Of the wild tomato species, Solanum, pennellii has been the most difficult for me to work with.
1- The seeds are very small. Therefore young plants are also small, and delicate. Highly susceptible to damping off.
2- The root system seems weak and fragile. Seems like a little disturbance kills the plant. The plants haven't sent out roots from the stem, so growing cuttings is problematic.
3- The type of soil it grows in has a huge impact on plant growth.
I managed to grow a respectable amout of seed this year in pots, that were shaded mid-day, and received morning and afternoon sunlight. The potting mix was 100% compost. Not just any compost, but a specific batch of compost, which is the only type of compost/soil that the plants did well in out of about a half dozen that I tried. Solanum pennellii seems to do better if it is misted rather than irrigated, so I watered with a jet of water across the plants, rather than irrigating the soil.
So for those of you attempting to grow Solanum pennellii, I highly recommend starting seeds in a dozen different potting mixes, and then choose to grow your population in the one in which it thrives. And minimize transplanting. Maybe it wouldn't be an issue in a high humidity climate, but for me, where it is super arid, I need every advantage I can get.
I have yet to successfully grow a S pennellii plant in an open field that receives once a week irrigation, and full sunlight. I did raise one plant this year in which I put the tomato and pot directly into a bed, so that the plant could have it's preferred soil, and stretch into the native soil as needed for additional
moisture. But the plants that were grown in pots only did better.
Solanum pennellii is self-incompatible, and the shape of the anthers is cylindrical rather than conical. The stigma is exerted to highly exerted. The leaf shape is highly conserved in the offspring of interspecies hybrids with domestic tomatoes.
I grew F2 hybrids this summer of a cross between domestic tomatoes and S pennellii (Thanks Andrew). There were huge differences in vigor between plants.