Photoperiodic Regulation of Bulbing in Onions (Allium Cepa L. Var. Yellow Granex)

Photoperiod plays a vital role in the growth, bulb formation, and yield of onion (Allium cepa L.). This study evaluated the effects of varying photoperiods on the growth and bulb development of onion cv. Yellow Granex. The experiment was conducted from February to May 2024 at the ACIAR–ICM project site, Department of Horticulture, Visayas State University, Visca, Baybay City, Leyte, Philippines. The treatments consisted of natural daylength (control) and photoperiod extensions of 8, 10, 12, 14, and 16 hours arranged in a Randomized Complete Block Design (RCBD) with three replications. Growth and yield parameters such as days to bulb formation, plant height, number of leaves, neck diameter, bulb weight, bulb length, and bulb diameter were evaluated. Results revealed significant effects of photoperiod on the horticultural characteristics and yield components of onion. The earliest bulb formation was observed under the 14-hour photoperiod, followed by the 16-hour treatment, while delayed bulbing occurred under the 8- and 10-hour photoperiods. Plants exposed to shorter photoperiods produced taller plants and more leaves, indicating prolonged vegetative growth. In contrast, longer photoperiods promoted earlier bulb initiation but reduced vegetative growth. The control and 12-hour photoperiod treatments produced significantly heavier and larger bulbs compared with shorter photoperiod treatments. The findings demonstrated that photoperiod strongly influenced bulb initiation, vegetative growth, and bulb development in onion. A 12-hour photoperiod was found to be favorable for bulb yield and quality under protected cultivation conditions.