Sowing time and temperature influence bulb development in spring-sown onion (Allium cepa L.)

Bulb size is an important factor in determining the productivity of onion (Allium cepa L.), and production of suitable bulb sizes is a priority objective in cultivation. Previous studies have reported that bulb development is controlled by photoperiod and temperature. Although many studies have investigated the influence of day length on bulb development, the effect of temperature has not received as much attention. It has been shown that transplanting time strongly affects bulb size, and our study showed that crop yield and bulb size of four commercially used cultivars in Japan significantly decreased from the earliest to the latest sowing in spring-sowing cultivation. This phenomenon was investigated using the commercially used onion cultivar 'Momiji No. 3' and a temperature gradient growth chamber, and our study revealed that a high temperature inhibited leaf initiation and decreased leaf number, which preceded bulb development and was closely related to final bulb size. We also show that plant growth, including parameters such as total leaf number, stem diameter, and bulb diameter, fresh, and dry weight, linearly increased with increasing accumulated temperature, which was calculated from the integrated values of the daily mean temperature; growth was suggested to have a strong positive correlation with temperature because the r-values were greater than 0.9. Therefore, for onion production, apart from choosing a suitable cultivar and sowing time for each production area, to attain the production potential of each cultivar it is important that an optimal temperature is maintained during plant growth and bulb development.