Watching mine I think its normal. It seems to be putting out new higher leaves about the same time the old lower ones die off. Not sure if thats what is happening but thats what looks like is going on.

I have two huge clumps of basjoo. Every year about this time as the days get shorter, the leaves turn yellow, then brown and dry.

How weird. I was just looking up information about petunias in Florida, what seasons they're good for and such and then I come back to GW and find a topic on them. My experience with petunias is limited, but I had some in a terracotta pot last spring/summer that seemed to do okay. By July they were definitely showing signs of heat stress, but regular feedings of MG's All-Purpose Water-Soluble fertilizer kept them alive. At the time I was still very new to this whole gardening thing and so 'regular' for me was like every other day they were getting a dose. This constant feeding is probably what kept them from dying off and is probably why I did not see what you're describing. At the current time, I have about 6 petunias planted around the base of my Knock-Out Rose and they seem to be doing okay so far. They get a fair bit more sun than what you describe though. They get near constant sun from sunrise until 4 pm with a very short break around 10:30 as they fall into the shadow of a nearby bush for about 20-30 minutes,then they're back in the baking sun. My biggest problem with these things is the near-constant need to dead-head them. I like working in my yard, but I don't like having to constantly fawn over one particular plant to keep it going (which is why I'm also not all that keen on having roses [the Knock-Outs were a gift]). These don't get fed nearly as much as the potted ones from last year, just a dose of Fish Fertilizer every two weeks or so, which is more for the roses than the petunias, but the petunias get the benefit. Aside from that there's not much special about where they're planted. It's native soil (sand) mixed with a bag of Black Kow and topped off with Pine Fines as mulch. All that said, I will not be actively looking to purchase petunias again for myself. The ones I have now were actually given to me by my mother who had purchased too many for her planters. However, I do intend to purchase and sprout seeds since my mom is a pretty big fan of them. Hence my internet search, I was trying to figure out when to sow seeds to have them ready for her next planter arrangement. And here they are:

With plants in general dark discoloration of leaf sections can be due to bacterial or fungal infections. You will have to determine yourself - using descriptions and photos on web sites or in print publications - what, exactly is happening because you are the one that is there examining the plants. Unless you live in the US - it's always a good idea when posting here to say where you (or the plants) are located geographically - and can therefore try taking samples of affected parts to your nearest USDA Cooperative Extension Service branch office for comment.