1966; Hancock et al., 2003). The capacity of the bud in cultivars to continue producing inflorescences throughout the growing season accounts for the variation in runner develop­ ment. According to other researches (Guttridge, 1985; Hytönen and Elomaa, 2011), this is connected to the differentiation of the meristem into a leaf rosette, also known as a branch crown or stolon. A branch crown serves as a platform for inflorescences, whereas a runner is a vegetative, extended shoot with a terminal daughter plant that may be employed for clonal multiplication (Samad et al., 2021). Based on physiological and production features, strawberry plants were split into various classes. As seen in figure 1, each group has a specific function, one for physiological characteristics and the other for productive attributes. Because they influence plant­ ing and harvesting dates, as well as seedling repro­ duction and harvesting techniques, physiological fea­ tures are important. For both fresh and processed products, the production features (qualitative and quantitative) are essential to satisfy local consumers’ needs and exporters’ demands. Fig. 1 ­ A list of different features that can be taken into account to describe a strawberry cultivar and eventually classify it. Strawberry cultivars are classified into physiological and productive traits, with physiological traits playing the most important role in determining runner performance and productivity. SD: short­day, LD: long­day, DN: day­ neutral, EB: everbearing, SB: seasonal berry. Al‐Madhagi ‐ Key of strawberry runnering 435 Based on how they react to the photoperiod for flower induction, strawberry cultivars are categorised as short­day (SD), long­day (LD), or day­neutral (DN) (Durner et al., 1984). The ideal method for plant mul­ tiplication, blooming behaviour, and fruit production is determined by this division, which is crucial. In the meanwhile, strawberry cultivars exhibit considerable addiction as a result of interactions between pho­ toperiodic temperature. This link has led to the inter­ change ability of the two terms when describing strawberry blossoming behaviour (Cai et al., 2017). Based on their respective production periods, strawberry cultivars were divided into two groups: everbearing (EB) and seasonal berry (SB). This divi­ sion may be thought of as an implementation of actual cultivar behaviour in response to temperature and photoperiod. While certain places of the world may only see one season of production from differ­ ent strawberry SB kinds, other locations may experi­ ence two seasons. As a result, the split of production cycles is inaccurate globally while being based on the same latitude (Table 1). The major gene that regulates blooming and run­ ner production is called Perpetual Flowering Runnering (PFRU) (Hytönen and Kurokura, 2020). Retentive, day­neutral (DN), continuous flowering, and long­day plants are all examples of everbearing (EB) cultivars (Cai et al., 2017). The words “day­neu­ tral” (DN) and “everbearing” (EB) are interchange­ able and refer to a physiological insensitivity to day­ duration in flower bud initiation and a realistic expec­ tation of strawberry producing. Weak, moderate, and strong day­neutral cultivars can be used to categorise the everbearing strawberry cultivars (Nicoll and Galletta, 1987). EB cultivars can produce multiple crops through­ out the year, regardless of day length, at a signifi­ cantly higher temperature than seasonal berry (SB) cultivars (Smeets, 1980). Everbearing (EB) strawberry cultivars, in contrast to SB strawberry cultivars, have been linked to early flowering and initiation at shoot tips, resulting in bet­ ter crown branching ability (Hytönen and Elomaa, 2011). As a result, EB strawberry cultivars tend to produce few stolons on a large scale (Darrow, 1966; Simpson and Bell, 1989; Dale et al., 1996), and fewer stolons than SB. Since branch crowns are ended by inflorescences, the quantity of branch crowns is essentially correlated with the quantity of inflores­ cences (Hytönen et al., 2004; Tenreira et al., 2017). In the meanwhile, branch crowns that form from buds in the leaf axils of the crowns of mature plants are divided to create economically viable EB straw­ berry plants. The EB strawberry cultivars are quanti­ tative LD plants at medium temperatures, day neu­ tral only at low temperatures (15°C), and qualitative LD plants at high temperatures (Pedraza et al., 2010; Samad et al., 2021). Short­day (SD) conditions, as seen in figure 2, cause EB cultivars to stop growing and become dwarfed throughout the summer season (Darrow and Waldo, 1934). For growth, bloom initia­ tion, and stolon formation in EB strawberries, a criti­ cal photoperiod of 15 hours at 18°C and 14 hours at 30/25°C day/night temperature is needed (Nishiyama et al., 2006; Sønsteby and Heide, 2007). Seasonal strawberry blooms (SB), which bloom in the spring and produce a fruiting crop in the summer, have been identified as short day (SD), once bloom­ ing, seasonal flowering (SF), seasonal berry (SB), sin­ gle crop, or June­bearing plants (Cai et al., 2017). In Table 1 ­ Shows the differences in runnering and cropping between Everbearing (EB) and Seasonal blooming (SB) strawberry varieties throughout the year Type Defined Cropping over the year Runnering Commercially propagated Everbearing (EB) Remontant A couple of crops Only non­ to a few runners Dividing of branch crowns day­neutral (DN) perpetual LD plants Seasonal flowering (SB) once flowering One fruit crop More runner From the plantlets seasonal flowering (SF) seasonal berry (SB) single cropping June­bearing short­day (SD) Adv. Hort. Sci., 2023 37(4): 433­449 436 response to the seasonally reducing photoperiod and temperature circumstances, SB strawberries begin to bloom in the late summer and fall, the year before blossoming and fruiting. The majority of SB cultivars are now regarded as facultative short­day (SD) plants as it has been established that they are mostly SD plants. At temperatures between 18 and 20 °C, they need SD for bloom induction, although at lower tem­ peratures, the majority of cultivars begin flowering on long days (LD) (Ito and Saito, 1962; Heide, 1977; Heide et al., 2013). The crucial photoperiod for SD induction is 14–15 hours (Darrow and Waldo, 1934; Konsin et al., 2001) and the minimum number of SD cycles required for induction depends on the cultivar (Heide et al., 2013). The flower­inducing effect of SD, on the other hand, is temperature sensitive, peaking at intermedi­ ate temperatures and decreasing rapidly at tempera­ tures above 21°C (Heide et al., 2013). 2. Propagation of strawberry SB strawberry plants are commercially propagat­ ed from plantlets that multiply from the runner nodes of mature plants because technique is quicker than seed propagation and daughter plants retain the traits of their mother plant (Li et al., 2020). This plantlet is often created by nodes borne by runners or stolons that sprout from buds in the crown’s leaf axils over the summer (Darrow, 1966). The runners often is elongated branch which have nodes and internodes running parallel to their length, with the bud at the first node usually being inactive (Ahmed and Ragab, 2003). It’s length is due to the cell divi­ sion and intermodal elongation in the plant are responsible for runner growth (Nishizawa and Hori, 1993). Therefore, one of the most important metrics is the number of runner and daughter plants pro­ duced by mature plants. 3. Environmental factors The environmental conditions are one of the most crucial factors impacting the generation of strawber­ ry runners. In all of the Fragaria genotypes exam­ ined, stolon formation and flowering induction can compete for space in the axillary meristems, and both developmental strategies are sensitive to envi­ ronmental factors (Brown and Wareing, 1965; Guttridge, 1985; Bradford et al., 2010; Hytönen and Elomaa, 2011; Heide et al., 2013; Hytönen and Kurokura, 2020). In response to any alteration in the environment that encourages flowering (flowering habit), the strawberry produces a crown or stolon (Fig. 2). The photoperiod, chilling periods, and tem­ perature are especially linked to these environmental or seasonal factors, and their interactions may have a major effect on strawberry dispersal (Andrés and Coupland, 2012; Salinas et al., 2017). Photoperiod Photoperiod is the duration of the daily exposure of an organism to illumination within hour (Cammack et al., 2008), it is defined as the period of time within a 24­hour time frame that light is available (Lanoue et al., 2019). One of the most crucial environmental factors for plants is light. Where, the plant is impacted by the length of the lighting period (photoperiod), the radia­ tion strength, and the type of illumination wave­ lengths (colours). Plants employ photosynthesis, a process that uses light as an energy source, to pro­ duce secondary compounds and carbohydrates. Additionally, photoreceptors produce light that is utilised to detect and keep track of environmental changes (Chen et al., 2004). When it comes to the photoperiodic control of flowering in wild strawber­ ries, phytochromes are crucial photoreceptors (Rantanen et al., 2014). The photoreceptors’ main module, the leaves, is capable of detecting a broad range of wavelengths, light intensities, and photoperiods. It controls the Fig. 2 ­ A schematic diagram shows the flower habit of strawber­ ry cultivars that flower all year round is the key to propa­ gation. Where: Everbearing (EB) behave as day­neutral (DN) plants at low temperature and are considered quali­ tative long­day (LD) at medium temperature and quanti­ tative long­day (LD) at high temperature. Seasonal Flowering (SF) behave like long­day (LD) at lower temper­ ature and like short­day (SD) at medium temperature. Al‐Madhagi ‐ Key of strawberry runnering 437 essential gene proteins that the plant’s developmen­ tal regulatory programme may use to transmit infor­ mation about timing and light (Valverde, 2011; Shim et al., 2016). Photoreceptors also enable plants to accurately monitor ambient light conditions and alter their development, morphology, and metabolic rates, including the start of blooming, in accordance with the particular environment in which they exist (Song et al., 2018; Roeber et al., 2022). FLOWERING LOCUS T (FvFT1) and SUPPRESSOR OF THE OVEREXPRESSION OF CONSTANS1 (FvSOC1), two significant genes in the photoperiodic regulation of blooming and runners in woody strawberries, have provided some information on the photoperiodic control of FvTFL1 in seasonal flowering woodland strawberry. FvTFL1 integrates photoperiod and tem­ perature signals to control flower induction, and higher FvFT1 mRNA levels are linked to earlier flow­ ering under a variety of environmental conditions including light quality, photoperiod, and tempera­ ture, while turning off this gene significantly delays flowering (Hytönen and Kurokura, 2020). The long­day photoperiod (LD) is one of the most important environmental elements affecting the growth and development of strawberries (Ito and Saito, 1962; Darrow, 1966; Heide, 1977; Okimura and Igarashi, 1997; Robert et al., 1999; Heide and Sønsteby, 2007; Al­madhagi et al., 2011; Hasan et al., 2011; Li et al., 2020). And in distinguishing strawber­ ry runner axillary buds (Hytönen et al., 2009). The effect of photoperiod on strawberry vegeta­ tive development and runner production has been widely discussed and has attracted the attention of numerous studies. Petioles length, leaf number, leaf area, and runner number and length all increase with LD photoperiod (Darrow, 1966; Sung, 1973; Plancher and Naumann, 1978; Nishizawa and Hori, 1993; Pipattanawong et al., 1996; Robert et al., 1999; Wiseman and Turnbull, 1999; Konsin et al., 2002; Serçe and Hancock 2005; Sønsteby et al., 2006; Hasan et al., 2011; Li et al., 2021 b). LD conditions promote cell division and cell elongation (Nishizawa, 1992; Nishizawa, 1994), due to an increase in the amount of endogenous gibberellins (GAs) that promote bud development in the plant (Taylor et al., 1994). The number of hours during the LD photoperiod that the plant must be urged to produce a runner depends on a variety of factors, including cultivars and temperature. The photoperiodic cycle of 10 h light and 10 h dark failed to develop runners, but 14 h light and 14 h darkness did, and runner plants growing in LD were induced to flower provided they remained connected to parent plants growing in SD (Hartmann, 1947). As shown in figure 3, the impact of photoperiod (P) on strawberry propagation depends on a number of different parameters, including culti­ var (C), cold storage (CS), temperature (T) and plant growth regulators (PGR). Photoperiod × cultivars (P×C) The number and length of runners was influenced not only by photoperiod but also by cultivars or genetics, and the interaction of the two (Pipattanawong et al., 1996; Serçe and Hancock 2005; Hasan et al., 2011). In terms of runner produc­ tion, the strawberry parents responded differently to the photoperiod. The study by Serçe and Hancock (2005) shows the different responses of the wild strawberry genotype, F. chiloensis ‘CFRA 0024’ (Central Chile) and ‘CFRA 0368’ (Alaska) and F. virginiana ‘Eagle 14’ (Ontario); they just found that only ‘Eagle 14’ and ‘CFRA 0368’ produced an appreciable number of runners while ‘Eagle 14’ did not show a consistent trend, while ‘CFRA 0368’ had the most runners under the 11 hour photoperiod. Clear photoperiod responses to vegetative devel­ opment and stolon production were observed in SB cultivars (Plancher and Naumann, 1978; Konsin et al., 2002; Sønsteby et al., 2006; Hasan et al., 2011), as well as on EB strawberry cultivars (Guttridge, 1969; Dennis et al., 1970; Serçe and Hancock, 2005). The difference between EB strawberry cultivars is Fig. 3 ­ The response of strawberries to the photoperiod (P) effect on runner development is influenced by a number of other parameters, including cultivar (C), cold storage (CS), temperature (T), and plant growth regulators (PGR). 438 Adv. Hort. Sci., 2023 37(4): 433­449 also evident: cultivars ‘Aromas’, ‘Tribute’, ‘Frederick 9’, and ‘Fort Laramie’ did not produce runners under either LD (16h) or SD (8 h), but ‘Quinalt’ produced 0.2 runner/plant under LD (16h) (Serçe and Hancock, 2005). Flower initiation and runner development occur independently of LD in EB strawberry cultivars (Piringer et al., 1958; Piringer and Borthwick, 1961; Guttridge, 1969; Dennis et al., 1970). There was also a difference between SB cultivars, where ‘Camaroga’ yielded the most plantlets (22.06 per plant) when grown under the 17 hour photoperi­ od (Hasan et al., 2011). According to Serçe and Hancock (2005), the LD photoperiod had a significant effect on runner pro­ duction, with a significant effect of an interaction between the SB cultivar and the photoperiod, with ‘Allstar’ and ‘Honeoye’ not producing runners in pho­ toperiod ranges of 8 ­11 h, and ‘Chandler’ producing runners in photoperiod ranges of 8 , 9, or 11 h. Strawberry SB developed more crowns than run­ ners during the SD photoperiod compared to the plant under the LD (18 h) photoperiod on the ‘korona’ SB strawberry (Konsin et al., 2001). The number of runners is also differs between cultivars, in strawberry ‘Seolhyang’ it increased dramatically after LD (16 h) (Li et al., 2020). Meanwhile, for SB ‘Camarosa’ and ‘Camaroga’ cultivars, there was no significant difference between the 15 h and 17 h LD photoperiods, and the LD (15 h) was determined to be extremely efficient (Hasan et al., 2011). Temperature (T) Strawberry flowering habits (EB or SB) have been found to be under either qualitative or quantitative genetic influence (Heide et al., 2013). Figure 2 shows the behaviour of strawberries under different tem­ perature conditions. Wherever, the value of 10°C is a base tempera­ ture of strawberries (Al­madhagi et al., 2018). In Fragaria vesca, higher temperatures were found to be critical for runner induction (Heide and Sønsteby, 2007). The temperature variation between day and night at the same average daily temperature is also significant in creation runners; in strawberry ‘Seolhyang’, 25/15°C day/night was the best temper­ ature for runners formation (Li et al., 2020). LT (11°C) and SD (10 h) enhanced branch crown growth in wild strawberry Fragaria vesca, but HT (>18°C) promoted runner initiation independent of photoperiod (Bedry, 2017). Regardless of temperature, all of the F. x ananas‐ sa EB cultivars showed very low runner counts. Temperature has an effect on runner formation in EB strawberries (Smeets, 1955; Smeets and Kronenberg, 1955; Serçe and Hancock, 2005). Controlling temper­ ature alone, as well as cultivating plants at varied temperatures, will not increase EB strawberry runner output (Samad et al., 2021). The EB cultivars responded differently to temper­ ature in terms of runner production, with the great­ est number of runner being 0.6 and 0.7 at 30°C in ‘Aromas’ and ‘Tribute,’ respectively. In contrast, below the T­ range of 18­30°C, neither ‘Ogallala’ nor ‘Quinalt’ produced a runner (Serçe and Hancock, 2005). A recent study Samad et al. (2021) found that run­ ner production in EB strawberries decreased signifi­ cantly at 20°C, with no statistical difference between 25 and 30°C. Runners was almost twice in EB straw­ berries ‘Murano’ than in ‘Favori’ during the season and it was significantly higher in the plants raised outdoors than in those raised in the greenhouse (Sønsteby et al., 2022). The explanation for this is that EB’s runner potential is cultivar dependant, and low temperatures (LT) put more energy into flower­ ing than runner development (Rivero et al., 2021 a; Sønsteby et al., 2021). Photoperiod × temperature (P×T) Overall, the effect of photoperiod on runner and crown production was influenced by temperature at the time of photoperiod application. LD and high temperature (HT) have been shown to improve run­ nering in all flowering classes of strawberry cultivars (EB or SB) (Serçe and Hancock, 2005). When the pho­ toperiod was 12 h or more and the temperature was above 10°C, runners began to multiply (Went, 1957; Darrow, 1966). The strawberry’s reaction to the photoperiod is affected by temperature (Darrow, 1936). Actual pho­ toperiod and temperature parameters varied by cul­ tivar (Went, 1957). The LD must exceed a certain value at HT for runner development in EB and SB strawberry cultivars (Darrow, 1937; Went, 1957; Smeets, 1980). Temperature and photoperiod pro­ mote runnering by inhibiting flower initiation and increasing the activation of vegetative buds on the rosette crown (Went, 1957; Leshem and Koller, 1965). Meanwhile, there was no runner development at SD LT, and LT at higher light intensities had to be suppressing flower initiation (Went, 1957). Strawberries can develop runners at a higher temper­ Al‐Madhagi ‐ Key of strawberry runnering 439 ature than at a lower temperature (Smeets, 1955; Smeets and Kronenberg, 1955; Went, 1957; Leshem and Koller, 1965; Smeets, 1980). The ideal temperature for strawberries varies by cultivars. Under both LD and SD photoperiods, straw­ berry cultivars differ in the optimal temperature for runner development, and the number of runners produced increased as the temperature rose from 20 to 26°C under 16 h LD, but decreased as the temper­ ature increased from 26 to 29°C (Bradford et al., 2010). EB produced many runners during the 15 h ­ 20 h LD photoperiod with a temperature of at least 22.7°C (Darrow, 1966; Rivero et al., 2021 a), no runners developed in ‘Marshall,’ at 10°C but did for 16 hours at 14°C, and for 12 hours at 17°C (Went, 1957). When the temperature dropped to 18°C, the LD photoperi­ od factor alone was sufficient for optimal leaf and inflorescence growth and development (Sønsteby et al., 2006). Photoperiod preconditioned plants pro­ duced significantly more branch crowns than control plants, but cold­stored tray­conditioned plants pro­ duced much fewer crowns (Sønsteby et al., 2006). A LD of at least 14 hours was required for runner production in 9 cultivars cultivated at 13, 16, and 21°C in EB strawberry cultivars, where the photoperi­ od LD up to 14 h being the key determinant, a specif­ ic temperature being required for a prolonged runner development duration (Darrow, 1936). Flower initia­ tion and runner formation in ‘Revada’ and ‘Rabunda’ occurred at 20 and 26°C regardless of the LD, and the length of runner formation was longer at 20 and 26°C than at 14°C, and at 16 and 24 h than at 8 h (Smeets, 1980). Runners are formed almost entirely in the vegeta­ tive phase of plant growth in SB cultivars, with LD × HT favouring runner production (Darrow and Waldo, 1934; Heide, 1977; Durner et al., 1984; Bradford et al., 2010). SB cultivar also behaves like EB plants at LT under LD circumstances (14 h) (Darrow and Waldo, 1934; Darrow, 1936). The Honeoye SB cultivar did not develop runners at 14 or 17°C, regardless of pho­ toperiod, and it did not produce runners under SD, independent of temperature (Bradford et al., 2010). In the number of runners of F1­hybrid ‘Delizzimo’ cul­ tivar was significantly higher at 26°C than at lower 12°C under both SD and LD conditions (Samad et al., 2022). Addition, higher temperatures increased the concentrations of sugars in the leaves in LD photope­ riod (Rivero et al., 2022). The EB trait can also arise when inflorescences are removed during the growing season, leading to the development of latent buds, as in the LD and HT traits causing flowering suppression (Sugiyama et al., 2004). As shown in Table 2, the runner formation rises at HT × LD in both SB and EB. Table 2 shows a rough summary of the influence of photoperiod and temperature interaction on the generation of runners. Despite the fact that the criti­ cal value of each variety is different, both groups agreed that the long day (LD) at high temperature is the best condition for the development of runners. Chilling hour and cold storage (C) Flower initiation in strawberries requires chilling (Ito and Saito, 1962; Darrow, 1966; Kinet et al., 1993; Lieten, 1997; Al­madhagi et al., 2018; Al­doubibi et al., 2021). For the best production and berry quality, both types of strawberries (EB and SB) required dif­ ferent amounts of chilling period before planting. The chill­hours are measured in degrees below than 5, 7, or 8°C (Yanagi and Oda, 1993; Risser and Robert, 1993; Bigey, 2002; Gallace et al., 2019). If the natural environment is not favourable, the refrigerator can be used to carry out cold treatments (Hamano et al., 2009). Chilling stimulates cell division and elongation by breaking dormancy (Lee et al., 1970; Yanagi and Oda, 1989). The effect of chilling (up to zero and less than 5°C) or cold storage (below zero °C) on runner develop­ ment has been connected to the type of strawberry (EB or SB), cultivars, degree of chilling, length of cold storage, and cumulative of natural chilling hours, according to the most recent study. Longer cold treatments (more than 500 hours) limit flower development (Taghavi and Aghajani, *SD (short­day) is less than 14 hours, LD (long day) is more than 14 hours, and DN (day­neutral) is 12 hours. √: producing runner, ×: non­producing runner. Low= less than 20°C. Table 2 ­ Interaction effect of photoperiod and temperature on runnering of strawberry Factors Cultivars Photoperiod Temperature EB SB SD* Low × × LD Low Some cultivars × DN Low × × SD high × × LD high √ √ DN high Some cultivars √ Adv. Hort. Sci., 2023 37(4): 433­449 440 2017), lead to the shorter flower differentiation (Lieten, 2006; Al­madhagi et al., 2018) and delay the re­initiation of fresh floral primordial in the spring (Guttridge, 1958; Gallace et al., 2019). Strawberry propagation could benefit from this approach. Chilling has been shown to increase runners gen­ eration in both EB and SB strawberry cultivars (Yanagi and Oda, 1990). Many runners were formed when the strawberry EB or SB cultivar was subjected to a lot of cooling hours (Bringhurst et al., 1960; Bailey and Rossi, 1965; Guttridge, 1969; Braun and Kender, 1985; Kahangi et al., 1992; Risser and Robert, 1993; Lieten, 1997; Tehranifar et al., 1998; Bigey, 2002; Hokanson et al., 2004; Taghavi and Aghajani, 2017; Al­madhagi et al., 2018). The sensitivity of the chilling duration varies between cultivars; SB strawberry cultivars are more sensitive than EB cultivars, and prolonged chilling inhibits blossom production in SB cultivars (Yanagi and Oda, 1990). After more than 1000 hours of chilling, EB culti­ vars formed runners (Hamano et al., 2009; Watanabe et al., 2009; Al­madhagi et al., 2018). Although the cultivar does not develop runners under normal con­ ditions, and does not produce runners when chilled for 0 h, 360 h and 720 h, long chilling hours (1080 h and 1440 h) in a cold room at 2°C will reduce the flower and promote more runner (Al­madhagi et al., 2018). For Japanese EB strawberry cultivars (‘Akihime’, ‘Askaruby’, ‘Sachinoka’, ‘Tochiotome’, ‘Toyonoka’, ‘Nyoho’, and ‘Yumenoka’), cold storage for more than 1000 hours interrupts dormancy, pro­ motes runner development, and increases leaf elon­ gation (Watanabe et al., 2009). The duration of the cooling period for current EB strawberries is related to the cultivars. Chilling tem­ peratures in EB cultivars start with runner develop­ ment in ‘Revada’ and ‘Rabunda’ cultivars that have not experienced natural hours of chilling (Smeets, 1980), as well as in ‘Rabunda’, ‘Ostara’, and ‘Kletter’ cultivars refrigated at 1°C for 1 and 2 months (Yanagi and Oda, 1990). Flowering degree and stolon production in EB strawberry cultivars Delizzimo and Favori had little or no effect when chilled at 2°C for six weeks (Rivero et al., 2021 a). Furthermore, for a one to four weeks of chilling at 1°C increased runners in the cultivar EB ‘Pajaro’, with no significant difference in the length of cold storage, while one or two weeks of cold storage resulted in a larger number of daughter plants (Taghavi and Aghajani, 2017). Longer cold storage duration improved runner production in the SB strawberry cultivars ‘Hokowase’ (Yanagi and Oda, 1990), ‘Korona’ and ‘Elsanta’ (Sønsteby and Heide, 2006), ‘Allstar’, ‘Chandler’, ‘Latestar’, ‘Northeaster’ and USDA selection B27’ (Hokanson et al., 2004) and ‘Sulhyang’ (Lee et al., 2020). SB strawberry cultivars stored chilled at 1°C for two months produced more runners than fresh plants that had never been exposed to cold (Hokanson et al., 2004). The degree of cold storage also influences runner quality. According to (Lee et al., 2020) Sulhyang’ plants held at ­5°C produced fewer daughter plants than those stored at ­2°C, and the quantity of daughter plants was modest. Long cold storage reduced vigour and glucose stores of mother plants (Lieten et al., 1995). Plants that have been stored cold for a long period should have a higher starch content and if possible, be culti­ vated in nurseries located at higher altitudes (López et al., 2002; Al­doubibi et al., 2021). On the other hand, naturally cool night­time temperatures at high­ er elevations help plants collect more starch. The quantity of chilling hours the plant experiences affects runner production; both insufficient chilling and excessive chilling have an effect (Hamano et al., 2009). Photoperiod × cold storage (P × C) As a result, exposure to prolonged photoperiods and longer cold storage duration improved runner production. The results of the previous study show that cultivars respond differently to photoperiod × cold storage and duration. After determining the cul­ tivar type (EB or SB), this interaction is linked to the length of cold storage LC and LD photoperiod (Sønsteby and Heide, 2006; Hamano et al., 2009; Watanabe et al., 2009; Rivero et al., 2021 a). Due to its insensitivity to the pre­chilling history and day duration, the EB strawberry ‘Rabunda’ showed con­ secutive flower development (Yanagi and Oda, 1989). Meanwhile, more efficient runners production can be achieved in EB strawberry cultivars by combining cold storage with LD photoperiod, where LD (16 h) increase runner production by about 10% in plants chilled at 4°C that for 1000 and 1500 hours in com­ parison to unrefrigerated ones (Watanabe et al., 2009). The same result was observed in EB strawber­ ry ‘Natsuakari’ and ‘Dekoruju’ treated with 1000 h chilling under 16 h LD (Hamano et al., 2009). EB strawberry cultivars ‘Natsuakari’ and ‘Dekoruju’ Al‐Madhagi ‐ Key of strawberry runnering 441 chilled for 1500 and 2000 hours (5°C) produced run­ ners above natural day length, but not below natural day length regardless of LD treatment. In contrast, after 5 and 10 weeks of precondition­ ing at 2°C no runners occurred under either LD 10 h or 20 h, while runners were common in SD, particu­ larly at 26°C and with 10 weeks of preconditioning (Rivero et al., 2021 a). Photoperiod enhanced the condition of Fragaria shoot cultures maintained at 4°C in SB strawberries (Reed, 2002). In SB strawberry cultivars ‘Korona’ and ‘Elsanta’, no cooling was required to re­establish nor­ mal leaf and inflorescence elongation and runner development under subsequent LD circumstances (Sønsteby and Heide, 2006). 4. Exogenous hormone Gibberellins Gibberellins are required for initiation of straw­ berry runners and inhibit GA production with PP333, AMO­1618, or prohexadione­calcium (Pro­Ca) (an inhibitor of the GA3­oxidase enzyme) (Rademacher, 2000), causes the formation crown branches and reduces runner development (Avigdori­Avidov et al., 1977; Nishizawa, 1993; Reekie and Hicklenton, 2002; Black, 2004; Hytönen et al., 2009; Grez et al., 2021). The GA20ox gene is mainly expressed in the axillary meristem dome and primordial, and in developing stolons. Runner less strawberries such as the wood­ land diploid strawberry (F. vesca) are caused by a mutation in the active site of a gibberellin 20­oxidase enzyme (GA20ox). As a result, GA3 stimulates run­ ners development in all genotypes and species of strawberries, including the EB types of F. vesca, F. vir‐ giniana , and the EB and SB of F. x ananassa (Agafonov and Solovei, 1972; Solovei, 1972 a; Verzilov and Mikhteleva, 1974; Soetarto, 1979; Choma and Himelrick, 1984; Braun and Kender, 1985; Deyton et al., 1991; Fouad et al., 1991; Ra et al., 1996; Dwivedi et al., 1999 a, b; Paroussi et al., 2002 a, b; Tenreira et al., 2017; Li et al., 2021 a; Godara et al., 2022). Overall, the effect of GA3 on runner growth was variable and dependent on GA concentration (Solovei, 1972 b; Mohammad et al., 1990; Rajesh et al., 2008), with GA3 at 50 ppm having no effect on runner growth in ‘Sparkle’ (SB) and ‘Ozark Beauty’ (EB) strawberries (Waithaka and Dana, 1978). According to Agafonov and Solovei (1974) GA3 administered to strawberries at a concentration of 0.005% improved the quantity of runners but decreased their quality. GA3 reduced runner growth at concentrations of 100 and 200 mg/L (Solovei, 1972 a). Application of 50 mg/L GA3 produced runner before flower in SD ‘Camarosa’ and ‘Camroga’ culti­ vars (Al­madhagi et al., 2012). The number of straw­ berry ‘Seolhyang’ runners was reduced by GA3 foliar spray, which showed a negative correlation between the concentration and number of runners (Li et al., 2020). Effect of GA3 on runner growth varies between cultivar (Solovei, 1972 a; Choma and Himelrick, 1984), with GA3 stimulating daughter­ plant formation in the EB cultivar but suppressing it in the SB cultivar (Waithaka and Dana, 1978; Choma and Himelrick, 1984). According to Kender et al., (1971) the response of three EB cultivars to GA3 at 50 increased runner development in cultivars ‘Ozark Beauty’ and ‘Superfection’, but had no effect on culti­ var ‘Geneva’. Due to longer internodes, EB strawberry ‘Tribute’ and ‘Selva’ cultivars treated with GA3 produced fewer daughter plants (Dale et al., 1996). Compared to NAA and CCC, GA3 produced the greatest vegeta­ tive growth and runner production at 90 ppm on ‘Sweet Charlie’ (Rajesh et al., 2008). GA3 use was related to the frequency of applied (Tafazoli and Vince­Prue, 1978; Duarte and Hermosa, 1998). GAs increased runner production when applied prior to the onset of dormancy and during the chill require­ ment stage (Honda, 1972), but did not increase the number of runners and hastened flowering when applied about a month before the appearance of flower buds, while hastened fruit maturation when applied at the flowers opening stage. Cytokinins Exogenous benzyladenine (BA) resulted in a greater numbers of runners in certain studies (Kour et al., 2017; Liu et al., 2019), while cytokinin and auxin coordinate the dormancy and expansion of axil­ lary buds in strawberries (Qiu et al., 2019). The influence of the exogenous hormone cytokinin on vegetative development has also been studied by several researchers, BA­type cytokinin has been observed by several researchers to enhance runner induction (Waithaka et al., 1978; Waithaka and Dana, 1978; Kour et al., 2017; Liu et al., 2019). 6­ BA also enhanced runner induction, with 50 mg/L being the most effective concentration (Li et al., 2020). Adv. Hort. Sci., 2023 37(4): 433­449 442 In ‘Sparkle’ (SB) and ‘Ozark Beauty’ (EB) strawber­ ries, foliar spraying with PBA at 200­600 ppm increased runner production (Waithaka and Dana, 1978). In contrast, BA alone had no effect on the genera­ tion of runners such as EB cultivar ‘Geneva’ (Kender et al., 1971), SB cultivars ‘Pajaro’, ‘Queen Eliza’, and ‘Paros’ (Momenpour et al., 2011) and ‘Redchief’ (SB) (Archbold and Strang, 1986). PBA caused axillary bud explants to grow into stolons (Waithaka et al., 1980). Interaction of exogenous hormone on runner devel‐ opment The effect of the combining hormones on straw­ berry runner development is based on a fight between them that prevents flowering. In EB ‘Geneva’ the use of both N6B and GA3 had a signifi­ cant impact on runner formation (Kender et al., 1971). In ‘Ozark Beauty’ (EB), a combination of PBA and GA3 had a stronger impact on runners and daughter plant development than PBA alone, and PBA reduced rooting of daughter plants, which GA3 could not overcome (Waithaka and Dana, 1978). When BA and GA3 were combined, petioles and stolon internodes were less thickened and elongated, resulting in greater leaf area than when PBA was used alone (Waithaka and Dana, 1978). The number of runners in the EB ‘Tribute’ and ‘Selva’ strawberries treated with GA3 and BA increased linearly when the benzyladenine (BA) con­ centration was increased up to 1800 mg/L, the rec­ ommend that BA at 1200 mg/L + GA3 at 300 mg/L in strawberries, under field or greenhouse conditions for runner formation (Dale et al., 1996). Application of 6­BA + ACC resulted in the maximum number of plantlets (six plantlets per plant) (Kirschbaum, 1998). In EB, GA3 at 50 ppm, BA at 50 ppm, or companion boosted the number of runners in ‘Miyoshi’ by 2­3 fold, whilst GA3 or GA3 + BA raised the number of runners by up to 8 and 4 times in ‘Enrai’ and ‘Summer Berry’, respectively (Pipattanawong et al., 1996). Gibberellic acid, when combined with benzylade­ nine, significantly increased runner development in the Geneva cultivar, but benzyladenine alone had lit­ tle impact (Kender et al., 1971). Photoperiod × PGR Day length photoperiod and gibberellin alone both increase runner production in strawberry culti­ vars with different genotypes and blooming habits. Strawberry plant susceptibility to exogenous gib­ berellins was enhanced by LD photoperiods (Tafazoli and Vince­Prue, 1978; Al­madhagi, 2012). The LD photoperiod increased the level of endogenous gib­ berellins, which promoted the growth of plant buds (Taylor et al., 1994). Meanwhile, the LD photoperiod had the same effect as gibberellin, leading to a greater number of epidermal cells, indicating that cell division and internodes length were increased (Nishizawa and Hori, 1993; Nishizawa, 1994). Suppression of GAs biosynthesis has been shown to promote crown branching, restrict runner produc­ tion, and improve flowering by increasing the num­ ber of possible sites for floral induction and differen­ tiation (Hytönen and Elomaa, 2011; Tenreira et al., 2017). In a prolonged photoperiod, exogenous GA3 com­ pletely reversed the effect of prohexadione­calcium when transferring GA3­treated plants from short to long days, on the other hand, it restored normal run­ ner development, this did not happen in plants that had not been treated with GA3 (Hytönen et al., 2009) The influence of photoperiod and exogenous hor­ mone interaction on the vegetative development of strawberries has been documented mainly with GA3. After exposure to the LD photoperiod, GA3 elicited comparable change in strawberries (Paroussi et al., 2002 a). The study by Soetarto (1979) discovered that dur­ ing the 24 h photoperiod GA3 at 150 ppm improved the stolon length of cultivar ‘Ostara’. SD, DN, and LD photoperiods plus GA3 (50 ppm) resulting in the greatest vegetative growth in the LD photoperiod with 50 ppm GA3 application and great­ est number of crowns/plant when plants in the LD photoperiod and treated with 1000 ppm CCC (Dwivedi et al., 1999 a). Plants grow faster when treated with GA3 in the LD photoperiod than in the SD photoperiod (Paroussi et al., 2002 a). By increasing the level of soluble sugar in ‘Seolhyang’ the strawberry cultivar, LD photoperiod (16 h) and 50 mg/L 6­BA break the dormancy of axil­ lary buds and produced runners (Li et al., 2020). Gibberellins can compensate for the effects of envi‐ ronmental variables However, chilled strawberry plants treated with GA3 in tropical countries (Kenya) produced about the same number of runners as those subjected to chill­ ing alone, but plants treated with BA produced signif­ icantly more runners than chilling alone (Kahangi et Al‐Madhagi ‐ Key of strawberry runnering 443 al., 1992). When the plant was exposed to chilled conditions in conjunction with the treatment of BA + GA3, the number of runners increased (Kahangi et al., 1992). GA3 induced and enhanced vegetative growth, equivalent to the impact of four to six weeks of chilling (Tehranifar and Battey, 1997). 5. Discussion and Conclusions The formation of strawberry runners was influ­ enced by the interaction of genetic (cultivars), envi­ ronmental (photoperiod, temperature, chilling hours or cold storage), and internal (hormones and carbo­ hydrate) factors. The most significant factor impacted by long­day photoperiod (LD) is the cultivar in strawberry runner proliferation. For the development of stolons in all flowering strawberry classes, LD and HT must inter­ act. Additionally, in order for runners to grow in EB and SB strawberry cultivars, LD must surpass a cer­ tain value at HT (Darrow, 1937; Went, 1957; Smeets, 1980). This may have been connected to the influ­ ence of photoperiod on photosynthesis and the metabolism of carbohydrates, which suggested that the amount of carbohydrates may rise during the cre­ ation of runners and that the amount of soluble sug­ ars was positively correlated with the number of run­ ners (Li et al., 2020). Everbearing (EB) strawberries absorbed more CO2 when temperature and irradi­ ance rose (Rivero et al., 2021 b). However, the accu­ mulation of photosynthates was not the only factor that affected the runner induction in cultivated strawberries (Li et al., 2021 b). The photoperiod also enhanced the amount of endogenous hormone as well as the synthesis and accumulation of starch, sugar, amino acids, and protein (Li et al., 2022). espe­ cially gibberellins that promote runner bud develop­ ment (Taylor et al., 1994). And the effect of the appli­ cation of gibberellins or photoperiod is the same result (Taylor et al., 1994). Meanwhile, more efficient runner production can be achieved in EB and SB strawberries by Gibberellins (GAs) Cytokinin and chilling period (CP) individually or in combination lead to production of stolons. In addition, CP or GAs enhances the effect of the pho­ toperiod LD. Long cold storage or the chilling (CP) also works on converting starch to soluble sugars (López et al., 2002; Al­madhagi et al., 2018; Al­ doubibi et al., 2021), and increased level of endoge­ nous gibberellins (Avigdori­Avidov et al., 1977). In fact, greater photosynthesis and respiration under the LD condition imply that more chemicals and energy are produced, which may account for the increased soluble sugar content in strawberry seedlings during runner production (Li et al., 2020). In which the respiration produced ATP and hydrolyzed the sugar for biosynthesis, resulting in altered levels and ratios of endogenous hormones, maybe with a focus on gibberellins and cytokinin, by transferring more sugar to axillary buds that are in high demand while restricting the quantity of sugar via the apical shoot (Mason et al., 2014). In strawber­ ry runners (non­dormant buds), as opposed to dor­ mant buds, the expression of genes involved in sugar metabolism and signalling was also increased (Qiu et al., 2019). In order to explain the changes in sig­ nalling between stages of bud release to sustained development, Cao et al. (2023) propose a model of apical dominance that combines auxin, sucrose, strigolactones, gibberellins, and cytokinin. The application of cytokinin helped to break the apical dominance and shift the auxin/cytokinin ratio (Al­madhagi, 2012; Qiu et al., 2019), which led to the growth of axillary buds to runner (Li et al., 2020). This cytokinin appears to promote runnering in early development stage, but prolonged, elevated it levels inhibit runnering. The application of AB­6 increased the level of free active of endogenous gibberellins and auxin in straw­ berry seedlings to a value higher than the free active of endogenous cytokinin (Al­madhagi, 2012) and increased the soluble sugar (Al­madhagi, 2012; Li et al., 2020). This ultimately converted polysaccharides into soluble sugars and stimulated axillary buds to produce runners. Additionally, the photoperiodic control the two genes (FvFT1) and (FvSOC1), that reg­ ulation of blooming and runners of woody strawber­ ries (Hytönen and Kurokura, 2020). This may be able to explain how the interaction of photoperiod and temperature influences the growth of runners by enhancing photosynthesis, elevating endogenous hormone levels, raising respiration, and raising the amount of soluble sugars. In order to fully develop a runner in SB strawberries or partially generate in EB strawberries, the plant makes advantage of the indi­ rect effects of photoperiod or cold storage as shown in figure 4. It can be concluded that, in all strawberry cultivars (EB or SB), soluble sugars may be required for axillary buds to emerge from their dormant case and pro­ duce runners, when this is impacted by the applica­ Adv. Hort. Sci., 2023 37(4): 433­449 444 tion of photoperiod LD or exogenous hormone. The optimal strategy is determined by optimizing the planting date for propagation or changing the propagation conditions, depending on the cultivars and the seasonal environment (photoperiod × tem­ perature) of the geographic region. Those factors are also important in the tissue culture technique as well as in the greenhouse or field. References AGAFONOV N.V., SOLOVEI E.P., 1972 ­ The effect of gib‐ berellin and chlorocholine chloride on runner growth and productivity in strawberry. ­ Doklady TSKhA: 211­ 216. AGAFONOV N.V., SOLOVEI E.P., 1974 ­ The use of growth substances in strawberry growing. ­ Khimiya v Sel’skom Khozyaistve, 12: 64­66. 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