Phenotypic and molecular characterization of vernalization sensitivity and pre-mature bolting in onion (Allium cepa L.)

Onion is a biennial crop, with bulb development occurring in the first year and flower induction in the second. Bulb formation is mainly controlled by day length and flowering by the vernalization. Flowering (Bolting) is a pre-requisite for seed production, but pre-mature bolting is undesirable during bulb production. In this study, onion germplasm (122 lines) was screened for pre-mature bolting (%) and days to pre-mature bolting in the early Rabi (November transplanted) and Rabi (January transplanted) seasons (2020-2021) and we found significant differences between genotypes for both traits. The extent of pre-mature bolting was higher during early Rabi (0-76.67%) than Rabi season (0-24.60%). Genotypes were divided into different classes for pre-mature bolting incidence viz. resistant (< 1%), tolerant (1-10%), susceptible (10-25%), and highly susceptible (> 25%). Furthermore, genotypes exhibited quantitative response to pre-mature bolting with cold exposure. Consistent results for both the traits were observed in the 51 representative genotypes during the subsequent season. Molecular characterization of onion lines with 36 simple sequence repeat (SSR) markers grouped pre-mature bolting susceptible and tolerant/resistant lines in distinct clusters highlighting the genetic differences between them. This study indicates that genotypes with strong vernalization requirement exhibit pre-mature bolting tolerance/resistance and variation in vernalization sensitivity differentiate genotypes at phenotypic and molecular level. The results of this investigation will help onion breeders in the development of pre-mature bolting tolerant/ resistant onion cultivars.