Optimizing both the fresh weight of the bulb and the overall yield is crucial to achieve profitable onion production. This study aimed to examine the effects of cultivar, plant density, and the amount of light interception on bulb weight and total yield. Two yellow onion cultivars, ‘Momiji No. 3’ and ‘Totana’, were grown at plant densities ranging from 10.4 to 31.2 plants per m2 (with plant spacings of 24, 16, 12, and 8 cm at a fixed row spacing of 24 cm) over two years. Bulb fresh weight increased as plant spacing widened, whereas total yield per unit area increased with plant density. This shows a trade-off between individual bulb size and yield. ‘Totana’ consistently produced larger bulbs and higher yield than ‘Momiji No. 3’ at each plant density. For both cultivars, regardless of plant density, the amount of integrated light interception post-transplantation strongly correlated with bulb fresh weight and dry matter production. High light interception is driven primarily by a greater leaf area index (LAI) and canopy development. Among tested conditions, plant densities of 10.4–20.8 plants per m2, with plant spacings of 24, 16, and 12 cm, in ‘Totana’ provided the optimal balance between bulb weight and yield, with bulbs exceeding 300 g per plant and total yields surpassing 4.0 kg·m−2. For ‘Momiji No. 3’, wider plant spacing (16–24 cm) increased bulb weight, and bulbs exceeding 300 g may be achieved under favorable environmental conditions. The results of this study suggest that optimizing the combination of cultivar selection and plant density, while considering light interception capacity, can simultaneously improve bulb size and yield under field conditions.
Oku et al. (Thu,) studied this question.