Cantaloupe (Cucumis melo L.) is a warm-season crop that germinates poorly at low temperatures. All cantaloupe cultivars are extremely sensitive to frost, but some germinate at low temperatures better than others. The base temperature where germination stops varies by cultivar but is generally considered to be approximately 12 degrees Celsius. Cultivar Persia 202 (P-202) is capable of germinating at relatively low temperatures, while Noy Yizre’el (NY) is sensitive. Dr. Greg Welbaum, professor of horticulture at Virginia Tech, has been collaborating with researchers from Israel to determine why some cultivars of cantaloupe germinate at low temperatures, while others do not.
Theory: Inhibition of respiration causes poor germination
One theory is that anoxia, or lack of oxygen intake, inhibits respiration and thus inhibits germination of some cantaloupe cultivars at low temperatures. Researchers hypothesized that other cultivars of cantaloupe respire faster at low temperatures, which improves their ability to germinate.
Theory: Seed coat structure affects levels of oxygen uptake
Researchers in Israel had identified a particular testa (seed coat) anatomy that seemed to be linked to anoxia in some genotypes. The testa in the temperature-sensitive genotype has fewer pores and a tightly sealed coat. Genotypes that germinate well at low temperatures have a more porous testa, with more intercellular spaces, that allows more oxygen to enter the seed, where it is used by the respiring embryos within. Welbaum collaborated with the Israeli researchers and offered to join the study, bringing his expertise in seed biology as well as specialized laboratory equipment to fully investigate the phenomena.
Testing the theories
Using an oxygen electrode, Welbaum and his colleagues measured the oxygen uptake in the different cantaloupe cultivars and verified how much oxygen the seeds were using with and without the seed coat. The technology and equipment allowed Welbaum to test the Israeli researchers’ hypothesis that the variance in seed coat porosity between cultivars was responsible for one cultivar germinating more poorly at cold temperatures than another.
Testing samples of the seeds with the oxygen electrode, Welbaum learned that the cultivars not only use less oxygen with the seed coat on, they also don’t use as much oxygen with the seed coat removed as other cultivars. Suddenly, Welbaum and his colleagues realized something other than the porosity of the testa was causing the seeds to take in less oxygen than genotypes that germinate well at low temperatures. It was clear that the embryo respiration rate also affected the seed’s ability to germinate at low temperatures and was an additional factor involved in low-temperature germination performance.
Beyond performance testing
Thus far, Welbaum and his colleagues have studied the performance of cantaloupe seeds under low-temperature conditions. The scientists have yet to discover what underlying genetics cause the differences between germination percentages in cultivars. Welbaum is currently seeking funding to continue this research.
“In order to identify the genes associated with low temperature response so they can be used for genetic improvement, more research is needed,” he reports. The professor cites numerous available techniques to compare and characterize the genetic differences to find out what genes are responsible for the temperature-sensitive germination. “I am excited that a Spanish research group is now sequencing the cantaloupe genome. This will make it easier to identify traits responsible for economically important characteristics like cold tolerance,” Welbaum adds.
In the past, Welbaum has used differential display PCR and various AFLP techniques, but he is primarily interested in using newer techniques for this study. Welbaum intends to consult with colleagues who specialize in molecular genetics to find the best techniques.
With the genes identified, other researchers will use marker-assisted breeding to develop cantaloupe cultivars with better low-temperature germination. After tracking the desired traits, Welbaum expects to turn the project over to plant breeders who will actually develop new cultivars by making the crosses and using molecular techniques to make sure each new trait transferred. With funding and luck, Welbaum and his colleagues will produce new cold-tolerant cultivars of cantaloupe in the next 10 years.
The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.