Study on the Relationship between Flowering Characteristics and Fruit Setting of Oil Tree Peony

This study investigated the association between floral and fruiting traits in oil tree peony to inform elite breeding and hybrid programmes. Floral and fruiting metrics of Paeonia ostii ‘Fengdanbai’ and Paeonia rockii ‘Zibanbai’ were recorded under natural conditions; data were analysed using independent-samples t-tests, Pearson correlations, stepwise multiple linear regression and membership functions. Paeonia ostii ‘Fengdanbai’ produced more pollen number per flower, but fewer ovules and seeds per fruit than Paeonia rockii ‘Zibanbai’ (p < 0.05). Pollen viability and stigma receptivity followed similar unimodal temporal curves in both taxa. Peak pollen viability occurred on day 3 after anthesis in P. ostii ‘Fengdanbai’ (86.95 ± 1.41%) and on day 2 in P. rockii ‘Zibanbai’ (85.91 ± 2.99%). Optimal stigma receptivity spanned days 3–5 and 2–5 post-anthesis, respectively, yielding a 3-day overlap favourable for synchronous pollination. Seed number per fruit correlated positively with petal number per flower (r = 0.49), ovule number per fruit (r = 0.68) and seed setting rate (r = 0.70, p < 0.01); carpel number also contributed (r = 0.31, p < 0.05). Stepwise multiple regression identified ovule number per fruit and seed setting rate as the most significant predictors of seed number per fruit (p < 0.001), together explaining over 99% of the variation in the model (adjusted R2 = 0.992). Membership function analysis, based on pollen viability, ovule number per fruit, and seed setting rate, suggested that ‘Fengdanbai’ exhibits superior comprehensive performance, though selection indices should be validated across broader germplasm. We propose ovule number per fruit and seed setting rate as primary selection indices, with pollen viability as a supplementary indicator, for consideration in breeding high-yield cultivars.