Well, your plant is definitely well fed, judging by the dark green color. Maybe a bit too well fed. Not sure what's causing the yellowing or what deficiency it could have if you're using a balanced fertilizer.. How much sun does it get? The pot looks a little small. Am assuming the potting soil drains well. Sometimes leaves turn yellow with over watering, but that shouldn't be a problem in Dallas with a fast draining mix. Am thinking the problem is environmental rather than nutritional, but just a guess. Also, doesn't seem to have many tomatoes, or blossoms. What variety is it? Shouldn't it be loaded with fruit/flowers? edit: ok, looking closer I do see a few tomatoes.

Most likely magnesium deficiency. I see it all the time with my alkaline soil pH. Advanced interveinal chlorosis, with some mild necrosis in some leaf tissue. A dose of epsom salts (1 tablespoon to a gallon of water) normally fixes it with in ground plants. In a container you may have to provide the supplement more often. If it looks like it will be a chronic issue you can always add about 1 teaspoon of it to your normal watering.

As to the cause - most likely the size of the container and the plant is root bound in it so nutrient uptake is retarded. Plants sold in those type set-ups are not intended to be grown in them permanently. They are supposed to be transplanted into much larger containers or the ground. If you wish to see it to maturity I'd suggest you do that ASAP.

Dave

What size pot is that? Could be weird photo perspective, but that pot looks far too small for that plant. Combine that with the fact that the heat is probably really ramping up right now, and you likely have unhappy roots. If that's an indeterminate plant, the issue is only going to get worse.

Also, what does your fertilizing look like? What, how often, how much?

I would also double-check the undersides of the leaves carefully with a magnifying glass for pests on the off chance that it could be mites or some sap-sucking pest, but to me, it looks far more likely to be nutritional/unhappy roots.