Femaleness in breeding glasshouse cucumbers

In growing parthenocarpic cucumber varieties the occurrence of bullnecks due to pollination by bees is a problem. To prevent the formation of these bullnecked cucumbers the possibilities of breeding female-flowered varieties were explored. Use was made of material derived from the gynoecious line MSU 713-5. Femaleness is caused by the gene Acr, which accelerates the production of female flowers. Within the type which, genetically, must be regarded as female, a variation occurs from total absence of male flowers to rather numerous axils with male flowers, mainly on the lower part of the stem. On the one hand this variation is also genetically controlled, and we may speak of “weakly” female and “strongly” female. On the other, environmental factors play an important part. The formation of male flowers in preponderantly female-flowered material is promoted by a long day, high night temperature and particularly, at least with our material, low light intensity; furthermore by a low pH of the soil and rich dressings of nitrogen. In order to be able to distinguish during selection between the weakly and strongly female plants, the environmental conditions should be favourable for the formation of male flowers. The maintenance of female plants is possible, because the formation of male flowers can be induced by spraying with gibberellin, after which selfing can be practised. Old plants respond less readily to gibberellin than young plants. Also, the response from strongly female plants is less pronounced than that from weakly female ones, which offers another possibility of selection. The extent to which male flowers may occur when growing female varieties depends on the cultivation method used. With the conventional method fruiting is forced to begin mainly on the laterals and any male flowers, if confined to the main stem, could be removed early. However, in view of fluctuations caused by the environment a rather pronounced female character will be desirable. Other cultivation methods are conceivable, e.g. short-term cultivation of larger numbers of plants per area unit, allowing the plants to come into bearing rapidly. The total absence of male flowers is then indispensable. With the aid of the femaleness it is also possible to simplify the production of hybrid seed, as hand pollination can be replaced by bee pollination. The combination gynoecious × monoecious, which in itself is the most favourable for this purpose, has the drawback that the resulting hybrid is usually insufficiently female. Therefore, use will generally have to be made of the combination gynoecious × gynoecious, spraying the latter parent with gibberellin.