Bulbing in Onions: Photoperiod and Temperature Requirements and Prediction of Bulb Size and Maturity

Bulb size and maturity are key characteristics of an onion cropand the onset of bulbing is an important determinant of these.In this paper we describe an experiment in which bulb and neckdiameter and leaf number were measured in onion crops (cultivarsPukekohe Longkeeper and Early Longkeeper) with different sowingdates planted at two different locations in New Zealand. A sensitiveindicator of earliest time of bulbing was developed using theratio of bulb and neck diameters and the statistical techniqueof cusums. Bulb diameter at bulbing was related to thermal timeaccumulated prior to bulbing. Bulbing only occurred when dualthresholds of a minimum thermal time of 600 degree days anda photoperiod of 13.75 h were reached. Mathematical relationshipswere developed between leaf number, sowing date, bulbing dateand bulb growth and maturity. Final bulb size could be predictedfrom bulb size at bulbing and number of leaves produced afterbulbing. Bulb maturity date could be predicted by number ofleaves after bulbing. Allium cepa L.; onion; temperature; photoperiod; bulb-neck ratio; leaf number; bulbing