Noticing this, I set them in a much shadier location. Within four days, the yellowness had gone away, and the purple patches were very diminished. The curling was better as well. Does anyone know what might have been causing this?

Basically, it helps to jack up the nitrogen in pots when you want to put little tomatoes in full sun.

spiced ham, I thank you for that amazingly informative, yet concise reply. It seems you know your tomato facts well. Very well :)

Jessicavanderhoff, thanks for your input as well.

Well, zones only refer to minimum temperatures, not maximums. While we never get 110 in the zone 6s I've lived in, where I am now easily stays in the mid-hi 90s for a good portion of summer. I am more comfortable with my tomatoes getting some afternoon shade, because they are still getting over 6 hours a day. Zone 6s can vary quite a lot with regard to high temperatures, just like say parts of New York state that are zone 7 because they freeze less only get into the 80s in the summer, and plant with the zone 5s because of nighttime temperatures. Every situation is different, zones bother me because I think they can be misleading :)

When they are small though, and this early, I agree they might not have been hardened off enough. spiced_ham explained it, but here is a link anyway that I think Anney here passes around that has the best explanation of it that I have ever read! I have it bookmarked. :)

Here in the lower 48, particularly in the South, shade and/or lower temps are often the answer for tomatoes that aren't doing well since indirect sunlight bathes the plants but the heat is reduced.

Yes, you are correct, and I was just thinking about the "too much sun" question without relating it to Zone 6. IOW, the OP's problem probably isn't related to heat and humidity in that zone, more likely sunburn or not hardened off enough as you mentioned.

I also plant as many tomatoes as possible in a spot that has afternoon shade from an oak tree about 3PM on. It is really cooler in that shade than in sunlight and the plants produce well there since they do have about 8 hours of direct sunlight -- their "down-time" doesn't start as soon, though eventually the shady spot is also too hot and humid for them to thrive.

At high temperatures plants undergo photorespiration. One of the photosynthetic pathways starts to put the wrong things together and the efficiency of photosynthesis is greatly reduced. This is such a problem in tropics and subtropics that a different type of photosynthesis has evolved (C4 pathway). Corn/maize and sugar cane are C4 plants, tomatoes are not.

At the same time (high temps) it is difficult to separate growth decline due to water stress from photorespiration. In addition, wilting increases photorespiration.

None of that corresponds to zones, which do not follow lines of latitude so you have to rely on experience at your location. I get better growth on the eastern side of the house than on the warmer southern side where the bricks of the house heat up behind the plants. The soil dries out very quickly ther as well. I'm sure a little shade would help tomatoes there.

I have a similar spot, because it is the only suitable spot in the entire yard. Only mine gets sun (right now, it does change) from sunrise-11 am, 3-7pm. I was going to hack down this mess of honeysuckle that is shading it for those mid-afternoon hours, but so far everything is full, no lankiness, and fruit is setting like crazy. I hope that when we start approaching 100 (we already have had a few days) it will be for the best.

Being a novice I don't know about hardening, but I figured at this size the plant would be able to take full sun. When I got home today, though, the Roma plant in particular was drooping badly and has some yellow and purple patches on its leaves. I had the plants inside next to a well-lit window, and they grew really well. Now that they're on the roof garden with full, all-day sun, The soil is still moist, but it was 86 her today.

Does anyone have any ideas whether it is an excessive heat issue, or excessive sunlight? Should I try moving them to get more shade during the day? If so, am I better opting for shade during morning, mid-day or afternoon?

Thanks in advance for any advice!

I'm in an extreme area with no humidity and high heat, and the tomatoes here in full all-day-long sun have smaller fruit, more splits, don't get as red, and don't get as sweet. Mine tend to have huge root systems so that might be the reason for no drooping.

And just an observation; I didn't learn from last year...

Dave