This is a complicated question to answer. For inbreeders like tomatoes and beans, there is very little genetic diversity left in heirloom varieties. They don't "adapt" very much from a conventional breeding perspective. Where there is some flexibility is in epigenetics. These are small changes that a plant can make in expression of genes to adapt to a given environment. Tomatoes in particular show epigenetic changes for adaptation to temperature extremes.

For outbreeders like corn, there is some room for selection, but it is usually limited by the years of selection that have gone into the variety. In other words, the more diversity has been reduced by inbreeding, the less adaptable the resulting plants become.

Thanks. I'm growing in Texas and working on aquaponics so I was hoping that I could selectively breed for best performers for my situation, while staying heirloom.

Heirloom varieties generally have much more genetic diversity than clonally propagated cultivars, but paradoxically there are many cases [particularly fruit trees] where clonally propagated varieties are hybrids from two very different varieties, and so the seeds from these plants could have very different qualities than their hybridized parent.

It is very difficult to try to say there is a generalized rule however, because it really depends on the plant variety's lineage, how it was bred in the first place.

In terms of trying to grow plants from seeds in climates that the parent variety is not well suited to grow in, usually the results would tend not to be good either way, whether it is a natural heirloom variety or not.

In order to be a heirloom a variety has to have not been crossed with anything for a very long time. That means that you have a genetic bottleneck that is very long. You can help alleviate this by seeking out the same variety from different sources and home there is some genetic variation in there (usually from poor seed keeping practices) that will help you adapt. You do have to worry that one of your sources might have gotten their germplasm from another source so you end up paying double for the same thing.

Alternatively you can try to cross similar varieties to get the traits you want, and then breed back to type to get the heirloom type. Indeed this happens and could be some of the source of genetic variation between different suppliers of heirlooms. That's not a particularly honest way to go, but it does happen.

What I think would be the best option for a market gardener with a good relationship with their customers would be to use heirlooms to breed new varieties. Establish a reputation for yourself as a renegade plant breeder, bucking the system and marrying old world breeding principles with new age technology to bring a superior, lovingly coddled product to your customers. If you come up with something distinctive and good, that basically leaves your customers coming back to you as their only source.

Thanks, the first paragraph is what I was thinking.

Just got some seeds in from university of Florida where they are mixing flavor of heirlooms with production and resistances of more modern types. Unfortunately I got a determinant and semi determinant from them where as I have always grown indeterminate tomatoes