Leaf curl is often a reaction to stress. It sounds like you just transplanted them which is enough to stress a plant, especially in heat. When I repot during hot weather I do so in the evening and then place the plant in a shady location for at least a couple of days.

They look very limpy, I would so spray leaves with water. Yeah, stress of transplanting can be offset but since you already done what I would do it to put them in shade for a day or so and spray leaves with water and once they perk up diluted liquid fertilizer like fish emulsion. I particularly like Neptune brand. When roots can not support the plant foliar feed is the best.

Possibly stress from transplant and heat. Should recover.

The only times that happened to me was when I forgot to water my seedlings on time.

When transplanting, I would really flood them. This way there will no chance for drooping. UNLESS you do it without hardening off properly and planting out in a sunny warm day.

When I don't harden my transplants they do this but recover.

Harden?

Harden?

To gradually accustom a plant to more difficult living conditions

Plants need to be "hardened-off", an adjustment process that takes 7-10 days to gradually adjust to a new environment prior to transplanting outside.

So were these plants purchased or did you grow them from seed? If bought were they outside or inside? If you grew them did you move them directly out from growing inside to the outside rather than giving them a period of adjustment?

Dave

PS: are you sure there was no damage to the stem done when transplanting?