Abstract
This paper reports the results obtained from the observations on the maturing process of plant grown under natural condition and on the effect of external factors on some characteristic phenomena which are found in relation to the bulbing of onion plant as a part of experiments carried out from 1959 to obtain the fundamental data of onion cultivation.
1. The process of bulb development was observed with Senshuki onion plants grown in the field under long day condition from April to July in 1959 and 1961.
The increases in height and rooting occurred rapidly from April and reached their maximum at the beginning of May, being followed by the remarkable thickening of bulbs.
As the bulb developed, the number of new roots decreased and their root activities declined. As a result the ratio of top weight to root weight increased gradually.
The top prostration was observed at the later stage of bulbing, and was significantly correlated with the increase in the ratio of top weight to root weight.
2. Onion plants were grown under short or long day condition to clarify the effect of external factors on the following phenomena concering bulbing.
(1) The increase in height was very slow under short photoperiod, wheareas the rapid increase occurred under long photoperiod, being followed by a remarkable bulb thickening.
Accordingly it is clear that a rapid increase in height indicates the initiation of bulbing, and that the two growth periods are noticed in the bulbing, that is, the rapid leaf growth period and the bulb thickening period.
(2) The morphological characteristics of bulb in onion plant were the scale leaf formation and the thickening of basal part of leaf sheath.
The former was induced by the long photoperiod at first, and then the latter by the aid of photosynthesis.
Measurement of transversal diameter at the base of leaf sheath is not proper for determining the time of scale leaf formation. The time of scale leaf formation may be determined by the following two ways: (1) To know the time to reach the maximum in height and (2) to examine the curve of the ratio of leaf blade to leaf sheath from outer leaf to inner one.
(3) Development and activity of roots were affected not only by day length and temperature, but also by the bulb thickening. Namely, both long photoperiod and high temperature arrested rooting and curtailed root activity. The thickened bulbs had little ability to develop their roots.
(4) The neck of bulb is the weak point where top falls down owing to the weakness of solid core of growing leaf blade which gives the neck rigidity before bulbing. It appeared that the top prostration may be affected by water supply from the roots too.
(5) It was observed that the tillering of the first order was formed even under short photoperiod, while the bulb division up to the third order was found under long photoperiod with the thickening of bulbs. It seemed that the tillering is not directly affected by the photoperiod but by the thickening of bulbs.
