Challenges and solutions in hand pollination for hybrid pepper (Capsicum annuum L.) seed production: a review

Abstract

Main conclusion

Successful hybrid seed production in pepper depends strongly on the stress sensitivity of reproductive processes, particularly pollen function and flower retention. Integrating physiological understanding of pollen biology, hormonal regulation of abscission, and targeted environmental and genetic interventions is essential to stabilize fruit set and improve hybrid seed yield under increasing climatic variability.

Abstract

Hybrid pepper (Capsicum annuum L.) F₁ seed production relies largely on controlled (manual) emasculation and pollination, while seed set at commercial scale is often unstable due to the pronounced environmental sensitivity of the reproductive phase. The present review discusses, within an integrated framework, the main constraints of manual pollination and hybrid seed yield as well as the possible solutions, with particular emphasis on (i) abiotic stressors (temperature, light intensity and light spectrum, water and nutrient supply, relative humidity), (ii) pollen biological and progamic processes, (iii) the hormonal regulation of flower and young fruit abscission, and (iv) genetic/male-sterility systems supporting hybrid purity. Based on the literature, pollen is the most stress-sensitive “weak link”: consistently high temperatures (above 32 °C) and unfavorable light regimes impair pollen development, viability, and pollen tube growth, while shifts in the auxin-ethylene balance in the abscission zone increase flower and fruit drop. Reduced assimilate availability (source-sink competition) and the hormonal dominance of developing fruits further intensify abortion, in protected cultivation potentially leading to cyclic fruit-set patterns. Although CMS/CGMS (cytoplasmic and cytoplasmic-genic male sterility) and GMS (genic male sterility) systems can reduce labor costs and improve genetic purity, their application is not suitable in all breeding and hybrid seed production scenarios; therefore, controlled pollination performed with manual emasculation remains of key importance. The review proposes a physiology-based, decision-support approach that integrates microclimate optimization (thermal and spectral management), pollen-based rapid phenotyping, and marker-based male-sterility identification to improve successful fertilization, seed formation, and hybrid seed quality. In this review, the most critical research gap is the lack of an empirically validated relationship between in vitro pollen stress assays and in vivo fertilization and seed-set success, as this could establish the predictive foundations of stress-tolerant, scalable hybrid seed production.

Graphical abstract

Introduction

Peppers (Capsicum annuum L.) are widely appreciated and consumed worldwide, with global production reaching approximately 5.8 million tons in 2023 (FAO 2025). During their extensive cultivation, plants face numerous abiotic challenges (Hosseinifard et al. 2022). The pepper seed market represents a substantial economic sector, clearly reflecting the prominent industrial importance of hybrid seed production within the cultivation chain (Tripodi et al. 2021; Naves et al. 2022). According to estimates of the Intergovernmental Panel on Climate Change (IPCC), the global average temperature is expected to increase by 3.7–4.8 °C by 2100 (Mumtaz et al. 2023), making the generative development, pollination, and fruit set of pepper particularly vulnerable. In addition to extreme abiotic factors, genetic diversity is a key requirement in every plant breeding program (Sood et al. 2023). F₁hybrid cultivars are widely used in pepper production, as this species exhibits pronounced heterosis (Guo et al. 2014). It is estimated that hybrids account for 80% of all pepper seeds used. F₁hybrids exhibit more uniform growth and generally achieve higher yields by 15–50%—depending on crop type—than parental lines (Fu et al. 2014; Meena et al. 2020). The production of F₁hybrid bell pepper seeds is one of the most profitable sectors worldwide; therefore, there is a strong demand for efficient and cost-effective approaches in commercial hybrid seed production (Cheng et al. 2018).

Currently, hybrid seed production largely relies on labor-intensive manual pollination and emasculation. In contrast, seed production based on male sterility can reduce costs, as it requires less labor and time for the preparation of female flowers. In addition, the application of male sterility can increase the genetic purity of F₁seeds by preventing self-pollination (Wang et al. 2006). Although cytoplasmic male sterility plays an important role in pepper hybrid production, its underlying molecular mechanisms remain incompletely understood (Zhang et al. 2015). Several DNA markers specific to cytoplasmic male sterility (Kim and Kim 2005), as well as SCAR (Sequence Characterized Amplified Region) markers derived from mitochondrial sequences, have been developed (Kim and Kim 2006); however, their practical application may be limited, as PCR amplification may occasionally occur at a low level in maintainer lines as well (Min 2009). Nevertheless, male sterility cannot be applied universally, particularly in breeding programs and in the production of specific hybrids; therefore, controlled pollination performed through manual emasculation continues to play an important role in pepper hybrid seed production (Shifriss 1997; Dhaliwal & Jindal 2014a, b).

The aim of this review is to discuss, within an integrated framework, the major abiotic stressors (heat, light, and water/nutrient supply) determining pepper fruit set and hybrid seed production, pollen biology and hormonal regulation, and the relevant findings on genetic and male-sterility systems, with particular emphasis on flower and fruit abscission, seed formation, and parthenocarpy.

Heat stress and reduced reproductive performance

Heat stress has numerous detrimental effects on plants, including protein denaturation, suppression of protein synthesis, protein degradation, and changes in the fluidity and integrity of lipid membranes (Howarth 2005; Zhang et al. 2022). The impact of heat stress varies among crops, species, and cultivars (Sita et al. 2017a, b), partly due to differences in ecological adaptation and species-specific temperature optima (Yamamoto et al. 2011). The effect of high temperature depends on the intensity and duration of the stress, fluctuations above the optimal temperature, and the developmental stage of the plant (Alghabari et al. 2015).

In pepper, heat stress can cause substantial yield losses during the vegetative and, most critically, the generative (reproductive) stages (Kafizadeh et al. 2008). Temperatures approximately 32 °C can severely impair pollination and fertilization, leading to reduced fruit set or complete fruit-set failure (Pérez-Jiménez et al. 2019). Gisbert-Mullor et al. (2021) determined that the pollen viability of flower buds smaller than 2.5 mm greatly decreased when they were exposed to 33 °C for 120 h. Plant responses to high temperature include short-term stress avoidance or acclimation mechanisms, such as leaf repositioning, evaporative cooling through transpiration, and modification of membrane lipid composition (Wahid et al. 2007). Exposure to moderately elevated temperatures may induce acclimation, conferring acquired thermotolerance against subsequent severe heat stress (Wahid et al. 2007; Senthil-Kumar et al. 2007).

Yamazaki and Hosokawa (2019) reported that the inbred C. chinense lines ‘Sy-2’ and ‘No. 3686’ showed a strong reduction in fruit set under high-temperature conditions, whereas their F1 hybrid maintained a high fruit set. Reciprocal artificial pollination experiments indicated that both female and male factors contribute to the enhanced fruit-setting capacity of the F1, which was further supported by a reduced heat-induced decline in pollen germination in the hybrid.

Generative organs are generally more heat-sensitive to heat stress than vegetative tissues; therefore, temperature stress occurring during the reproductive phase can have a disproportionately large impact on fruit set and yield (Hedhly and Hormaza, 2009; Jagadish et al. 2010a, b; Sita et al. 2017a, b). The reproductive response to heat stress involves several stages with differing sensitivities, including gametogenesis, the progamic phase from pollination to fertilization, and early embryo development. High temperatures often promote flower and young fruit abortion (abscission), which represents a direct physiological cause of yield loss (Warner & Erwin 2005; Koti 2005). In pepper, yield reduction induced by heat stress is typically not primarily associated with impaired vegetative growth, but rather with disturbances in reproductive processes—particularly pollen development, pollination, and early fruit retention (Peet et al. 1998; Pressman et al. 2002a, b; Wubs et al. 2009a, b).

Pollen as a critical “weak link”

Pollen is one of the most heat-sensitive plant tissues (Hedhly et al. 2009; Giorno et al. 2013). Consistently high temperatures impair pollen development and metabolism, reduce viability and germination capacity (Aloni et al. 2001; Pressman et al. 2002a, b; Firon et al. 2006; Wubs et al. 2009a, b; Müller & Rieu 2016; Kotak et al. 2007), and thereby directly decrease fruit set. Fruit set itself can serve as a screening trait for heat tolerance; however, it does not allow separate assessment of individual floral and pollen parameters and is highly influenced by environmental variability. The heat sensitivity of pollen is manifested not only in reduced germination rates but also in a reduction in pollen tube growth rate and the occurrence of structural and morphological abnormalities, which further decrease the probability of successful fertilization (Hedhly et al. 2004; Sato et al. 2002a, b; Müller & Rieu 2016).

In vitro pollen heat-stress assays have proven to be effective tools for evaluating thermotolerance. For example, Heidmann et al. (2016) demonstrated that the tomato pollen germination was examined between 37 and 40 °C, and it was shown that heat treatment at 38 °C negatively affected pollen germination in tobacco and tomato, which is consistent with previous publications. However, in pepper, only limited empirical evidence is available that directly links in vitro heat-stress responses to in vivo pollination, fertilization, and seed-set success. Moreover, the relationship between in vitro pollen traits and in vivo fruit set is often non-linear, as the physiological status of female tissues, processes occurring during the progamic phase, and interactions with fluctuating environmental conditions can substantially modify fertilization success (Hedhly et al. 2004; Peet et al. 1998; Wubs et al. 2009a, b).

Breeding for heat tolerance: complex genetic background and the role of wild relatives

Managing heat stress in cultivation is challenging; therefore, genetic improvement for heat tolerance is regarded as one of the most effective long-term strategies, although it requires the simultaneous evaluation of multiple physiological and reproductive traits (Lin et al. 2021). Wild relative species represent important genetic resources for improving heat tolerance (Driedonks et al. 2018a, b). Heat tolerance is a complex, polygenic trait resulting from the interaction of multiple genes (Jagadish et al. 2010a, b; Grover et al. 2013; Opeña et al. 1992; Lucas et al. 2013; Dickson 1992), and the expressed phenotype is often strongly dependent on the applied temperature regime (Ye et al. 2012).

Breeding for heat tolerance is particularly challenging during the reproductive phase, as generative traits—such as pollen viability, fruit set, and seed formation—are highly sensitive to environmental variation and often exhibit low to moderate heritability (Jagadish et al. 2010a, b; Müller & Rieu 2016; Sita et al. 2017a, b).

The substantial genetic diversity of wild Capsicum species and their adaptation to extreme environments are promising; however, introgression breeding in pepper has progressed more slowly than in other Solanaceae species, partly due to crossing barriers, reduced fertility in interspecific hybrids, and linkage drag (Aguilar et al. 2009; Carrizo García et al. 2016; Cheng et al. 2016; Oyama et al. 2006; Votava et al. 2002; Khoury et al. 2020; Parry et al. 2021).

Recent QTL-driven studies in Capsicum challenge the traditionally pollen-focused framework of reproductive heat tolerance. Yamazaki et al. (2025) identified a major-effect locus, qFS6 on chromosome 6, that was robustly associated with enhanced fruit set under elevated temperature conditions in greenhouse experiments yet showed no detectable linkage to pollen viability or dispersal traits. This decoupling indicates that, in certain genetic contexts, improved reproductive success under heat stress may arise through mechanisms other than male gametophytic function, highlighting the potential importance of processes such as ovary persistence and the regulation of abscission responses.

Nevertheless, several studies have demonstrated that alleles derived from wild relatives can effectively enhance heat tolerance, particularly when selection focuses on reproductive performance-related phenotypes, such as pollen viability, fruit set, and seed retention under elevated temperatures (Driedonks et al. 2018a, b; Jagadish et al. 2016; Tripodi et al. 2021). Key genes and genomic regions associated with reproductive heat tolerance in pepper and related Capsicum species are summarized in Table 1.

Table 1.

Genes and genomic regions associated with reproductive heat tolerance in pepper and related Capsicum species

Functional category	Gene/genomic region	Species/source	Reproductive trait affected	Physiological role under heat stress	References
Heat shock response	HSP70	Capsicum annuum	Pollen viability	Stabilization of proteins and maintenance of cellular metabolism during pollen development under elevated temperature	Pressman et al. 2002a, b
Heat shock response	Small heat shock proteins (sHSPs)	Capsicum annuum	Pollen development	Prevention of protein aggregation and protection of pollen cellular integrity during heat stress	Firon et al. 2006
Transcriptional regulation	HSFA2	Solanum lycopersicum (Solanaceae model)	Fruit set	Activation of heat-responsive transcriptional networks in reproductive tissues	Jagadish et al. 2010a, b
Oxidative stress regulation	APX (ascorbate peroxidase)	Capsicum annuum	Pollen germination	ROS detoxification, limiting oxidative damage in pollen under heat stress	Sita et al. 2017a, b
Carbohydrate metabolism	Cell wall invertase (INV)	Capsicum annuum	Pollen tube growth	Maintenance of carbohydrate supply required for pollen tube elongation and fertilization under heat stress	Firon et al. 2006
Hormonal homeostasis	GA2ox	Capsicum annuum	Fruit set, seed formation	Regulation of gibberellin levels influencing reproductive organ retention and early fruit development under stress	Tripodi et al. 2021
Reproductive development (stress-sensitive node)	CaOFP20 (Ovate Family Protein)	Capsicum annuum	Male fertility, pollen formation	Regulation of floral organ development and male fertility; altered function can exacerbate reproductive failure under heat stress	Cheng et al. 2018
Wild introgression loci	Heat tolerance–related QTLs	C. chacoense, C. baccatum	Fruit set	Enhancement of reproductive stability at elevated temperature through introgressed alleles from wild relatives	Driedonks et al. 2018a, b; Tripodi et al. 2021

Light intensity and light spectrum: fruit set, assimilate supply, and hormonal regulation

The spectral composition of light plays a key role in fruit set. A low red:far-red (R:FR) ratio or a very high blue:red (B:R) ratio reduces the phytochrome photostationary state (PSS), which in turn promotes flower and fruit drop in sweet pepper (Chen et al. 2022; Chen et al. 2024a, b; Legris et al. 2019; Kong et al. 2018, 2019). Flower and fruit drop represent hormonally regulated abscission processes, in which ethylene and abscisic acid act as promoters, whereas auxin and gibberellin have inhibitory roles (Crane 1969; Taylor and Whitelaw 2001; Sawicki et al. 2015). A high B:R ratio may also influence fruit set through light-dependent modulation of hormonal balance, partly mediated by blue-light receptors (Chen et al. 2024a, b).

Light intensity is one of the primary determinants of generative organ retention (Ascough et al. 2005). Reduced light intensity and shading increase flower abscission and reduce fruit formation in many crop species. According to Ferree et al. (2001), five days of 80% shading during flowering reduced grape fruit set from 47.3% to 29.2%. Zhu et al. (2011) found that, after 19 days of shading, about 98% of the young fruits abscised, whereas under normal (control) conditions less than 10% dropped. According to Aloni et al. (1996), for example, in some cultivars flower abscission increased to 86–100% when the maximum light intensity was reduced from 920 µmol m⁻² s⁻¹ to 200 µmol m⁻² s⁻¹. The effects of photoperiod and light spectrum are often closely coupled with total light quantity, making their individual contributions difficult to separate (Demers et al. 1998).

Fruit sets are also strongly dependent on carbohydrate availability. Low sugar and starch contents promote flower abscission (Aloni et al. 1996), whereas supplemental sugar supply can reduce abscission, partly through the activity of key enzymes involved in sucrose metabolism, such as sucrose synthases and invertases (Aloni et al. 1997; Nielsen et al. 1991; Ruan et al. 2012). A very high proportion of blue light can reduce leaf starch accumulation and overall dry matter production, thereby limiting fruit set through restricted photoassimilate supply (Warrington & Mitchell 1976; Wang et al. 2016; He et al. 2017; Larsen et al. 2022).

Mechanistic processes underlying fruit abortion under far-red (FR) light include reduced sucrose accumulation, altered invertase activity, and interactions between carbohydrate status and hormonal signaling pathways (Li et al. 2022; Chen et al. 2024a, b).

Pollen, flower biology, and fruit set: fundamentals and stress sensitivity

Flower biology, pollen production, and seed formation

Pollen is a central component of plant reproduction, and its long-term, viability and preservation are crucial in breeding and genetic resource maintenance programs. Pepper pollen is relatively short-lived, making cryopreservation relevant in some programs (Ren et al. 2019; Jia et al. 2022). Pepper is often partially cross-pollinated (7–65% outcrossing) (Herath et al. 2020), and the availability of male-sterility systems supports the widespread use of hybrids cultivars.

Successful fruit set depends on the synchronization of anthesis, pollen viability and germination capacity, and stigma receptivity (Peña-Yam et al. 2019; Dhall et al. 2011). Pollen production often correlates with another characteristic and may be closely associated with seed formation; high pollen viability increases the likelihood of successful pollination (Agnihotri and Singh 1975; Dafni 1992; Minnaar et al. 2019). Experiments have demonstrated that both open and manual pollination can increase fruit set, fruit size, and seed number compared to controls (Bergonia and Cervancia 1992; Deprá et al. 2014; Krell 2018).

Seed set can serve as a direct indicator for pollen fertilization success as incomplete fertilization can lead to deformed and uneven fruits (Torrey 1967; Patterson 1990). The correlation between seed number and fruit size suggests that the fertilization of more ovules stimulates fruit growth, and the amount of pollen reaching the stigma may also be a determining factor (Serrano Roldán and Guerra-Sanz 2006).

Pollen germination, pollen tube growth, and selection

A strong correlation exists between pollen germination rate and pollen tube elongation (Salem et al. 2007; Gajanayake et al. 2011). During the progamic phase, intense pollen competition occurs within the stigma, which can serve as the basis for gametophytic selection (Mulcahy 1979). Under stressful conditions, pollen originating from genotypes better adapted to elevated temperature may be preferentially successful in fertilization (Zamir 1982; Hedhly et al. 2004, 2005; Pasonen et al. 2002).

Temperature and carbohydrate metabolism in pollen function

High temperatures can reduce pollen quantity, alter pollen morphology, impair viability, germination capacity, and pollen tube growth rate, thereby directly limiting fruit set (Prasad et al. 2002). Koti (2005) found that high temperature reduced the number of pollen grains produced in soybean by approximately 34% compared with the control. Sato et al. (2002a, b) found that the number of released pollen grains decreased under temperatures above the optimum (28/22 °C, day/night). This caused stress in the processes of pollen development and release, resulting in less pollen reaching the site of fertilization. An increase of 2.5 °C above the optimal mean daily temperature can reduce fruit set by up to approximately 40%. Aloni et al. (2001) found that under heat stress, pollen metabolism slows down, resulting in higher sucrose and starch concentrations, while germination capacity decreases. Elevated CO₂, however, can increase assimilate availability, thereby improving pollen germination under high-temperature conditions. The development of the male gametophyte occurs within the anthers and requires the coordinated function of multiple tissue layers, including the tapetum (Goldberg et al. 1993; Ma 2005). The developmental period around meiosis is particularly heat-sensitive; under elevated temperature, morphological abnormalities and defective pollen formation may occur (Iwahori and Takahashi 1964; Peet et al. 1998; Sato et al. 2000). In pepper, reductions in fruit set under heat stress are primarily attributed to pollen damage, and to a lesser extent to impairment of maternal tissues (Han et al. 1996; Peet et al. 1998). Carbohydrate metabolism plays a key regulatory role in pollen development (Clement et al. 1994; Pacini 1996). In the days preceding anthesis, starch reserves are typically depleted and sucrose accumulates, and heat stress can disrupt sucrose utilization and associated metabolic fluxes (Dinar and Rudich 1985; Taub et al. 2000). In certain cultivars (e.g., Mazurka), heat stress reduced pollen germination and seed number, even when fruit number remained largely unchanged, highlighting the decoupling of fruit set and seed set under stress conditions (Aloni et al. 1996).

Hormonal regulation and an integrated model of fruit set and abortion

Auxin-ethylene balance and the abscission zone (AZ)

In pepper, flower and fruit abortion represent major physiological constraints on fruit formation and are regulated by the abscission zone (AZ) formed in the pedicel, together with the balance between auxin and ethylene signaling (Taylor and Whitelaw 2001; Wubs et al. 2009a, b; Botton et al. 2011; Chen et al. 2022). Under high-temperature conditions (approximately 32–40 °C), flower and fruit abortion frequently increase and are associated with enhanced ethylene production and reduced indole-3-acetic acid (IAA) levels in the AZ (Huberman et al. 1997). In bell pepper, ethylene promotes, whereas auxin inhibits flower abscission; accordingly, exogenous auxin applications in some studies improved vegetative growth, fruit parameters, and pollen viability (Wien et al. 1989; Kaur et al. 2017). However, the magnitude and direction of auxin effects may vary among cultivars (Motsenbocker and Arancibia 2000).

As illustrated in Fig. 1, environmental stressors and reproductive status converge at the pedicel abscission zone, where the maintenance or decline of auxin flux represents a central physiological switch between organ retention and abortion. Exogenous factors such as high temperature, altered light conditions, and resource limitation, together with endogenous signals linked to pollination and fertilization, modulate auxin transport through the pedicel and thereby determine ethylene sensitivity within the AZ, ultimately controlling whether the flower is retained or shed.

Fig. 1.

Environmental and endogenous control of flower abscission in sweet pepper: the central role of the pedicel abscission zone. Exogenous stress factors and endogenous reproductive signals converge at the pedicel abscission zone, where changes in auxin flux determine ethylene sensitivity and the decision between flower retention and abscission. Successful pollination and seed-derived auxin maintain polar auxin transport through the AZ, whereas environmental stress reduces auxin flux, enhances ethylene responsiveness, and promotes abscission

High temperatures can reduce auxin concentration and polar auxin transport in the pedicels of flowers and young fruits, while simultaneously increasing ACC levels, thereby enhancing ethylene biosynthesis (Wien et al. 1989; 1993; Huberman et al. 1997). Ethylene sensitivity of the AZ is further modulated by auxin status and carbohydrate availability, indicating close crosstalk between hormonal and metabolic signals.

Maupilé et al. (2024) showed that at 1.5 days after anthesis, fruit set in pollinated or parthenocarpic flowers is initiated by a reduction in ethylene (ET) and abscisic acid (ABA) levels, or by an increase in brassinosteroids (BR). At 3 days after anthesis, elevated auxin (IAA) and cytokinin (CK) levels stimulate gibberellin (GA) biosynthesis, thereby promoting fruit development. In contrast, in non-fertilized and non-parthenocarpic flowers, ABA and ET levels remain high after flowering, suppressing the accumulation of IAA, CK, and GA and ultimately leading to flower abscission.

Light quality, hormone transport, and “dominance”

A low red:far-red (R:FR) ratio and increased far-red light availability can enhance flower and fruit abortion by modifying auxin biosynthesis, transport, and distribution (Küpers et al. 2020). These effects are closely linked to assimilate competition and hormonal dominance, whereby auxin export from earlier-developing fruits and the apical shoot can suppress the development and retention of later-forming flowers and fruits (Bangerth 2000; Wubs et al. 2009a, b; Walker & Bennett 2018). According to Bangerth’s (1989; 2000) “correlative-driven abscission” concept, hormonal dominance among developing sinks—primarily mediated through the auxin–ethylene balance—plays a central role in regulating abscission processes.

Other hormones and assimilate supply

In pepper, comparatively less information is available on the role of abscisic acid (ABA); however, studies in other species indicate that ABA concentration often positively correlates with fruit senescence and may act as a mediator linking assimilate limitation to enhanced ethylene biosynthesis (Guinn & Brummett 1987; Gómez-Cadenas et al. 2000). Gibberellins are essential for fruit set, and inhibition of gibberellin biosynthesis or signaling can markedly increase the abscission of young fruits (Serrani et al. 2007; Webster & Spencer 2000). Overall, hormonal regulation and assimilate availability function as tightly interconnected and mutually reinforcing drivers of flower and fruit abortion (Guinn 1976; Wien et al., 1989b; Aloni et al. 1997).

Ovate family proteins (OFP) and hormonal networks

Ovate Family Proteins (OFPs) are plant-specific transcriptional regulators characterized by a conserved OVATE domain and were originally identified in tomato as key regulators of fruit shape (Liu et al. 2002; Tang et al. 2018). Subsequent studies indicate that OFP proteins also play broader roles in pepper, influencing vegetative and reproductive development, floral organ morphogenesis, pollen viability, and the maintenance of male fertility (Tsaballa et al. 2011; Borovsky et al. 2021). Yin et al. (2025) demonstrated that overexpression of the pepper CaOFP20 gene in tomato resulted in style elongation, anther developmental abnormalities, and reduced male fertility, effects that were associated with altered expression of genes involved in gibberellin metabolism (Yin et al. 2025). These findings suggest that OFPs function as regulatory nodes linking developmental programs with hormonal homeostasis, thereby indirectly influencing reproductive success under stress conditions.

Fruit drop (flower and fruit abscission): significance, mechanisms, and triggers

Low fruit set, caused by high rates of flower and young fruit abscission, is a major yield-limiting factor in sweet pepper (Wubs et al. 2009a, b; Chen et al. 2024a, b). In protected cultivation, up to 70–80% of generative organs may be shed, resulting in cyclic yield fluctuations and unstable market supply (Heuvelink et al. 2004). Abscission is an evolutionarily conserved developmental process that enables organ detachment through the integration of hormonal, environmental, and developmental cues (Estornell et al. 2013; Tonutti et al. 2023).

At the molecular level, abscission is a genetically programmed and tightly regulated process. In model systems such as Arabidopsis thaliana, several distinct phases can be distinguished, ranging from abscission zone (AZ) formation to cell wall remodeling and final sealing of the separation surface (Kim 2014; Kim et al. 2019). The AZ consists of specialized cell layers, in which degradation of the pectin-rich middle lamella and the formation of a protective boundary layer are key steps (Lee & Kwak 2018; Patharkar & Walker 2018).

Triggering factors

In bell pepper, flower and fruit abscission are influenced by multiple interacting factors, including high temperature, reduced light intensity, water and salt stress, plant density, insufficient pollination, application of plant growth regulators (PGRs), and unfavorable source–sink relationships (Reyes-Castro et al. 2024). Stress conditions such as shading and high temperature can reduce photosynthetic performance and vegetative growth (Turner & Wien 1994a,b; Aloni et al. 1996). However, several studies indicate that abortion is not driven solely by reduced photosynthesis, but rather by altered dry matter partitioning and reduced assimilate supply to generative organs (Turner & Wien 1994b; Aloni et al. 1996).

Flower and fruit abortion are commonly interpreted within two conceptual frameworks: (i) competition for assimilates (source-sink limitation) and (ii) hormonal dominance mediated by auxin export and ethylene signaling (Marcelis et al. 2004; Bangerth 1989; 2000). These mechanisms are not mutually exclusive, as reduced assimilate availability can trigger hormonal responses that activate the AZ and promote abscission.

Seedlessness and parthenocarpy: stress, hormones, and breeding value

According to Rylski & Spigelman (1982) low night temperatures (approximately 12–15 °C) can markedly reduce flowering and fruit set in pepper by impairing pollen viability, leading to flower abortion or the formation of parthenocarpic (seedless) fruits, and at a low night temperature (15 °C) most of the pepper fruits were seedless: on average, about 34% of the fruits were parthenocarpic. Polowick & Sawhney, (1985) likewise found that low day/night temperature combinations (for example, 18 °C during the day and 15 °C at night) strongly affected flower and fruit development, reduced male gamete (pollen) viability, and consequently decreased fruit set and seed number, thereby increasing the proportion of seedless fruits. Seedless fruits are often smaller and morphologically irregular, although the extent of parthenocarpy shows substantial cultivar dependence (Shifriss & Eidelman 1986; Tiwari et al. 2007).

Seedless fruit formation is frequently induced through the exogenous application of plant growth regulators, including auxins, gibberellins, and cytokinins, which can promote fruit set under stress conditions (Heuvelink & Körner 2001; Silveira et al. 1986). However, certain treatments (e.g., GA₃) may negatively affect pollen viability and cause abnormal floral development, limiting their practical applicability (Pressman et al. 1998; Sawhney 1981).

From a biological perspective, fruit development is normally initiated by pollination and fertilization, with the primary function of the fruit being seed protection and dispersal (Lord and Russell, 2002). From an economic standpoint, however, seedless fruits may be desirable in several crops (Varoquaux et al. 2000). Hormones produced by developing seeds, particularly auxin, act as central regulators of fruit growth, making the hormonal and genetic control of parthenocarpy an important target in breeding programs (Sotelo-Silveira et al. 2014; Joldersma & Liu 2018).

Genetics, heterosis, and male-sterility systems

Hybrid breeding and Capsicum diversity

In pepper, hybrid breeding remains partially underexploited, partly due to limited understanding of heterosis and reciprocal effects. Increasing climatic uncertainty enhances the value of genetic diversity within the Capsicum genus for developing resilient and market-adapted cultivars (Tripodi et al. 2020; Devi et al. 2021). Of the 41 described Capsicum species, only five are domesticated; domestication and selection have generated extraordinary diversity in fruit morphology and quality traits (Pickersgill 1997; Barboza et al. 2019). Although many Capsicum species are predominantly self-pollinating, heterosis has been investigated only to a limited extent, despite reports of increased yield, biomass accumulation, and altered gene expression in selected hybrids (Garcés-Claver et al. 2007; Singh et al. 2014; Yang et al. 2021).

Male sterility (GMS and CMS/CGMS) in seed production

Genetic male sterility (GMS) represents a key tool in Capsicum hybrid breeding, as it can substantially reduce labor costs and prevent self-pollination (Meena et al. 2018; Jeong et al. 2018). GMS is often controlled by single recessive nuclear genes (Swamy et al. 2017), allowing efficient introgression into breeding lines using molecular markers (Dhaliwal & Jindal 2014a, b; Zhang et al. 2020). The genetic basis of cytoplasmic-genic male sterility (CMS/CGMS) was first described by Peterson (1958); in pepper, sterility is commonly induced by the interaction of sterile cytoplasm (S) with a recessive nuclear gene (Rf1), while fertility restoration depends on a dominant Rf1 allele (Shifriss 1997). In some genotypes, an additional recessive gene (Rf2) contributes to complete male sterility (Novak et al. 1971).

Water and nutrient supply, humidity: abortion and seed set

Nitrogen supply and abscission

In bell pepper, flower abscission can be higher under low nitrate (NO₃⁻) levels; correlations have been reported between growth parameters and the plant’s carbon and nitrogen content (Riga 2014). Increasing total nitrogen (N) concentration (3 → 6 → 9 mM) enhanced flower and fruit set; in some systems, a partial ammonium proportion during the vegetative stage was beneficial, whereas nitrate dominance during fruit development proved more favorable (Xu et al. 2001).

Flower and fruit abortion increases linearly with decreasing photoassimilate availability and may be triggered by leaf pruning, high plant density, reduced radiation, or rapid growth of early-developing fruits (Marcelis et al. 2004). The most sensitive window typically occurs in the weeks surrounding anthesis.

Relative humidity and temperature

Baër and Smeets (1978) reported that fruit drop rates were largely unaffected by constant relative humidity (RH) levels of 55%, 80%, or 95%; however, at 95% RH, fruit set was reduced. Other studies observed negative correlations between flower and fruit number and nighttime RH, while daytime RH also influenced fruit drop (Bakker 1989). At high temperatures (≈33 °C), Erickson and Markhart (2001) found no strong RH effects on flower buds and flowers, suggesting that heat-induced abortion is not solely driven by humidity conditions.

Water supply and cultivar effects

Under water deficit, some cultivars (e.g., ‘Blue Star’) exhibit pronounced increases in flower and fruit abortion (Jaafar et al. 1994), whereas others show moderate or non-significant responses (González-Dugo et al. 2007; Fernández et al. 2005). Increasing irrigation frequency has been shown to promote earlier flowering and higher yield in several experiments (Sezen et al. 2006; Tanaskovik et al. 2015). Water stress can reduce the concentration of reducing sugars in flower buds, potentially impairing pollination and enhancing flower drop (Ningoji et al. 2024).

Conclusions and recommendations

The ecological and evolutionary consequences of climate change are likely to manifest through the high sensitivity of reproductive processes. Sexual reproduction is central to plant fitness, and environmental stress during gametogenesis and fertilization can influence adaptation across generations via gametophytic selection and phenotypic plasticity. Epigenetic mechanisms, including DNA methylation, may contribute to the persistence of stress effects and, in some cases, to transgenerational responses. Because both pollen and the female gametophyte are directly exposed to environmental conditions, the reproductive phase represents a critical vulnerability point, where stress-induced fertility reduction acts both as a risk factor and as a potential selective filter.

Fruit set in pepper and the success of hybrid seed production emerge from the tight integration of environmental, physiological, hormonal, and genetic processes. Temperature and light conditions directly affect pollen development, viability, and fertilization success, while light intensity and spectral composition modulate the retention of generative organs through photoassimilate availability and photoreceptor-mediated signaling. At the organ level, the auxin-ethylene balance, together with associated ABA and gibberellin pathways, functions as a critical regulatory switch within the abscission zone. These hormonal signals interact closely with source-sink dynamics and correlative dominance, providing a mechanistic explanation for cyclic patterns of fruit set observed under stress conditions. At the genetic level, diversity within the Capsicum gene pool, together with the application of GMS and CMS/CGMS systems, underpins the efficiency, genetic purity, and economic sustainability of hybrid seed production.

Looking ahead, progress in stress-resilient pepper breeding will increasingly depend on the integration of physiological insight with applied breeding strategies. Key advances are expected from the empirical linkage of rapid, pollen-based stress phenotyping to fruit-set and seed-set performance, the targeted optimization of light spectrum and intensity in protected cultivation, and the reliable marker-assisted deployment of male-sterility systems. In parallel, improved strategies for the introgression of adaptive alleles from wild Capsicum relatives, while minimizing linkage drag, will be essential. Together, these approaches provide a foundation for developing robust, climate-adapted hybrid seed production systems capable of maintaining yield stability under increasing environmental variability.

Abbreviations

ABA

Abscisic acid

ACC

1-Aminocyclopropane-1-carboxylic acid

AZ

Abscission zone

B:R

Blue:red light ratio

CK

Cytokinin

CMS

Cytoplasmic male sterility

CGMS

Cytoplasmic-genic male sterility

ET

Ethylene

FR

Far-red light

GA

Gibberellin

GMS

Genetic male sterility

IAA

Indole-3-acetic acid

INV

Invertase

OFP

Ovate family protein

PGR

Plant growth regulator

PSS

Phytochrome photostationary state

QTL

Quantitative trait locus

R:FR

Red:far-red light ratio

ROS

Reactive oxygen species

sHSP

Small heat shock protein

Author contributions

Ingrid M. Gyalai: writing—original draft, validation, conceptualization. Orsolya Kedves: writing—review and editing, validation, supervision, conceptualization. Alfonz Kedves: writing—review and editing, validation, conceptualization. Zoltán Kónya: validation, conceptualization. Flórián Kovács: writing—review and editing.

Funding

Open access funding provided by University of Szeged. University of Szeged Open Access Fund, Grant ID: 8412 (Szegedi Tudományegyetem, 8412, Ingrid Melinda Gyalai).

Data availability

Not applicable.

Declarations

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.