Abstract

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°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°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°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.

Key words:  Growing degree days (GDD), onion, bulb, broken-stick analysis.