I'd suggest heat stress. Probably enhanced by problems with the roots getting adequate water.

Purple, when it's that diffuse color you have in your pictures, is usually caused by a buildup of anthocyanins in the leaves; that's what causes the purple in phosphorus deficiency. It indicates a disruption of the normal photosynthetic process, where the leaf builds sugars and sends them into the phloem to get used by the rest of the plant -- anthocyanins are a byproduct which is normally broken down, but when there's some monkey wrench in the works (such as when it's too cold, or there isn't enough phosphorus) it builds up. I see it sometimes in plant parts which have been cut off from the plant by sudden girdling, so that the sugar-building process grinds to a halt in midstep.

All of that is a long-winded way of saying that I agree with Jean; it may have been heat or water stress, which is causing those leaves to have trouble keeping up the regular flow of water, nutrients, and sugars that they usually have. It would also explain why the leaves are yellowing and dying off, as the purple is just the first sign that things aren't ticking along as they should for those particular leaves.

Make sure the plants are moist, shade them a little if they get very hot afternoon sun, and don't worry about it further unless something like this starts to happen all over the plant. Keep an eye out for purple/bronze patches that kill the areas between the veins, because that's a symptom of the spotted wilt virus... but I don't think this is nearly that dire.

--Alison

Thank you all for your advice! Heat / water-stress makes sense. Lately we've had some hot days, and I've been noticing when I check the plants around late morning / early afternoon that the ends of some of the branches are saggy / droopy, leading me to conclude they aren't getting enough water via the capillary strips. So I've been dumping water into them to saturate them until it begins to run out the bottom. The droopy ends would perk up after that, so I assumed this was the right thing to do.

Today I noticed a few droopy ends again. However the soil at the top of the planter (remember these are upside down planters) was still quite moist though it didn't seem so at the bottom, and they hadn't pulled much water from the resevior. So again I dumpted water into them directly and, this time, replaced the capilary strips. Might I be overdoing it? Any further advice here?

I'm beginning to wonder of the whole capillary strip system isn't failing to work any more...perhaps as the container has become rootbound? When I fertilized (organic, every 4 weeks) last weekend, I was unable to mix it into the soil at the top of the planter very much due to root density.

Keep in mind that when the soil dries out too much, the roots start to die... if that happens too much, the root system becomes too small for the plant and it wilts even when the soil is moist. A common mistake with potted plants is then to overwater them trying to keep them from wilting, which kills even more roots. :/ So lay off the water, keep the soil evenly moist but not wet, shade the plants if you can, and let them recover their root systems. I sometimes resort to something like Superthrive in this situation, which seems to accelerate the root-building process, but they will manage okay without it.

How big are these containers, and how big are the plants? Not being able to scratch in fertilizer really does sound like they're rootbound... I generally figure that five gallons is the smallest size pot a full indeterminate tomato plant can handle, and most hanging pots are not that large.

--Alison

The planters I'm using hold 17 quarts of soil...so 4 1/4 gallons. Yeah, I'm coming to the conclusion this is a problem. This is the second year I've used these and I think the same thing happened last year. The first half of the season they grew like crazy and sometime in July I started seeing that purpling pattern and the plants gradually did worse and worse. Last year I chalked it up to them getting some disease, but now I think it was probably the same root-boundness / watering problems.

Ironically my plants have been even growing even more vigorously this year (which may be due to this Mycogrow, a mycorrhizal supplement I added to the soil) and hense have become even more solidly rootbound than I remember them being last year.

At least I've gotten some nice big ripe tomatoes from it (just picked my first 4 this morning), as I did last year, but I think maybe one just has to expect an early deterioration and end to the season with these things. I probably won't use them again next year.

The maker of the planter recommends determinant varieties, but my plants, I think, are indeterminant--at least one surely is. (The garden center evidentaly got their plants mixed up because niether plant is producing the color tomatoes I expected, so I really don't know what they are.) Between the plant hanger, and the metal railing around the deck these are on, I manage to tie up and support branches as necessary.

Anyway, thanks for the reply, Alison. I'll try to keep the watering as even and moderate as possible and see how it goes.

--Eric