Physiological studies on the bulbing and dormancy of onion plant. III.

Effects of external factors on the bulb formation and development

Abstract

The present paper describes in detail the effects of external factors on the bulbing in onion plants.
1. As the photoperiod became longer, the plant height rapidly arrived at the maximum, being followed by the remarkable thickening of the bulb.
The final size of bulbs at harvest time significantly correlated with the size of seedlings at the beginning of bulb formation.
2. The effect of the long photoperiod during the course of development was nullified by the interruption with the short photoperiod, and the bulbing phase was reversed to the vegetative growth phase. The number of days necessary for the reversal of bulbing phase to vegetative growth phase increased gradually with the thickening of bulbs, but it is possible to reverse the phase even after the top prostration of the plants.
3. The higher the temperature under long photoperiod, the earlier the bulb development. It was not considered, however, that the high temperature compensates partly day length as the low temperature acts as a part of short days for short day plants.
4. The growth of leaves was promoted by the high temperature, but if the temperature was higher than 25°C, the duration of leaf growth was shortened and the final length of a leaf was rather short.
It is considered that the minimum temperature available for leaf growth is 10°C, and the favorable temperature is ranged from 17 to 25°C. This tendency was more remarkable under the long photoperiod than under the short photoperiod.
The Aichi-Shiro seedlings elongated more remarkably than the Kaizuka-Wase and Imai-Wase seedlings and arrived at its maximum height very early.
Consequently the height of the plants was in the following order;
Aichi-Shiro <Kaizuka-Wase<Imai-Wase
5. It is very noticeable that the bulbing was found under the long photoperiod even at 10°C, the minimum temperature for leaf growth.
6. Under the long photoperiod the initiation of bulb formation was not affected by the light intensity, but the thickening of bulbs was inhibited by the low light intensity.
7. The bulb formation was not affected by nitrogen top dressing under the long photoperiod, but a heavy nitrogen application considerably inhibited the thikening. Under the critical day length, the nitrogen application retarded the initiation of bulb formation, though producing the large bulbs at later harvest time.
8. When the long photoperiod was given to the different sized plants, their height began to increase and arrived at their maximum at the same time. This result showed that the initiation of bulb formation was not affected by the size of plant at the beginning of long day treatment, but the thickening was considerably affected by the size of the seedlings.