Temperate fruit tree species (apple, pear, peach, nectarine, almond, grapevine) are naturally grown in regions where winter temperatures remain low for a certain period. The sustainability of production in these species largely depends on the amount of chilling accumulated during the winter months. To adapt to low winter temperatures and initiate synchronous growth in spring, temperate fruit species have evolved a complex dormancy mechanism
(Lang et al., 1987; Faust et al., 1997). Endodormancy, the most critical phase of dormancy, is the period during which internal physiological factors suppress bud growth and a certain duration of low-temperature exposure is required to overcome this state (
Erez, 2000;
Campoy et al., 2011). The amount of low temperature required for a genotype to break endodormancy is defined as the “chilling requirement,” which varies significantly among species and cultivars (
Hauagge and Cummins, 1991;
Fadón et al., 2020). Failure to meet the chilling requirement results in delayed and irregular bud break, prolonged flowering periods, poor fruit set and significant yield losses (
Petri and Leite, 2004;
Atkinson et al., 2013).
In recent years, increasing winter temperatures associated with global climate change have reduced chilling accumulation in many production regions, thereby directly threatening critical phenological processes such as bud break, flowering and fruit set
(Luedeling et al., 2011; Luedeling, 2012). This decline in winter chill has been documented in temperate fruit growing areas of India, where insufficient chilling has led to irregular flowering, reduced fruit set and increased susceptibility to pests and diseases
(Rimpika et al., 2021). Irregular flowering and yield losses observed under insufficient chilling conditions are making the cultivation of high-chill cultivars increasingly risky
(Atkinson et al., 2013; Gonzalez-Martinez et al., 2025). While this situation complicates sustainability in traditional production areas, it has also spurred growing interest in adapting temperate fruit species to subtropical and tropical regions (
Janick, 1974;
Edwards, 1987a;
Penso et al., 2020;
Clavijo et al., 2026).
In regions such as Brazil, Indonesia, the Philippines, Venezuela, Thailand, Florida and Taiwan, the production of apples, pears, peaches and grapes is gaining increasing economic importance. However, natural chilling accumulation in these regions is generally insufficient and dormancy-related physiological constraints limit the cultivation of high-quality cultivars with high chilling requirements. Lee
et al. (2025) demonstrated that even in low-chill peach cultivars under subtropical conditions in Taiwan, interannual variability in chilling accumulation significantly disrupts the synchronization of bud break. This finding indicates that the use of low-chill cultivars alone may not be sufficient and highlights the need for alternative biological tools to ensure sustainable production. In this context, the use of rootstocks with different chilling requirements (
Fazio and Robinson, 2022), the application of dormancy-breaking chemicals (hydrogen cyanamide, potassium nitrate) (
Edwards, 1987a) and the manipulation of the rest period through artificial defoliation and pruning techniques (
Edwards, 1987a) are among the prominent strategies. In addition, biotechnological approaches such as the induction of epigenetic modifications through mobile RNAs transported from rootstock to scion
(Kapazoglou et al., 2021) have recently been considered promising alternatives.
Developing new low-chill cultivars for climate adaptation has important limitations, including restricted genetic resources, the long time required for cultivar development and potential losses in fruit quality. Furthermore, it limits the use of existing high-quality cultivars under new climatic conditions. An alternative and complementary approach involves the strategic use of rootstocks to modulate scion dormancy dynamics and chilling responses.
In modern fruit production, grafting and the use of rootstocks play a decisive role not only in providing tolerance to soil-borne stresses but also in regulating scion vigor, yield, fruit quality and tolerance to abiotic stresses
(Warschefsky et al., 2016). Although the effects of rootstocks on vigor and yield have long been recognized
(Lordan et al., 2017; Shahkoomahally et al., 2021; Reig et al., 2022), their role in dormancy dynamics and chilling requirements has been relatively less explored.
In recent years, an increasing number of studies have provided strong evidence that rootstocks can influence the scion’s dormancy behavior and chilling requirements. More recent research has shown that different geneva rootstocks can significantly alter the chilling requirement of the ‘Gala’ apple cultivar and that some rootstocks can promote high bud break even under low-chill conditions (
Fazio and Robinson, 2022). In almond,
Abedian-Chermahini et al. (2026) reported that the use of the ‘Arabica’ interstock on GN15 rootstock delayed flowering of the ‘Mamaee’ cultivar by approximately 20 days, thereby reducing frost risk.
Emerging evidence suggests that rootstocks can affect scion phenology through two fundamentally different pathways: (i) true modulation of the scion’s intrinsic chilling requirement, which lowers the threshold for endodormancy release, or (ii) phenological shifts that advance or delay bud break timing without altering the underlying chilling requirement. Distinguishing between these mechanisms is critical for both scientific understanding and practical application. The hypothesis that low-chill or non-dormant rootstocks can reduce dormancy depth, alter hormonal thresholds and promote bud break under lower chilling accumulation is gaining increasing importance.
This review uniquely integrates physiological, molecular and practical perspectives to provide a comprehensive understanding of rootstock-scion interactions in dormancy regulation. The specific objectives of this review are: (1) to elucidate the physiological and molecular mechanisms by which rootstocks influence scion dormancy, (2) to evaluate rootstock-mediated dormancy management strategies in tropical and subtropical systems, (3) to present case studies from key production regions and (4) to establish a conceptual framework distinguishing true chilling requirement modulation from phenological shifts, thereby guiding both fundamental research and applied breeding programs.
Mechanisms by which rootstocks influence dormancy and chilling requirement
The influence of rootstocks on the dormancy depth and chilling requirement of scion cultivars is highly complex and cannot be explained by a single factor; rather, it represents the integrated outcome of multiple signals transported from roots to shoots
(Warschefsky et al., 2016). In this section, four major mechanisms that play a decisive role in overcoming low chilling accumulation under tropical and subtropical conditions are summarized: hormonal signaling; water relations and hydraulic conductivity; carbohydrate metabolism and energy allocation and molecular and epigenetic regulation.
Hormonal signaling: ABA/GA balance
Plant hormones synthesized in the roots and transported in the scion xylem sap are among the most critical factors regulating the timing of bud break and the depth of dormancy
(Aloni et al., 2010). Rootstock induced differences in ABA signaling and aquaporin expression have been shown to affect stomatal conductance and photosynthetic rate, as demonstrated in mandarin citrus grafted onto different rootstocks under drought stress
(Yulianti et al., 2021). In particular, the balance between abscisic acid (ABA) and gibberellins (GA) plays a central role in the maintenance and release of endodormancy
(Cooke et al., 2012). It is hypothesized that low-chill rootstocks may promote earlier bud break by reducing chilling accumulation, either by accelerating the decline of ABA levels or by enhancing GA activity in scion tissues.
Lordan et al. (2017) demonstrated that ABA and cytokinin concentrations in xylem sap of ‘Honeycrisp’ apple grafted onto different rootstocks varied significantly depending on rootstock genotype. The B.9 rootstock exhibited high ABA content, whereas the G.11 rootstock showed elevated cytokinin levels and these hormonal differences were associated with bud break uniformity.
Hezema et al., (2021) reported that under osmotic stress conditions, tolerant rootstocks (B.9, V3, G41, G935) accumulated significantly higher levels of ABA compared to sensitive rootstocks (M.26, B118) and upregulated genes involved in ABA-dependent signaling pathways (SnRK, DREB, ERD, MYC2). A similar mechanism has been confirmed in citrus;
Primo-Capella et al. (2022) showed that under cold stress, the cold-tolerant ‘Carrizo’ rootstock increased ABA concentration in roots, which was transported to the scion and enhanced the expression of genes associated with cold adaptation.
Water relations and hydraulic conductivity
Rootstock root system architecture and vascular characteristics (
e.
g., xylem vessel diameter, root density) determine the water uptake capacity and water potential of the scion
(Basile et al., 2003). Indeed, in mandarin citrus, the Citrumelo rootstock provided higher stomatal aperture and transpiration rates compared to other rootstocks, indicating superior hydraulic conductivity under water deficit
(Yulianti et al., 2021). Under low-chill conditions, early and efficient water transport may accelerate the transition of buds into metabolic activity.
Clavijo et al., 2026 reported that under subtropical Florida conditions, the high-chill rootstock ‘MP-29’ provided higher transpiration rates and stem water potential in ‘UFSun’ peach compared to the low-chill rootstocks ‘Okinawa’ and ‘Flordaguard’. This finding indicates that even a high-chill rootstock can improve phenological performance under low-chill conditions by enhancing the scion’s water uptake capacity. However, the specific anatomical and molecular traits underlying this enhanced capacity have not been fully elucidated in the context of dormancy regulation.
QTL analyses in grapevine have revealed that the genetic regions controlling scion transpiration and acclimation to water deficit differ and are closely associated with ABA signaling
(Marguerit et al., 2012). Similarly,
Soar et al., (2006) demonstrated that stomatal conductance in ‘Shiraz’ grapevine grafted onto different rootstocks is inversely related to xylem sap ABA concentration under drought stress. Although their study primarily addressed water-deficit responses, the rootstock-mediated modulation of ABA signal transport they observed may also represent a potential mechanism influencing the timing of budbreak and phenological transitions under non-stress conditions.
Hydraulic conductivity is governed by root anatomical traits such as xylem vessel diameter, vessel density and root cortical cell size, which vary among rootstock genotypes
(Basile et al., 2003; Clavijo et al., 2026). Rootstocks with larger xylem vessels may exhibit higher axial conductance, facilitating rapid water transport to the scion during bud swelling and early shoot growth. Conversely, rootstocks with high root density and fine root branching may enhance water uptake from the soil, particularly under fluctuating soil moisture conditions. These architectural differences translate into differences in stem water potential and transpiration rates, which can influence the timing of bud metabolic reactivation.
At the molecular level, aquaporins (plasma membrane intrinsic proteins, PIPs) play a key role in regulating root hydraulic conductivity. Rootstock genotype affects aquaporin gene expression, which in turn modulates water flow across root membranes
(Yulianti et al., 2021). In peach,
Clavijo et al. (2026) observed that the high-chill ‘MP-29’ rootstock maintained higher stem water potential and transpiration under subtropical conditions, likely due to superior hydraulic conductance.
Importantly, water delivery to buds affects turgor pressure, which is a physical signal for cell expansion and metabolic activation. Adequate hydration facilitates the hydrolysis of storage carbohydrates and the transport of metabolites to bud meristems, thereby supporting the energetic demands of bud break. Conversely, restricted water supply can delay or impair these processes, even when chilling requirements have been fulfilled. Thus, rootstock-mediated differences in hydraulic conductivity can accelerate or delay bud break by modifying the water status of bud tissues, independent of, or in concert with, hormonal signals.
Furthermore, hydraulic signals interact closely with ABA signaling. Mild water stress induced by limited hydraulic conductance can elevate ABA synthesis in roots, which is then transported to the scion where it promotes stomatal closure and modulates bud dormancy depth. Conversely, rootstocks with high hydraulic efficiency may maintain low ABA levels, thereby promoting earlier growth under non-stressful conditions. This interplay between hydraulic and chemical signals
(Soar et al., 2006; Marguerit et al., 2012) suggests that rootstock effects on scion phenology are mediated by a complex integration of water delivery and hormonal balance.
Carbohydrate metabolism and energy allocation
Rootstocks play a critical role in the storage of photosynthates (starch in roots) and their transport to buds in the form of soluble sugars (sucrose, glucose, sorbitol, raffinose) during spring
(Foster et al., 2017). Under low-chill conditions, early and efficient carbohydrate mobilization is essential to meet the energy demands of buds and to provide osmoprotection.
Primo-Capella et al. (2022) reported that under cold stress, leaves of the tolerant ‘Carrizo’ rootstock accumulated higher levels of total soluble sugars, particularly raffinose, compared to the sensitive
C.
macrophylla rootstock. Raffinose family oligosaccharides (RFOs) are well known for their membrane-stabilizing and antioxidant roles.
Saini (2025) found that in ‘Fuji’ and ‘Gala’ apple trees exposed to natural frost events, the B.9 rootstock maintained more stable levels of sucrose, glucose and sorbitol compared to M26. This suggests that tolerant rootstocks improve the scion’s energy balance and osmoprotection, thereby mitigating the adverse effects of low chilling and frost stress. It is therefore proposed that low-chill rootstocks may contribute to earlier bud metabolic activation by regulating carbohydrate metabolism.
The mobilization of carbohydrates from root and stem reserves to bud tissues is a prerequisite for bud break, as it provides both the energy required for cell division and elongation and the osmotic solutes necessary for water uptake and turgor generation
(Foster et al., 2017). Inadequate carbohydrate supply, or delayed transport, can result in weak and asynchronous bud break even when chilling requirements have been fulfilled. Conversely, rootstocks that facilitate early and efficient sugar accumulation in buds may reduce the apparent chilling requirement by providing both energy and osmotic protection, thereby promoting earlier and more uniform bud break (
Saini, 2025).
Furthermore, sugars such as sucrose and glucose are not only metabolic substrates but also function as signaling molecules that interact with hormone pathways, particularly ABA and GA
(Cooke et al., 2012). High sugar levels can antagonize ABA signaling and promote GA biosynthesis, thereby facilitating the transition from dormancy to active growth. Thus, rootstock-mediated differences in carbohydrate allocation and sugar signaling are likely to influence the timing and uniformity of bud break under low-chill conditions, independent of or in interaction with hormonal signals.
In summary, rootstock effects on carbohydrate metabolism and energy allocation are integral to dormancy regulation and bud break performance. Rootstocks that enhance starch storage, promote early sugar mobilization and maintain favorable sugar-to-ABA ratios can effectively reduce the metabolic and osmotic barriers to bud break, thereby contributing to the successful cultivation of temperate fruit species under tropical and subtropical conditions.
Molecular and epigenetic signals
Recent studies indicate that communication between rootstock and scion is not limited to hormones and metabolites but also involves the transport of small RNAs (siRNA, miRNA) and mRNAs across the graft union, which can regulate gene expression and trigger epigenetic modifications in the scion
(Kapazoglou et al., 2021; Shivran et al., 2022). This mechanism may explain why rootstock effects can have long-lasting or persistent effects over multiple years rather than being limited to a single growing season. However, it is important to distinguish between the well-established principle of mobile RNA transfer across graft unions and the still-hypothetical application of this mechanism to dormancy regulation in fruit trees.
Primo-Capella et al. (2022) showed that the cold-tolerant ‘Carrizo’ rootstock upregulated genes associated with the ABA signaling pathway (PP2C, PYL, SRK2E) in the scion. In contrast, these genes were suppressed in the sensitive
C.
macrophylla rootstock. Similarly,
Saini (2025) demonstrated that under frost stress, the B.9 rootstock enhanced the expression of cold-responsive genes, such as MdCBF4 and MdHSFC1, in the scion, as well as of genes regulating carbohydrate metabolism. These findings indicate that rootstock effects extend beyond hormonal signaling to include deep molecular reprogramming. In tropical and subtropical regions, rootstocks used for successful cultivation are thought to facilitate adaptation to low-chill conditions by inducing such gene expression changes in the scion.
Recent studies have identified specific miRNAs, such as miR156 and miR172, that regulate flowering time and phase change in woody perennials
(Kapazoglou et al., 2021). In apple, miR156 levels are associated with juvenile-to-adult transition and rootstock-induced changes in miR156 abundance could potentially affect dormancy release and bud break timing. However, direct evidence linking specific mobile miRNAs to dormancy regulation in grafted fruit trees is still lacking and this remains an important research frontier.
In addition to DNA methylation, chromatin remodeling (histone modifications) is increasingly recognized as a key epigenetic mechanism that may mediate rootstock-scion interactions. Histone acetylation and methylation can alter the accessibility of dormancy-associated genes to the transcriptional machinery, thereby modulating their expression. Although chromatin remodeling has been studied in model plants, its role in graft-induced epigenetic changes in woody perennials has not yet been demonstrated and warrants further investigation.
Rootstock selection and dormancy management strategies
Even with optimal rootstock selection, additional management practices are typically necessary to ensure reliable bud break and yield in tropical and subtropical regions. The cultivation of temperate fruit species in tropical and subtropical regions, where natural chilling accumulation is insufficient or absent, requires specialized strategies that go beyond conventional practices and aim to prevent or bypass endodormancy (
Edwards, 1987a;
Erez, 2000). This approach, referred to as “dormancy management” or “continuous cropping,” aims to maintain uninterrupted cycles of growth and fruit production throughout the year (
Erez and Lavi, 1985). The success of this strategy depends on three main cultural practices-control of vegetative growth, artificial defoliation and induction of bud break-as well as appropriate rootstock selection.
Control of vegetative growth and promotion of flower bud formation
In tropical regions, continuously high temperatures and humidity can promote excessive vegetative growth, thereby inhibiting flower bud formation
(Notodimedjo et al., 1981). Therefore, controlling vegetative growth is the first critical step in dormancy avoidance strategies. In apples and pears, bending long shoots horizontally breaks apical dominance, promoting lateral bud break and spur formation. In peach, practices such as water stress (as applied in Peru) or the use of growth regulators like paclobutrazol can suppress shoot growth and stimulate flower bud formation (
Edwards, 1987a;
Erez, 1987). Growth regulators such as daminozide (1000-2000 mg/L) can also promote earlier terminal bud formation in apple trees, advancing flowering.
Rootstocks play an indirect but significant role in this process. Dwarfing rootstocks (
e.
g., M9 in apple, ‘MP-29’ in peach) limit excessive vegetative growth and thereby support flower bud formation.
Prevention of dormancy through defoliation
One of the most critical components of dormancy avoidance strategies is the removal of mature leaves after each harvest cycle. This practice prevents the plant from entering dormancy and initiates the next growth cycle (
Edwards, 1987a). Defoliation can be carried out manually or using chemical agents. In Indonesia, apple leaves are manually removed within four weeks after harvest (
Verheij, 1990), whereas in Venezuela, peach trees are defoliated using 2% sodium chlorate (
Edwards, 1987a). Chemical defoliants such as 10% urea, 10% copper sulfate, DNOC-mineral oil mixtures and 1-2% cyanamide have also been found effective. The timing of defoliation is critical: early application may inhibit flower bud formation, while late application may reduce bud break.
At this point, rootstocks play a determining role. In contrast to the successful applications reported by
Janick (1974) and
Verheij (1990) in Indonesia,
Saltveit (1983) found that defoliation did not induce bud break in apple trees grafted onto M26 and M7a rootstocks. Variations in chilling requirements of the rootstocks used may explain this difference.
Couvillon et al., (1984) suggested that the ‘Chinese crab apple’ rootstock used in Indonesia has a low chilling requirement and likely enhances flowering and vegetative responses to defoliation by reducing the scion’s chilling requirement. Therefore, the success of defoliation practices is directly linked to the use of low-chill or non-dormant rootstocks.
Induction of bud break
Following properly timed defoliation, bud break typically occurs spontaneously within 1-4 weeks. However, under dry conditions, irrigation can improve the uniformity of bud break. Various chemicals used in subtropical regions to compensate for insufficient chilling (
e.
g., mineral oil-DNOC, potassium nitrate, thiourea and hydrogen cyanamide) have also been successfully applied in tropical regions to stimulate bud break (
Edwards, 1987a;
Erez, 1987). In temperate fruit production, these dormancy-breaking agents have been widely adopted to compensate for insufficient chilling in warmer years
(Rimpika et al., 2021). Hydrogen cyanamide (1-3%) is currently the most widely used and effective chemical.
The effectiveness of these applications may vary depending on the rootstock-scion combination. For example, low-chill rootstocks may enhance the response to cyanamide by reducing endogenous ABA levels, whereas high-chill rootstocks may require higher doses. This interaction should be considered when designing chemical application protocols for specific rootstock-scion combinations. It is suggested that low-chill rootstocks enhance the response to these chemicals and promote more uniform bud break.
Strategic role of rootstocks: A system-defining component
Rootstock selection plays not merely a supportive but a system-defining role in the success of dormancy avoidance strategies in tropical and subtropical regions. Rootstocks are used to control excessive vegetative growth, influence the timing and uniformity of bud break and improve overall adaptation. The unidentified ‘Chinese crab apple’ rootstock used in Indonesian apple production has been observed to produce nearly continuous shoot growth throughout the year without entering a distinct endodormancy phase (
Verheij, 1990). This non-dormant behavior underpins successful systems that enable two harvests per year in East Java. Similarly, in peach cultivation, low-chill rootstocks such as ‘Okinawa’ (150 CU) and ‘Flordaguard’ (150-300 CU) are widely preferred due to their nematode resistance and adaptation to subtropical conditions (
Clavijo et al., 2026).
Orchard establishment and management practices in tropical regions also differ. The use of dwarfing rootstocks is encouraged to manage excessive vegetative growth. High-density planting (row spacing as low as 1 m) can limit vegetative growth by increasing competition among plants. Instead of pruning, shoot bending is often preferred. In regions with high rainfall, cultivation on raised beds may be adopted to reduce the risk of disease (
Edwards, 1987a). When combined with appropriate rootstock selection, these practices enable the successful cultivation of high-quality, high-chill temperate fruit cultivars under tropical and subtropical conditions.
The main dormancy management strategies and their interactions with rootstock genotypes are summarized in Table 1.
Successful cultivation examples in tropical and subtropical regions
Across various tropical and subtropical regions of the world, there are striking examples demonstrating that temperate fruit species with high chilling requirements can be successfully cultivated under insufficient natural chilling through appropriate rootstock selection and well-adapted cultural practices. These examples clearly highlight the strategic role of rootstocks in modulating dormancy and managing chilling requirements.
Indonesia: A pioneering case in apple cultivation
One of the most remarkable examples of temperate fruit cultivation in tropical regions is apple production in the Batu region of East Java, Indonesia (8°S, approximately 800-1200 m altitude), which has been successfully practiced for decades (
Janick, 1974;
Edwards, 1987b). In this region, high-chill apple cultivars are grafted onto a local, low-chill and non-dormant rootstock known as ‘Chinese crab apple’ (likely a hybrid of
Malus sylvestris or
Malus toringoides), enabling two harvests per year.
In apple, rootstock selection significantly influences scion performance under subtropical conditions. For instance, ‘Scarlet Gala’ and ‘Red Fuji’ grafted onto EMLA 7 and EMLA 111 rootstocks exhibited superior growth and yield compared to those on EMLA 9 under high density planting (
Bhatia and Kumar, 2009).
Verheij (1990) reported that when grown on its own roots, this rootstock produces active shoots almost year-round and does not enter a distinct endodormancy phase. This characteristic is transmitted from the rootstock to the scion, preventing the scion from entering dormancy and enabling continuous growth and fruiting.
Couvillon et al., (1984) suggested that this rootstock has a low chilling requirement and likely enhances flowering and vegetative responses to defoliation by reducing the chilling requirement of the scion. This example is particularly important as it demonstrates that a non-dormant rootstock can enable the successful cultivation of high-chill scions even under tropical conditions.
Fig 1 illustrates this continuous growth cycle in comparison with the single annual cycle observed in temperate regions
(Notodimedjo et al., 1981).
Venezuela: Defoliation and rootstock integration in peach cultivation
In Venezuela, high-chill peach cultivars are successfully grown in the colonia tovar region near Caracas (10°N, approximately 1800-2200 m altitude) (
Edwards, 1987b). In this system, artificial defoliation-the basis of dormancy avoidance-is achieved by applying 2% sodium chlorate after harvest. Following defoliation, bud break typically occurs spontaneously within 2-3 weeks, initiating a new growth cycle.
The rootstocks used in this system have low chilling requirements and help regulate scion vegetative growth.
Edwards (1987b) reported that low-chill rootstocks such as ‘Okinawa’ are preferred in successful peach production systems in Venezuela. This example represents a successful model of dormancy avoidance in which chemical defoliation is effectively integrated with rootstock selection.
Thailand: Table grape production
Table grape cultivation in the vicinity of Bangkok, Thailand (14°N, low altitude), represents another successful example of temperate fruit production under tropical conditions (
Edwards, 1987a). Although low-chill cultivars (
e.
g., ‘Cardinal’, ‘Perlette’) are commonly used, rootstock selection plays a critical role in the cultivation of high-quality cultivars.
In grapevine, dormancy avoidance relies on pruning-induced defoliation followed by the application of hydrogen cyanamide to ensure uniform bud break (
Edwards, 1987a).
Miele (2019), in a study examining the effects of different rootstocks on the phenology of ‘Cabernet Sauvignon’, reported that the ‘Dogridge’ rootstock exhibited the highest chilling requirement and was therefore unsuitable for tropical regions. In contrast, rootstocks such as ‘3309 C’ and ‘101-14 Mgt’ promoted earlier bud break due to their lower chilling requirements. These findings highlight the importance of selecting low-chill rootstocks for grape cultivation in tropical environments.
Florida (USA): Peach cultivation and rootstock performance under subtropical conditions
The subtropical climate of florida presents significant challenges for peach production. Clavijo-Herrera
et al. (2026) evaluated the low-chill peach cultivar ‘UFSun’ grafted onto five rootstocks with varying chilling requirements (‘Okinawa’ ~150 CU, ‘Flordaguard’ 150-300 CU, ‘Barton’ ~400 CU, ‘P-22’ ~500 CU and ‘MP-29’ ~750 CU) over two years period. No significant effects of rootstocks were observed on photosynthetic rate (
Pn), stomatal conductance (
gs), or intercellular CO
2 concentration (C
i), suggesting that rootstocks influence water and nutrient uptake rather than directly altering photosynthetic capacity.
The results indicated that the high-chill ‘MP-29’ rootstock provided higher transpiration rates and stem water potential compared to low-chill rootstocks and also increased leaf nitrogen (N) content. Additionally, ‘MP-29’ enhanced leaf sulfur (S) and zinc (Zn) concentrations, while the medium-chill ‘Barton’ rootstock resulted in higher phosphorus (P) and magnesium (Mg) levels (
Clavijo et al., 2026). These findings demonstrate that a high chilling requirement in rootstocks does not necessarily limit scion performance under low-chill conditions and may even confer certain advantages.
Indeed, rootstocks with higher water uptake capacity (such as ‘MP-29’) exhibited less negative stem water potential (
Ψstem) and higher transpiration rates (E), which were positively correlated with leaf nutrient concentrations (N, K, Ca, S, Fe) (
Clavijo et al., 2026).
Shahkoomahally et al., (2021), using the same experimental system, reported that ‘MP-29’ produced the smallest fruits (104 g) but achieved the highest soluble solids content (13.1%), total phenolic content (749.8 mg GA/L), total antioxidant activity (1.60 mmol TE/L) and total anthocyanin levels (15.03 mg/100 g). The authors also emphasized that strong vegetative growth flushes during summer in subtropical regions increase nutrient demand, necessitating adjustments in fertilization and irrigation programs (
Clavijo et al., 2026).
These findings underscore that rootstock selection in subtropical regions should not be based solely on chilling requirement. Still, they should also consider the overall influence of rootstocks on water uptake, nutrition and fruit quality.
Taiwan: Phenological synchronization and rootstock use in subtropical conditions
In Taiwan, low-chill peach cultivars such as ‘Tainung No.4 Ruby’ (~100 CU) and ‘Tainung No.7 HongLing’ (~77 CU) have been successfully cultivated when combined with appropriate rootstocks
(Huang et al., 2024; Lee et al., 2025). In a three-year phenological study,
Lee et al., (2025) showed that in years with high chilling accumulation (280 CU), the difference between apical and basal bud break was relatively small (14 days). In contrast, in years with low chilling (120 CU), this difference increased significantly (22 days). This finding indicates that insufficient chilling disrupts bud break synchronization along the shoot, while adequate chilling promotes more uniform flowering.
In this context, the phenological flexibility provided by rootstocks plays a critical role in mitigating the negative effects of interannual variability in chilling accumulation. In successful production systems in Taiwan, local peach rootstocks such as ‘Kutaw’ have been widely used
(Huang et al., 2024). This example demonstrates that, in addition to the development of low-chill cultivars, appropriate rootstock selection is indispensable for successful fruit production under subtropical conditions. The chilling requirements of these and other very low-chill peach genotypes, estimated using different chilling models, are presented in Table 2
(Ming-Te et al., 2010).
Discussion: Reduction of chilling requirement or phenological shift?
The evidence presented in this review demonstrates that, in the cultivation of temperate fruit species in tropical and subtropical regions, rootstocks have a pronounced effect on scion dormancy dynamics, bud break timing and overall phenology. However, a fundamental question arises regarding the nature and underlying mechanisms of these effects: do rootstocks truly reduce the chilling requirement of the scion cultivar, or do they rather shift the timing of phenological events such as bud break and flowering? The distinction between these two concepts is of great importance for both basic science and horticultural management strategies in tropical and subtropical regions. In this discussion, we synthesize the evidence for each mechanism, propose an integrated framework and address the practical implications of interannual climatic variability-a critical factor in subtropical systems.
Modulation of chilling requirement (True reduction)
Modulation of chilling requirement refers to a reduction in the amount of low-temperature exposure required for endodormancy release when the same scion cultivar is grafted onto a different rootstock. In other words, the rootstock may lower the scion’s dormancy-breaking threshold, enabling bud break with less accumulated chilling. This phenomenon is highly valuable for production under limited chilling conditions in tropical and subtropical regions. Studies in apple provide some of the strongest evidence supporting this mechanism.
Fazio and Robinson (2022) grafted the ‘Gala’ scion onto different Geneva rootstocks and demonstrated that, under the same low chilling accumulation (585 hours), certain rootstock genotypes (G.213, G.814) resulted in significantly higher bud break compared to others, strengthening the hypothesis that rootstock genotype can modify the scion’s endodormancy release threshold.
Finetto (2004) also showed that the M9 rootstock induced 50% bud break with lower chilling accumulation than M26 and M27, suggesting that M9 and some Geneva rootstocks may reduce chilling requirements by approximately 50-100 hours. In grapevine,
Miele (2019) reported that the ‘Dogridge’ rootstock leads to higher chilling requirements, whereas ‘161-49 C’ results in lower chilling requirements. These studies collectively demonstrate that chilling requirement is not an absolute trait and can be modulated through rootstock effects.
As discussed in Section 2.1, hormonal signals (particularly ABA and cytokinins) produced in roots and transported to the scion appear to play a central role in this modulation. Studies in citrus further indirectly support this hypothesis;
Primo-Capella et al. (2022) showed that the cold-tolerant ‘Carrizo’ rootstock significantly increases ABA concentration in roots during cold stress and this ABA is transported to the scion, where it enhances the expression of cold-adaptation genes (CBF regulon), suggesting that the ‘Carrizo’ rootstock may influence chilling requirements by lowering the scion’s cold-response threshold or by accelerating its response dynamics.
Phenological shift (Timing modification)
On the other hand, there is also strong evidence that the effects of rootstocks are not related to a reduction in chilling requirement but rather to a shift in the timing of phenological events such as bud break and flowering. In this case, the scion continues to require the same amount of chilling accumulation; however, its response during ecodormancy, or the rate of bud development after chilling fulfillment, is modified by the rootstock.
The 14-20-day flowering delay achieved with interstocks in almond is one of the most striking examples of this phenological shift (
Abedian-Chermahini et al., 2026). This delay has been associated with the repression of flowering gene expression (FT, SOC1, LFY) and changes in hormone levels, including ABA and IAA. Although it remains unclear whether the chilling requirement itself has been altered, it is evident that flowering time has been substantially shifted. In subtropical regions, delaying flowering to avoid spring frosts is strategically important and this mechanism can be effectively exploited through rootstocks.
In grapevine,
Miele (2019) demonstrated that different rootstocks significantly affect bud break, flowering and ripening dates. The ‘Cabernet Sauvignon’ scion grafted onto ‘Dogridge’ showed the latest bud break, whereas the same scion on ‘101-14 Mgt’ exhibited the earliest bud break. In apple,
Lordan et al., (2017) showed that scions grafted onto G.935, G.814 and G.11 rootstocks exhibited earlier and more synchronized bud break, while bud break was significantly delayed on B.72020 rootstock. These examples suggest that rootstocks can modify the rate of bud development during ecodormancy through their effects on carbohydrate allocation and hydraulic conductivity, without necessarily altering the endodormancy threshold itself.
Integrated function and strategic implications in tropical/subtropical cultivation
Current evidence indicates that rootstocks act through two distinct but complementary mechanisms. In the first, the rootstock directly modifies the depth of endodormancy in the scion through mobile signaling molecules originating from the root system-such as ABA, cytokinins, carbon signals and small RNAs-which can reprogram dormancy at the bud level
(Albacete et al., 2015; Notaguchi and Okamoto, 2015). Rootstocks such as G.213 and G.814 in apple and ‘Carrizo’ in citrus, have been reported to alter chilling requirements of the scion
(Fazio et al., 2014; Goldschmidt, 2014). In this context, rootstocks function not only as growth regulators but also as active biological controllers of phenological threshold values.
The second mechanism is phenological shift occurring after chilling requirements have been fulfilled. In this process, rootstocks modify phenological timing without altering chilling requirements by affecting growth rate, carbon allocation, hormonal balance and expression of flowering-related genes
(Cooke et al., 2012; Foster et al., 2017). The delayed flowering observed in almond through the use of interstocks represents a typical example of this mechanism.
From a horticultural perspective, these two mechanisms lead to different strategic outcomes. A reduction in chilling requirements is a critical adaptive mechanism for sustaining production in tropical regions where chilling accumulation is severely limited. In particular, rootstocks that do not enter dormancy or that have very low chilling requirements enable stable phenological cycles under tropical conditions. In contrast, phenological shifts provide important management advantages in subtropical regions, such as avoiding spring frost, improving flowering synchrony and optimizing harvest timing. Therefore, this mechanism serves as an important adaptive tool against climatic variability in subtropical fruit production systems.
Buffering interannual climatic variability and implications for modeling
As noted in the case studies from Taiwan
(Lee et al., 2025) and Florida (
Clavijo et al., 2026), subtropical climates exhibit more variable winter temperature regimes than temperate climates, leading to substantial fluctuations in chilling accumulation and its interannual consistency. Even in orchards established with low-chill-requiring cultivars, phenological irregularities may occur, increasing production risk. In this context, rootstocks provide a critical buffering function by modifying the scion’s sensitivity threshold to environmental signals, ensuring bud break even in years with insufficient chilling. Some high-chill-requiring rootstocks (
e.
g., ‘MP-29’ in peach) may exhibit improved water and nutrient uptake capacity under low-chill conditions, enhancing the overall physiological status of the tree (
Clavijo et al., 2026;
Shahkoomahally et al., 2021).
The recognition that rootstocks can either modulate chilling requirements or shift phenological timing has significant implications for phenological modeling. Current models assume fixed chilling requirements for a given genotype; however, our review indicates that these parameters may vary depending on the rootstock. Incorporating rootstock-specific coefficients into phenological models could improve their predictive accuracy and help growers make more informed decisions about cultivar and rootstock selection for specific regions. For breeding programs, identification of rootstock genotypes that consistently reduce chilling requirements or provide reliable phenological shifts should be prioritized.
