Cucurbit Genetics Cooperative Report 2:46-47 (article 28) 1979
Use of In vitro Culture for Inducing Germination of Cucumis Seed
D. L. Visser and J. Franken
Horticultural Plant Breeding, Wageningen, The
Several 10-year old seed samples of accessions of wild Cucumis species in the breeding collection of the Institute for Horticultural Plant Breeding did not germinate under standard conditions, which included heat treatments as used for commercial cucumber seed. Soaking the seeds in GA-3 and scarification of the accessions remained dormant.
Therefore, we tried a modified embryo culture technique, similar to that reported by George and Crowder (1). Seeds were sterilized for 20 minutes in 2% sodium hypochlorite solution and washed in sterilized water. The seedcoats were removed and apparently healthy embryos were placed in sterile conditions on a modified medium of Murashige and Skoog (2) with sucrose (2%), caseine hydrolysate (5x10-4),indole acetic acid (1x10-7), thiamine (4 x 10-7), and Difco bacto agar (0.8%). After one week, 13% of the explants were growing well. The other embryos grew slowly or did not grow at all. Remarkably, the slow growing embryos did not develop a tap root. Only some lateral roots developed to a certain extent. About 50% of the embryos grew into normal plants; the others degenerated after some time. Using this technique, we have been able to raise plants of several accessions of the species Cucumis africanus, C. myriocarpus, C. figarei, and C. metuliferus, that would otherwise have been lost.
Within C. sativus we encountered germination problems with the seed of tetraploid x diploid crosses. The fruits on tetraploid plants developed normally after such crosses, but the seeds were at best only partly filled. Eight weeks after pollination, the embryos were up to 6-7 mm in size (normal length is about 11 mm) and hardly any germinated. This phenomenon has been observed by many researchers when attempting to produce triploids, and it certainly has hampered the progress of triploid and aneuploid research in cucumbers. Therefore, the in vitro culture technique was used here also. The complete mature fruit was disinfected with 96% alcohol, seeds were extracted, and embryos were dissected out under sterile conditions. Each fruit typically contained 4-10 embryos of 4-7 mm long and a great number of embryos of about 1-2 mm, which often were brown. The larger embryos developed excellently, and 60% grew into plants which could be transferred into soil after only one week. The small embryos (1-3 mm) failed to grow. More than 90% of the dissected embryos grew, and out of several crosses, more than 100 plants were obtained. The ploidy level of the plants was checked by examination of pollen and general morphology, and all appeared to be triploid. In the same way, plants have been obtained of triploid x diploid crosses and 20 healthy plants are now being checked for aneuploidy.
The in vitro culture technique is a useful contribution to increase the yield of triploid plants to a suitable level for research programs, and it appears helpful for saving specific species accessions with inferior seed quality.
- George, B. F. and L. V. Crowder. 1970. The use of a modified embryo culture technique in cucumber breeding. HortScience 5:329. (abstr.).
- Murashige, T. and F. Skoog. 1962. Revised medium for rapid growth and bioassays with tobacco tissue culture. Physiol. Plant. 15:473-497.