B. STORAGE OF ONION - THE BIOLOGICAL BASES OF ONION STORAGE
The duration of the developmental cycle of onion depends on the climatic conditions. In subtropical regions and in Central Asia the developmental cycle from seed to seed takes 11–12 months, in the steppe belt of the USSR it is completed in 2 years and in the middle belt of the USSR in 3 years: here they obtain sets from seeds in the first year, in the following year the sets grow into large bulbs, used either for consumption or for seed production, the so-called mother bulbs, which are planted out after winter storage. In more northern regions, where seeds do not mature or give a poor yield, onions are multiplied vegetatively.
In our experiments in 1935–1938, bearing in mind that the onion frequently fails to mature, we looked for ways to hasten the bulb formation and the onset of the rest period. As a result we established that the dormancy could be induced by subjecting the onion to dry conditions; by the mutual competition of plants because of a crowded stand (in this case the dormant period commences irrespective of the availability of nutrients and water in the soil), and, at last, by pulling growing plants out of the soil and let them mature. At the expense of organic matter translocated from the leaves, plants pulled out of the soil will form bulbs which can be stored and used for vegetative propagation, provided they have grown in a day length of not less than 14 hours. The optimum temperature for a rapid and completely translocation of organic matter from the leaves is 20–25°C.
Bulbs which form as a result of translocation of organic matter from the leaves on plants which have been pulled out of the soil a month after germination, contain less dry matter and sugar than bulbs which form on older plants (in one experiment bulbs from 35 days old plants contained 12.5% dry matter, 50 days old - 13.8% and 80 days 17.4%, and 5.96, 7.58 and 10.71% total sugars respectively).
When growing onions from sets, the yield does not depend on the age of the sets, but only on their weight and the storage temperature.
Much work has been done by Soviet and foreign research workers on the influence of day length and temperature on the growth of onion plants, bulb formation and seed yield. From these experiments one can conclude that onion is a long day plant when considering both bulb and seed formation. Our own experiments to investigate how bulb formation depends on day length and temperature when onion is grown from seeds or by vegetative propagation from sets or larger bulbs, have shown that in continuous light all varieties form bulbs earlier but the bulb size is