THE EFFECT OF THE PERIOD OF LOW TEMPERATURE BEFORE OVERWINTER ON BOLTING AND FLOWERING IN EXTREMELY EARLY MATURING ONION VARIETIES

Extremely early maturing onion varieties have fewer and variable bolting number compared with medium and late maturing onion varieties for seed production. And these varieties are sensitive to and at variance with environmental factors making seed production difficult. Generally in onions, flower bud formation is determined by its reaction to low temperature around 10°C and the mother bulb vegetative growth. This study was conducted to find out the effect on bolting and flowering in extremely early maturing onion varieties of the duration of low temperature. Two extremely early maturing onion cultivars, ‘Kwiguem’ and ‘Marusino 330’, were planted by 1 mother bulb per pot on October 4, 2004. They were grown in the station’s open field to check on their responses to low temperature, and then they were cultured in the greenhouse at 20°C on Nov. 15, Nov. 30, Dec. 15, and Dec. 30. The number of hours for temperatures above and below 10°C (8.1-11.0°C) on the field was summed up, and the results were 74.6 h on Nov. 15, 146.7 h on Nov. 30, and 155.5 h on Dec. 15 and 30. The bolting ratios of those treated on Dec. 15 were 72.3 and 74.8% for ‘Kwiguem’ and ‘Marusino 330’, respectively, higher than those treated on Nov. 15 as 52.8% for ‘Kwiguem’ and 43.4% for ‘Marusino 330’. Similarly, the flowering ratios on Dec. 15 were 56.1% for ‘Kwiguem’ and 46.1% for ‘Marusino 330’, higher than those treated on Nov. 15 as 6.8 and 22.8%, respectively. The C-N ratio was analyzed on Jan. 30 to determine its effect on flower bud formation. ‘Kwiguem’ treated on Dec. 15 had the highest at 26.8%, while those treated on Nov. 15 had the lowest at 13.3%. Thus, bolting and flowering in extremely early maturing onion varieties depend more on longer hours and higher C-N ratio in low temperature.