Effects of Temperature on Bulb- and Tuber-formation in Bulbous and Tuberous Crops

VI. On the Bulb Formation in Bulbous Iris

Abstract

A great number of informations on the periodicity of flowering in plants have been obtained in this half century. While few works have been done on the periodicity of bulb formation. In the previous paper, it was reported that there were two thermo-reactive processes-induction of physiological states for corm formation and development of corms as the thickening growth of stems-for corm formation of reesia.
It has been known for bulbous iris that when store at 8° to 13°C of low temperature soon after digging, those do not sprout but consume their own food materials to form daughter bulbs within the mother bulbs. Hence the present study was designed to clarify the effect of temperature on both induction and process of bulb formation in bulbous iris, in using the bulbs of“Dominator”and“Wedgwood”iris.
The results obtained were summarized as follows:
1.When iris bulbs were stored under various constant temperatures, new daughter bulbs were formed by 5°, 10° and 15°C storages, respectively but were not formed under 0° and 20°C storage.
2. After treated with various low temperature (0°, 5°, 10° and 15°C) for various days, the bulbs were stored under 17°C in the thermostat. The bulbs exposed to 5° and 10°C for 30 days formed a central bulb within the mother bulb and did not sprout, even if planted on November 6 in a wet sand bed. Bulb formation was delayed by 0° and 15°C of chilling and in this case, the longer the duration of chilling, the more bulb formation was accelerated.
3. After the bulbs were exposed to 5° and 10°C of chillings for 45 and 60 days they were stored under 5° to 25°C. When the bulbs were stored at 15° to 20°C, bulb formation was accelerated and large bulbs were produced. Whereas, when the bulbs were stored under less than 10°C and more than 25°C, the bulb formation was delayed and a new bulb was scarcely formed in 5°C storage until December 16.
4. When the bulbs were exposed to 30° to 35°C for 10 to 30 days after the chilling treatment, the development of new daughter bulbs suppressed as the duration of high temperature lengthened.
5. When the bulbs of various sizes were stored at 17°C after the chilling treatment, the daughter bulb was formed within the mother bulb even in 2.5g of small bulbs, regardless of bulb size.
6. From the results obtained in this experiment, it seems that in order to form new bulb, bulbous iris must pass through the same two thermo-reactive processes as freesia does. It is evident that such a low temperature storage as 5° to 10°C is apparently effective for the induction of physiological states for bulb formation and somewhat high temperature, 15° to 20°C promoted the development of a new bulb. Further, the chilling effect on induction of bulb formation increased with lengthening of chilling duration up to a certain limit. Exposure to high temperature, 30° to 35°C followed chilling destroyed the physiological states for bulb formation.
7. Based on the previous results on garlic, freesia and oxalis and on the results mentioned above on bulbous iris, it seems that the acquisition or acceleration of the bulb-and tuber-formation ability by a low temperature treatment is the general characteristic in the bulbous and tuberous crops which produce the bulbs or tubers in early summer, and this phenomenon resembles to vernalization for flower formation.