Genetic Factors and in Vitro Manipulations Influence Seed Dormancy in Cucumber

Seed lots with the genetic background of 'Baroda' and 'Marketer' cucumber (Cucumis sativus L.) containing all possible combinations (DF Df, Dfdf, dfdf) of df (which increases dormancy) and Df (wild type) were used. Dormancy was not solely due to the genotype dfdf and clear effects of genetic background were apparent. The df allele in the homozygous state induced a strong dormancy in 'Baroda', but the Df gene could not restore normal germination. However, Df did reduce the dormancy period to 85 days. In 'Marketer', df did not delay germination. Any treatment (punc- turing, removal, cutting) that damaged the inner integument allowed 'Baroda' dfdf to germinate, indicating an intact integument was essential for maintaining dormancy in this cultivar. All 'Baroda' dfdf embryonic axes without the cotyledons germinated in 5 days. 'Baroda' dfdf seeds with intact integuments imbibed adequate water to germinate but remained dormant, suggesting that the effect of the integument on dormancy was not related to imbibition. Cucumber seeds normally germinate a few days after being extracted from freshly har- vested mature fruits. However, Shifriss and George (1965) reported seed dormancy in an Indian cultivar known in the United States as 'Baroda'. Seeds remained dormant for 6 to 12 months, but dormancy could be over- come by exposing the seeds to 50C and 90% relative humidity for 6 days. Whereas most cucumber cultivars are day neutral, Shifriss and George (1965) also ob- served that flowering in 'Baroda' was af- fected by photoperiod. They showed that the df allele caused delayed flowering in 'Bar- oda' and was recessive to the Df allele for normal flowering found in other cucumber cultivars. Seed dormancy and flowering be- havior in 'Baroda' were not independent characteristics. They suggested that the Df locus was either linked with a gene (or sev- eral) affecting seed dormancy or the Df gene interacted with other genes to affect germi- nation. This study was designed to determine 1) how the delayed flowering (df) gene affects seed dormancy; 2) if seed dormancy could be overcome by manipulations of the seed in vitro; and 3) whether seed dormancy is as- sociated with specific seed tissues. In the 1960s (Shifriss and George, 1965) the df gene was identified in 'Baroda' (BA) and transferred to the normal-flowering 'Marketer' (MK). Through backcrossing, the Df gene from 'Marketer' was also put into 'Baroda' (W.L.G., unpublished). The plants were self-pollinated over many generations