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 (puncturing, 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. Table 1. Seed germination of various genotypes of ‘Baroda’ and ‘Marketer’ cucumber. Cucumber seeds normally germinate a few days after being extracted from freshly harvested 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 overcome 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 observed that flowering in ‘Baroda’ was affected by photoperiod. They showed that the df allele caused delayed flowering in ‘Baroda’ and was recessive to the Df allele for normal flowering found in other cucumber cultivars. Seed dormancy and flowering behavior in ‘Baroda’ were not independent characteristics. They suggested that the Df locus was either linked with a gene (or several) affecting seed dormancy or the Df gene interacted with other genes to affect germination. 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 associated 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 Received for publication 17 Sept. 1990. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact. 1076 to obtain near-isogenic lines. These genotypes, homozygous for alleles Df and df, were used in these studies. Seeds of each genotype (MK DfDf, BA DfDf, MK dfdf, BA dfdf) were germinated in petri dishes; the seedlings were transferred to a glasshouse and grown at Urbana, Ill., under the natural long days of April to August. One flower on each plant was self-pollinated, and another flower on the same plant was outcrossed. Reciprocal crosses were made in all 16 pair-wise combinations. Fruits were harvested 45 days after anthesis and the seeds extracted, cleaned, and air-dried. At various times after harvest, 30 seeds of each genotype were germinated in petri dishes for 15 days. Delayed germination in ‘Baroda’ was not explained completely by the df gene. DfDf genotypes did not germinate normally (Table 1). If the only gene regulating dormancy in cucumber were dfdf, self-pollinated MK dfdf (Table 1, cross 3) should have shown dormancy, but did not. BA dfdf was dormant (Table 1, cross 2), but BA DfDf was also somewhat dormant (Table 1, cross 4). It is evident from cross 3 that the df gene did not cause dormancy with the MK background. These results indicate an interaction between genetic background (i.e., cultivar) and dormancy induction by the df gene. With ‘Marketer’ background, neither seeds homozygous for the Df allele (Table 1, cross 1) nor those homozygous for df allele were dormant (Table 1, cross 3) Among heterozygous seeds, Dfdf (with ‘Baroda’ background) seed dormancy appeared to be influenced by the maternal parent (Table 1, crosses 15 and 16). Dormancy was broken more rapidly in seeds from a cross where the female was DfDf than for seeds where the female was dfdf. Hence, the df gene may interact with cytoplasmic factors, or there may be a maternal effect in which the offspring phenotype depends on maternal genotype. This effect, however, was not observed with the ‘Marketer’ background (Table 1, crosses 13 and 14). If, however, the dormancy-inducing factor(s) is caused by a cytoplasmic mechanism or maternal effect that is independent of the df gene, then crosses with “Baroda’ as the female parent should produce seeds with a higher degree of dormancy. However, in comparing crosses 8 vs. 7, 10 vs. 9, and 12 vs. 11, there was less dormancy when ‘Baroda’ was the female parent. From these studies, it is clear that dormancy is not due solely to genotype dfdf, and, in fact, the allele has no effect on seed dormancy with some backgrounds. There seems to be an interaction of df gene with the three genes that actually are responsible for germination (Shifriss and George, 1965). Genotype dfdf induced strong seed dormancy in ‘Baroda’, and the Df gene could not restore normal germination in this line. In ‘Marketer’ background, df gene did not deHORTSCIENCE, VOL. 26(8), AUGUST 1991 Table 2. Effect of cotyledon size and inner integument on germination of ‘Baroda’ dfdf cucumber seedZ. Table 3. Effect of seed inner integument on water uptake of ‘Baroda’ dfdf and ‘Marketer’ DfDf