Abstract
1. On account to know how to cultivate onions at the regions of diffrent altitude, studies on the effects of day length, temperature and age of plant on bulb formation were conducted. Three varieties, i. e. Aichi-shiro (early), Senshu-ki (medium), and Sapporo (late), were planted at different times in Nagano (360m) and Sugadaira (1, 250m) in 1950 to 1952.
2. The fall sown onions started the development of bulbs after the daily mean temperature had exceeded 10°C in spring. The most favorable temperature range was 18-20°C for both fall-and spring-sown onions.
3. In fall-sown onions. bulb formation started much later than the dates when the day lengths became sufficiently long for bulb formation. It seems that the low temperature limited bulb formation of onions in such cool regions in Nagano Prefecture.
4. Despite enough daylength and temperature for bulb formation were provided in early stages of growth in spring-sown onions, bulb formation was much delayed as compared with the fall-sown onions. It seems that age of seedlings was one of limiting factors in this case.
5. The least ages of seedlings necessary for bulb formation were 4, 5, and 6 leaves in Aichishiro (early), Senshu-ki (medium), and Sapporo(late), respectively.
6. It may be possible to increase yields of spring-sown onions by raising seedlings in hot bed so as to allow them to start bulb formation earlier in the season.
