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Proper vernalization temperature of mother onion bulbs stimulates early flowering and produces a heavier yield of seed. Moreover, onion cultivars seem to differ in their optimal temperature requirements depending on the locality to which they are adapted. Most studies on the effects of storage temperature on inflorescence initiation in onion have shown that the optimum temperature for flowering is in a range between 5˚C to 13˚C for 90 to 120 days and bolting resistant varieties require a longer cold stimulus (154 -185 days) in comparison to normal spring sown cultivars. Flowering is greatly reduced or suppressed at temperatures in the range of 15.5˚C -30˚C and low temperatures of −3˚C to 0˚C. Studies on both bulbs and seedlings have shown that, depending on genotype, flower initiation starts only after a juvenile phase, when the plant has a minimum number of 4 -14 leaves, including leaf initials. The initiation of inflorescence is favoured by a large set or bulb size. The post planting environmental conditions greatly affect bolting percentage of bulbs stored at an inductive temperature. Maximum seed yield in onion has been reported from large mother bulbs and early planting time. The emergence of inflorescence is faster at high temperatures (20˚C -30˚C) and long days (14 -16 hours).

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