A review of the effects of onion varieties with different photoperiod requirements and their origin on selecting the suitable sowing date.

Onion (Allium cepa L) varieties grow at different latitudes. Onions need cool temperatures in the early stages of growth, and high temperatures and long days during bulbing. Sowing of seed in autumn, winter, and early spring results in more leaf growth and subsequently higher bulb yield than sowing in late-spring and summer. Onion varieties show different reactions to temperature and day length. Short-day varieties need shorter day lengths and low temperatures for bulb induction compared to long-day varieties. As the required minimum day length of an onion variety is met with increasing temperatures, bulbs will be initiated. Short-day varieties reached the bulbing phase at day lengths of 11–12 h. Cultivation of short-day varieties originating from low and equatorial latitudes in areas with higher latitudes resulted in early maturity. All onion varieties, regardless of photoperiod requirements, require low temperatures in the early growing season and seedling stage. Cultivars with similar photoperiod requirements in terms of morphological and genetic traits are located in different groups. Carbohydrates are transferred from leaves to bulbs over long days and during the bulbing stage. There is a close relationship between sugar and flowering genes like AcFT2. Region, variety, and genetics of varieties influence onion bulbing. [ABSTRACT FROM AUTHOR]