Studies on the Flower Bud Formation in Onion Plants

III. Effects of Physiological Conditions on the Low Temperature Induction of Flower Buds in Bulbs

Abstract

Some experiments were carried out to elucidate the effects of bulb size, removal of scaly leaves and foliage leaves and dormant condition on low temperature induction of flower buds in onion bulbs.
The results were summarized as follows:
1. As the index of bulb size, bulb weight was used effectively in the following experiments.
It was found that the larger the bulb size, the shorter the period of low temperature (9°C) exposure required for flower bud formation.
There was recognized, however, the presence of the critical bulb size at which the period of low temperature required was minimized.
Thus, in “Senshuki” the critical bulb size appeared to be about 100-150g in weight and the minimum period of low temperature exposure to be about 40 days, while in “Sapporoki” they appeared to be about 30-50g and 30 days respectively; the latter being recognized to be more sensitive to low temperature exposure.
2. Bulbs, when their scaly leaves were romoved in various degrees before planting, required longer periods of low temperature exposure according to the severity of the treatment. The periods required were shorter, however, as compared with the intact bulbs of the same size corresponding to that of the treated bulbs.
The removal of foliage leaves after planting also increased the period of low temperature required.
3. The removal of scaly leaves and/or of foliage leaves resulted in remarkable decrease of carbohydrate contents of bulbs, which appeared to be related to the increase of low temperature requirement in treated bulbs.
The results mentioned above may be explained well by assuming the accumulation of the precursor in the bulbs, which changed into the so-called florigen at low temperature exposure. This precursor appears to accumulate in higher density in inner scales and its concentration seems to change in parallel with carbohydrate contents.
4. When bulbs immediately after, or 30 days after, harvest were exposed to low temperature without water supply, they almost did not respond to, or required much longer period of, low temperature exposure for flower bud formation.