With an annual production of about 250 000 to 300 000 tons, onion (Allium cepa L.) is the most important outdoor vegetable crop in the Netherlands. The greater part, namely 70 to 80 %, has to be exported, and is meeting increasing competition. For this reason and in connection with changed growing and handling methods, including mechanization, there is a need to breed improved varieties.
Breeding-work with the biennial onion, however, takes much time and shortening of the breeding cycle would be very profitable. This has been the purpose of the present research. By means of literature data, all phases of growth and development .have been investigated. Flower induction and initiation appeared to be dependent on temperature, but not on day length.
At first we tried to promote rapid flowering of the bulbs by means of temperature treatments (chapter 3.1). This did not give satisfactory results. Development was very irregular and slow and only exceptionnally all the plants became generative (table 3). Nevertheless, we obtained more information about the factors required for development. We showed that the optimum temperature for flower induction for the cultivar "Rijnsburger" is 9° C rather than 13° C (graph 4). Moreover, short heat treatments before and after the storage period stimulate the regrowth (graph 2), but counteract the floral development (antivernalisation and devernalisation). This explains why the yield of seeds after storage of the mother bulbs under supra-optimal temperatures is lower than after storage under sub-optimal temperatures.
Abortion of the young scapes (flower stalks) occurs exclusively under long days and to a greater extent at high temperatures (table 6). Our observations indicate that abortion is dependent on the distribution of assimilates between the scape and an axillary bud (photo 3). A "bulb/flower competition hypothesis" was formulated in the sense that, just as with onions grown for bulb production, in long days and at higher temperatures this axillary bud swells, which results in abortion of the scape (photo 4).
Hitherto rarely different stages of development in onion in relation to optimal conditions have been distinguished. For a good development different conditions of day length and temperature appeared to be required. For that reason an outline was proposed that consists of three phases:Thermo-phase
. Flower induction and initiation are dependent on the temperature, irrespective of day length.Competition-phase
. Under long day and high temperatures, abortion may occur but at low temperatures (about 9° C) the flower scape develops, even in long days.Completion-phase
. For a good further development long days are necessary and a high temperature is beneficial.
Based on this information an annual breeding cycle has been realized by using onion plants (chapter 3.2). Early in September the seeds were sown in a greenhouse under natural short day conditions, in which bulbing and the associated rest period are prevented. With supplementary light during the daytime and day and night- temperatures of 16° C and 13° C respectively, in about 100 days strong plants were obtained.
In the middle of December this plant material was transplanted and kept under long days at 9° C, under which circumstances flower induction and initiation were promoted and bulbing and abortion were completely inhibited (photo 7). In April the temperature was raised to 18° C and in May to at least 21° C. Under these temperature conditions and natural long days without supplementary light, all the plants developed into good seed plants, the seeds of which were harvested by mid July. In the beginning of August, 11 months after the initial sowing, these new seeds could be sown.
An extra gain in time of at least one month is possible, by using unripe seeds with a maximum moisture content of 50% (chapter 3.3). By using a higher light intensity during the raising period and further improvements in the day length and temperature regime during the competition and completion phases, it is probably possible to obtain four generations in three years.
Finally, some possibilities for application of the results in breeding projects have been discussed (chapter 4).