Combination Effects of Sowing and Transplanting Time on Harvest Time in Some Onion (<i>Allium cepa</i> L.) Cultivars with Different Photoperiod Requirements in Hokkaido

Abstract

The combination effect of sowing and transplanting time on harvest time, yield, and external bulb quality was investigated to achieve earlier shipment of onions in northeastern Hokkaido, Japan. A total of five cultivars with different levels of photoperiodic responses for bulb formation were used in both the 1996/97 and 1997/98 trials. The seeds were sown on a monthly basis in a soil bed in an unheated plastic greenhouse from December to March, and the seedlings were transplanted outdoors from late April to mid-May. The daily minimum soil temperature was kept at almost 0°C, even when the air temperature in the plastic tunnel inside the greenhouse fell to −14.4°C in midwinter. Seedling emergence took approximately 20 and 35 days in the 1996/97 and 1997/98 trials, respectively, when the seeds were sown in December and January. However, these seedlings grew slowly, and the leaf length, fresh leaf number, and leaf sheath diameter of the seedlings at transplanting time were all greater when the onion seeds were sown earlier. In addition, the bulb ripening time advanced with earlier transplanting. The cultivars, 'Kitawase No. 3' and 'Kitahayate', with an intermediate photoperiodic response, were harvested in early August if they were sown in December and January and transplanted from late April to early May. Moreover, with this combination of sowing and transplanting time, these cultivars produced an acceptable yield and bulb appearance in terms of marketable quality. The cultivar, 'Sonic', a typical short-day cultivar, had the earliest harvest time. However, the yield was very low due to the short period of leaf growth. On the other hand, for the cultivars 'Okhotsk No. 1' and 'Kitamomiji 86', which belong to a long-day photoperiodic response group, although the yields tended to increase under early sowing or early transplanting, they were not harvested by early August. On the basis of these observations, a new cropping type for early sowing and early transplanting will be adopted in northeastern Hokkaido by using the 'Kitawase No. 3' and 'Kitahayate' cultivars for early shipment in the domestic fresh onion market.