Source–sink modulation associated with wood vinegar and mineral fertilizer shapes fruit size and yield formation in bell pepper (Capsicum annuum L.)

Wood vinegar (WV) has gained attention as a low-cost biostimulant with potential to enhance crop performance when integrated with mineral fertilizers. However, its effects on yield and fruit development in bell pepper remain insufficiently understood. This study evaluated the effects of sole and combined applications of WV and synthetic fertilizer (SF) on plant height, growth rate, flowering time, fruit number, yield components, and fruit morphometrics of Capsicum annuum L. under pot-based conditions. Five treatments were arranged in a completely randomized design: an unfertilized control, sole WV, SF at the recommended rate (RRF), and WV combined with SF at 50% below, or 50% above the RRF. Vegetative growth parameters, including plant height and growth rate, were not significantly affected by treatment. In contrast, flowering occurred earlier under combined WV and SF applications (26.25–27.25 d) compared with the control (31.5 d). Sole WV application reduced fruit number (2.82 fruits per plant) but increased fruit weight (7.43 g fruit−1) relative to the control (5.50 g fruit−1), indicating enhanced fruit development rather than increased fruit set. Synthetic fertilizer primarily increased fruit number, reaching 4.34 fruits per plant at the recommended rate. The combined application of WV and SF produced the highest yield per plant (20.52–22.12 g) and improved fruit size and morphology, including increased fruit length (6.64–6.74 cm) and fruit volume. Overall, these results demonstrate that WV mainly enhances fruit filling and size traits, whereas SF promotes fruit initiation, and that their combined application optimizes reproductive efficiency and marketable yield in bell pepper production.