Growth responses of tropical onion cultivars to photoperiod and temperature based on growing degree days

Onions (Allium cepa L.) are widely produced within the tropics, but little is known about the Eritrean cultivars Hagaz Red 1 and 2 (HR 1, and HR 2) and Red Creole (RC). Responses of the onion cvs. HR 1, HR 2 and RC to photoperiod and temperature on bulbing were compared. Plants were grown in growth rooms under combinations of photoperiod (11.5, 12, and 12.5 h) and day/night temperatures (25/12, 30/15 and 35/18 degree C). Growth responses were determined by growing degree day (GDD) base and the broken-stick regression model. There were relationships between bulb initiation and rate of leaf area growth under 12 and 12.5 h. Under the 12 h photoperiod, cultivars needed 343, 482, and 597 GDD before bulb initiation and 405, 432, and 431 GDD to increase rate of leaf area development at 25/12, 30/15, and 35/18 degree C, respectively. Under a 12.5 h photoperiod, cultivars needed 344, 423, and 432.2 GDD to initiate bulbing and 140, 411, and 579 GDD to increase leaf growth rates at 25/12, 30/15, and 35/18 degree C, respectively. Temperature induces variations in leaf number, plant height, leaf area, and affect bulbing and these responses may be used in the development of superior cultivars for tropical conditions.