Pollination Method Affects Gold Kiwifruit (Actinidia chinensis) Fruit Weight Predominantly Through Seed Set: Convergent Mediation Evidence from Multiple Seasons (2018–2025) and Two Orchards

Pollination is the rate-limiting reproductive step in dioecious fruit crops. However, the developmental pathway through which pollination interventions translate into fruit weight remains unclear. Here, we tested whether seed set was the dominant pathway linking three pollination methods, namely, artificial pollination (AP), pollen dispenser-assisted bee pollination (BTD), and bee-only pollination (BT), to fruit weight and soluble solids content (SSC) in gold kiwifruit (Actinidia chinensis cv. ‘Haegeum’). A total of 2940 individually measured fruits were collected across two orchards in southern Korea (2018–2025), comprising 2022 main experiment (n = 1800), supporting trials (2018–2020, N = 540, including a six-treatment factorial design in 2020), and a 2025 replication of 600 fruits. Across the calibration range (seed number 100–700), each additional seed contributed 0.063–0.072 g of fruit weight (R2 = 0.68–0.72). The slopes were statistically indistinguishable among AP, BTD, and BT. Mediation analysis was triangulated through three estimators: partial correlation (49.8–63.1% mediation), product-of-coefficients with 10,000 bootstrap resamples (70.3–77.4% indirect-effect mediation), and model-free stratification that converged on the same conclusion. When seed number was matched (500–700 range), treatment differences in fruit weight collapsed to non-significance (F = 0.34, p = 0.714). The two mediation percentages reflect different denominators, that is, standardised association vs. unstandardised total effect, and both estimators agree qualitatively. SSC was not mediated by the seed number (<8%). The small residual SSC pattern may reflect block-level confounding rather than a method-intrinsic effect. A random-effects meta-analysis across six-year site combinations returned pooled Hedges’ g = +0.43 (95% CI: −0.02 to +0.89, I2 = 95.1%), with two years in which BTD significantly exceeded AP. In 2025, BTD achieved the Ferguson (1984) ≥1000-seed commercial standard in 79% of fruits. These findings identify the seed set as a method-independent reproductive bottleneck, reframing pollination system evaluation around pollen deposition-to-seed set conversion rather than around the delivery mechanism and support an efficiency-focused approach to orchard pollination management.