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

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. 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.