Cucurbit Genetics Cooperative Report 1:5 (article 4) 1978
Cucumis hardwickii: A Source of Germplasm for the Cucumber Breeder
E. K. Horst and R. L. Lower
North Carolina State University, Raleigh, NC 27650
Cucumis hardwickii, an annual monoecious cucurbit species resembling C. sativus, hybridizes readily with C. sativus, producing a fertile F1 with no observed reduction of fertility in the F2. C. hardwickii sets larger numbers of fruit per plant than C. sativus and does so in a manner not typical of C. sativus. Fruit with developing seeds do not inhibit growth and seed development of later fruit as occurs in C. sativus.
Plants of the C. hardwickii line1 we are using are indeterminate, seem to lack apical dominance and have more and longer laterals than lines of C. sativus. C. hardwickii plants are typically slow growing in early stages of growth and remain vegetative for extended periods of time. After the slow growth period, C. hardwickii enters a rapid phase of growth surpassing total vine length of C. sativus cultivars. Preliminary observations of field plantings of C. hardwickii in North Carolina revealed that number of days to flowering was sensitive to length of day or photoperiod and possibly temperature. Plants grew well vegetatively but did not flower until early September regardless of whether planted in May or July.
Cultivars containing C. hardwickii germplasm might result in genotypes with higher yields per plant than present C. sativus cultivars. The development of a plant type suitable for mechanical harvesting as well as improved yield is an important consideration of our breeding program. In a genetic study, C. hardwickii was crossed with two C. sativus cultivars, 'Gy14', a gynoecious, indeterminate line, and 'PG', a monoecious, determinate line. Plants of paternal and F1, F2, and BC1 generations were spaced 0.6m apart and were allowed to set a full complement of fruit. 'Gy14' had 1-6 fruit/plant. Number of fruit for C. hardwickii was not determined due to inhibition of flowering by long-day conditions. (In other seasons C. hardwickii has averaged 80 fruit/plant.) The F1 ('Gy14' x C. hardwickii) averaged 20 fruit/plant. Backcrosses to 'Gy14' and C. hardwickii ranged from 0-40 and 0-250 fruit, respectively, while the F2 plants had 0-90 fruit. Backcrosses to 'PG' and C. hardwickii ranged from 0-27 and 0-97 respectively. The F2 plants had 0-99 fruit. Because of lack of fruit number data for C. hardwickii , further genetic analysis was not attempted. However, data were taken for all populations for number of laterals. C. hardwickii had a mean of 11.4 laterals/plant. 'PG' had 2.4, while the F1 had 12.9. The estimation of heterosis was 87%. The backcross to C. hardwickii had 10.6 laterals, while the backcross to 'PG' had 5.2, and the F2 had 8.8. Environmental variance was 4.38, additive variance was 12.46, and dominance variance was -1.19. Heritability was 80%.
1 G. W. Bohn (USDA, Brawley, CA 92227) supplied our original seed as LJ 90430.