I think eggplant could tolerate a few days, especially if its not a lot of consecutive days, of temps as high as 110-120 F as long as the plants have plenty of water. I know that there have been years when I did not use my lexan and brick smaller greenhouse during the summer, and just sealed it up, and weeds that started growing between the bricks managed to survive incredibly hot temps in there and seemed to even thrive. I'm sure it got up to probably 130 or hotter, and they lived. Of course, these were plants like Dock, Sweet Annie, Amaranth/Pigweed, and Sow Thistles.

Maybe someone from Arizona or Texas will give you their 2 cents worth.

Personally, in Zone 3, I think you'd be a lot better off growing heat lovers like eggplant in the greenhouse than outside.

The major problem I've had with greenhouse cultivation is insects and especially spider mites, which thrive in that type of environment. So, be on the lookout for those.

Eggplant handles 115 degrees here (cooler at night). Don't know if fruit quality would suffer. Second the comments on insects.

For best flavor, treat the fruits like sweet corn - use it soon after picking.

It might stop production for a short while, but if its only for a day or two then no problem.

I am in SE Texas and we had regular 95 degree plus days and the plants thrived.

If it is getting 115 to 120 degrees though this might be pushing it especially if its staying there every day.

The plant itself can tolerate the heat but will it set fruit with temps that high? I don't think so. Blossom drop temp ranges apply to eggplant just as to peppers and tomatoes.

Hand pollination may help some but if the humidity is also high enough to render the pollen non-viable even hand pollinating won't help.

Dave

If you moved it outside during sunny days, and then returned it to the greenhouse in early evenings (and leave it in there on cloudy/cool days) it would probably do better.

I tried growing a couple eggplant in my greenhouse one summer hoping to get earlier fruit, but they quickly came down with spider mites. Never get spider mites when planted outdoors.

I grow Dusky, Ichiban, and Neon. They produce a lot sooner than the typical Italian eggplant like Black Beauty.

Well, in reading some of the other comments, the two things that come to mind for me are to increase the ventilation to as high a level as possible -- one of those really big floor fans like they sell at Home Depot perhaps, to really push air through there, combined with some type of mist emitters hooked to an automatic timer/controller unit. Probably would cost maybe $150 to do the whole thing for a greenhouse the size of my larger hoophouse, which is 10 x 32.

I think it would help a LOT in terms of keeping the plants comfy on hot days.

When I lived in Bakersfield, CA, my eggplants lived through the summers with no problems. We'd have many days hitting 105, and a couple days each summer hitting 110-112 (blech). One summer we didn't have lows below 100 for over a month, and the eggplant did fine as long as it had plenty of water, but it won't set fruit. So, those temps are OK for the plant, but not optimal for fruit set. I'd do as denninmi suggests and max out the ventilation on your hottest days.

They may tolerate so much heat, but would rather to be cooler.
My eggplants last year produced more after the peak of summer.
As far as surviving, they will do fine at mid to upper 40s and higher.