Yesterday it was 104F in Denver, much hotter than our 85F average. My container plants are suffering. I'm losing some, others look like they have leaf burn, some have mildew, others just plain died. Some of the dahlias are suffering more than others, and the osteospernum look pretty bad too.
When we get sudden extremely hot weather, I usually try to move my containers into an area where they get a little more shade in the afternoon. Is that an option for you? When temperatures suddenly get really hot like that (vs gradually getting warmer and warmer) I've found that plants can sunburn. Watering is important too of course, I'd make sure you're watering thoroughly when you do water (vs just giving them sips) to make sure that the potting mix really gets wet all the way through. And of course you'll also need to water more often.
If you can't move them, can you rig up a patio umbrella or shade cloth or something along those lines to protect them from the sun during the hottest part of the day? I've found once plants get used to hot weather things tend to go better, it's just that first really hot spell after cooler weather that seems to cause the biggest problem.
you might want to try something like this http://shadeforplants.com or maybe go to a local dollar store and purchase some childrens umbrellas for a quick short term solution. A lady down the street where we use to live used small umbrella's on her handing plants that she had on the side of the fence. At first I had to look twice when driving by, then I realized what was on there.
Just curious, what do sunburned leaves look like? My lemon tree's leaves have been looking a little more yellow than normal and I haven't done anything differently. It wasright at 100F on Sunday, Thursday it is supposed to be 104F.