Studies on the bulb formation in onion plants

Abstract

This experiment was carried out to ascertain the photo-environmental factors except light quality on bulb formation of onion plants using cultivar Kaizuka-wase. Plants were grown under short-day condition until the start of the treatment.1. Plants were shaded to reduce the daylight of natural day-length (astronomical day-length 14h 20 m-14h 30m). One group was grown in full sunlight, two others under one layer of white or black cheesecloth, and the 4th under two layers of black cheesecloth. Those admitted 100, 70-75, 30-40 or 10-20 per cent of the full light respectively.Not only their bulb formation was retarded by 10-20 per cent of full light, but also failed to bulb by 30-40 per cent of light. Moreover, by 10-20 per cent of light, the symptoms of long-day induction did not appear in the pore of innermost leaf (Table 1).2. Plants were exposed to the reduced sunlight for 8 hours, followed by suppelementary incandescent light of about 2000 lux for 16 hours. Plants grown in full sunlight had the highest bulbing-ratio, followed by plants grown under 70-75, 30-40 and 10-20 per cent of full sunlight, the plants that received the lowest intensity, also, developed their bulb (Table 1).3. Plants were exposed to full sunlight for 8 hours and followed by incandescent light of about 2000, 750 or 240 lux for 16 hours. The higher the supplementary lighting became, the higher the bulbing-ratio became, and even under the lowest intensity their bulb well developed (Table 1).4. Onion plants were exposed to 16-, 20- and 24-hours day-length (sunlight of 8 hours, followed by supplementary incandescent light) for 10, 12 or 15 days and then followed by 8 hours day-length. Measurements were made at 20, 22 and 25 days after the beginning of long-day exposure.When total duration of light given during long-day was ′240 hours′ as follows: 24 hrs-10 days, 20 hrs-12 days and 16 hrs-15 days, at same duration after the beginning of long-day, the longer the day-length became, the more the bulb developed, in spite of same duration of given light.But, at same duration after the beginning of short-day or at 10 days after the beginning of short-day exposure, such as 20, 22 and 25 days after the beginning of long-day exposure for 24, 20 and 16 hours day-length respectively, there was no difference between the reciprocity (day-length times duration of long-day) (Fig. 1).5. Plants were given 8 hours of sunlight and 16 hours of darkness, the dark periods being interrupted with incandescent light of 1000 lux at different times after the beginning of dark period for 8 hours. Plants interrupted at 2 hours after the beginning of dark period had the highest value of bulbing-ratio, on the contrary, plants interrupted at 4 hours (middle of night) had the lowest value (Fig. 2).6. Plants were given 8 hours of sunlight and 16 hours of darkness, the dark periods being interrupted with incandescent light of 1000 lux for 50 per cent of the time in light-dark cycles of various lengths. Plants received additional light of 16 hours served as 24 hours day-length control.The light-dark cycles are as follows: 30-32, 60-16, 120-8, 240-4, 480-2 and 960-1 (length of one light-dark cycle, in minutes-number of cycles) In secondary experiment, plants were given 12 hours each of light and darkness, the light-dark cycles are as follows: 30-24, 60-12, 90-8, 120-6, 180-4, 240-3, 360-2 and 720-1.The shorter the cycles became, or the more the number of cycle became, the more the bulb formation was induced, besides, it was found that relation between length of cycles or number of cycle and bulb formation was linear logarithmic (Fig. 3 and Fig. 4).