Inflorescence initiation pattern as affected by air temperature in fall-transplanted intermediate-day onion.

Owing to fall-transplanted onions (Allium cepa L.) grown in temperate zones overwinter, cultivars and agricultural practices have been developed to decrease bolting. Because of climate change, there frequently occur warm winters and low temperatures in spring. Physiological disorders, previously not shown often, deteriorate quality and yield of onion bulbs. This study evaluated the effects of spring temperature on bulb development and inflorescence initiation of intermediate-day onions. Thirty plants (3 replicates of 10 plants) were randomly collected at approximately 5-day intervals from 16 March to 5 June. Plant growth and bolter characteristics were measured, ad the number of lateral leaves and centers, and number of bulb scales in bolted and unbolted onion were counted. Bolting incidence increased three times, first in mid-March, second in mid-April, and last in early-May. Mean daily air temperature preceding increases in bolting incidence were 6.8°C (0.8–13.1°C, min.-max.) in the first 10 days of March, 10.8°C(2.8–18.5°C) from 5 April to 15 April, and 11.8°C (4.7–18.2°C) from 17 April to 28 April. Inflorescences induced after onset of bulbing did not develop into normal spathes and flowers but degenerated into immature umbels and did not produce individual florets. When temperatures after inflorescence induction or bulb initiation, was lower than the temperature required for bulb or inflorescence development, the lateral bud occasionally developed into leaves with leaf blades or swollen sheaths. These results provide information on onion bulb development and inflorescence physiology and to predict onion bolting or splitting in the field. [ABSTRACT FROM AUTHOR]