Studies on the bulb formation in onion plants

VIII. Internal factors in reference to bulb formation

Abstract

The experiments were conducted to clarify internal factors in reference to bulb formation such as the role of leaf and of root, translocation of stimulus of long-day and plant age, using onion plants, cv. Kaizuka-wase. Plants were grown under short-day condition until the start of the treatment.
1. Three leaves of four-leafed plants were defoliated so that only one leaf variously aged remained on the plants.
When those leaves were exposed to 24 hours daylength for 14 days, the bulbing was no difference among ages of leaf except for leaf area (Table 1 and Fig. 1).
2. Four-leafed plants were grown in each the following treatment for 8 days. (1) Entire plants were exposed to 24 hours day-length. (2) One, two or three leaves covered with aluminum-foil were daily exposed to sunlight only for 8 hours, and the other parts of the plants not covered with foil were exposed to 24 hours day-length.
After localized photoperiod-treatment, the leaves exposed to long-day or to short-day were defoliated in obedience of experimental schedule, followed by short-day for 5 days (Table 2).
After defoliation, the plants with LD-leaves developed their bulb, but the plants with only SD-leaves did not, even though their appearence indicated symptoms of LD induction, viz. emergence of subsequent leaf from pore of innermost leaf. Moreover, the plants with both SD-and LD-leaves bulbed as more as the plants with only LD-leaves, without distinction of the LD- or SD-leaves, the more number of leaves the greater the bulb development.
3. When one half of tillered plant was exposed to 24 hours day-length and the other tiller to 8 hours day-length, the tiller exposed to LD developed to bulb but the other tiller did not (Fig. 2).
Moreover, when two leaves surrounded tillered parts were exposed to 24 hours day-length and tillered parts were exposed to 8 hours day-length, tillered parts developed their bulb too.
From these results, it may be concluded that stimulus of long-day exposure in each part of tillered plant did not translocated each other.
4. In two lots, the roots of onion plants were removed immediately before long-day or immediately after long-day exposure. The entire plants transplanted immediately before long-day exposure and non-transplanted plants were served as control.
Plants of four lots were exposed to 24 hours day-length for 8 days and followed by day-length of 8 hours for 10 days.
Non-transplanted or transplanted controls and the plants removed after long-day exposure well developed their bulbs, but the plants removed before long-day exposure failed to bulb (Table 3).
5. Plants of three-leaf stage and of four-leaf stage were exposed to sunlight for 8 hours and followed by incandescent light of 2000, 750 and 240 lux for 16 hours. In each light intensities, the older plants developed larger bulb than younger plants. (Table 5)
Ten days old plants (loop stage) were exposed to 24 hours day-length for 8 days and followed by 8 day-length for 10 days From above result, it was assumed that the youngest age to induce bulb was 1st foliage-leaf stage. (Table 4 and Fig. 4).