Friday, November 18, 2016

Frosty Physalis

This has been a long and relatively warm fall season. There have been a few frosts, but most days the sun came out and it warmed up nicely. This is all over now that it is snowing. It might melt away and we might have some warmish days still, but after it has snowed I really can't convince myself that winter hasn't begun.

I grew a couple different types of tomatillos (Physalis ixocarpa) this year. I collected seeds for the first one from a medium-sized, dark purple fruit I got from a local CSA. The plants didn't thrive, but they did pretty good considering the lack of care I gave them. The plants have lots of purple pigment on the stems and the fruit husks, but the fruit themselves remain bright green on the plant. Once the fruit are picked and the husk is removed, the green fruit start developing intense purple pigment in response to light. The original fruit was purple all the way through, but no fruit this year have such a pigment pattern. I'm hoping the coloration of these plants indicates they are hybrids and so the darker color may turn up again in the F2 generation next year.

I collected seeds for the second type from a very large, green fruit I got at the grocery store. It didn't have a name associated with it, but it matches one of a few commercial varieties grown in Mexico. The plants did about as well as the other type. The fruit they grew weren't anywhere near as large as the original fruit, but they were much larger than the fruit from the other variety.

A couple days ago, I noticed the large-green variety had been killed back by one of the recent frosts... while the purple variety showed absolutely no damage. The above photos were taken at the same time and well illustrate the differences in frost-sensitivity.

Tomatillos are profligate outcrossers, due to the self-incompatibility mechanisms in their flowers. A consequence of this is that it is much harder to develop stable strains than it is with tomatoes and peppers, because you always have heterogeneity in at least the incompatability locus genes (and likely any nearby genes). The flip side of this is that it should be relatively easy to mix up the genetics of different strains that you happen to be growing next to each other.

I'd really like to develop a strain with the large fruit of the Mexican strain, but with the purple color and frost-resistance of the CSA strain. I expect I'll be able to find some hybrid seedlings with extra purple color among the masses I can grow out from the seeds in a few of the green fruit. These hybrids would have all of the alleles I'm interested in, though it would probably take several years to stabilize them in the homozygous condition.

A close relative of the tomatillo is the Cape Gooseberry (P. peruviana). I grew out this plant from a couple different seed sources. The plants ended up looking identical to each other early in the season, so I assumed they were essentially the same.

After I noticed the different frost-sensitivities of the tomatillo types, I checked on the Cape Gooseberry plants. Several plants were in perfect health (at left), while one was heavily damaged from frost (at right). The condition of the damaged plant indicates it was harmed by a more recent frost than the green tomatillo, suggesting that it has a greater resistance (even though it wasn't enough to matter).

There doesn't seem to be much trait diversity in these plants to do much breeding with. I'll probably only save seeds from the more frost resistant plants to grow next year.

If I had only grown the large-green tomatillo, I wouldn't have realized there was variation in frost resistance that could be bred with. This highlights the importance of collecting diverse germplasm when starting a breeding project. That I collected seeds from a local CSA and grocer shows it doesn't have to be a very difficult process.

If no or very limited genetic diversity is available, like in P. peruviana, it can take more dramatic efforts to collect sufficient germplasm. International travel or nurturing wide-flung collaborations may be necessary. I like the concept of mutation breeding, the use of chemical or radiological means to damage DNA of the plant to generate usefull diversity to breed with. Each approach has its own costs and difficulties, so the direction a breeder chooses will depend on what makes sense for them and the specific plant they're working with.