Floral phenology, insect pollinators and seed quality of 36 genotypes of Stevia rebaudiana Bert. cultivated in Italy

The complex reproductive mechanisms and the poor seed germination represent serious obstacles towards large scale establishment of Stevia rebaudiana Bert. The main goal of this study was to evaluate differences in the reproductive development of 36 F1 open-pollinated genotypes during flowering formation, pollination and seed production in order to identify the best genotypes, environmental condition and plant-pollinator interactions, for seed production and quality. Germination characteristics of seeds obtained from different genotypes grown under greenhouse without pollinators, were analyzed. Reproductive phenology, from the beginning of flowering to seed maturity, have been studied and the numbers of corymbs quantified for every genotype. Pollinators’ diversity and abundance have been quantified over the whole flowering period. In Central Italy the flowering stage spans from beginning of September to the end of October, whereas seed formation and maturity lasts until the end of November. Flowering and seed formation overlap each other. The rainy conditions during flowering and seed formation hampered pollinators’ activity and the seed filling. All F1 genotypes showed a large variability in terms of flowering time, length of flowering and number of corymbs per plant. All genotypes were visited by insects during anthesis. Main pollinators were Hymenoptera and Diptera. Honeybees and hoverflies represented the most abundant species, followed by sweatbees and blowflies. Seed germination rate and mean energy time differed among the genotypes. Further differences were found in the seed quality as consequence of pollinator abundance and spectrum. Overall, our findings suggest that the genotype choice, cross-pollination, and environmental conditions during the reproductive stage, together with support of pollinators are important parameters for achieving high seed yield and quality in S. rebaudiana.