I do not believe it to be heat stress..

looks like you over loved your plants

too much of something
leaves curling down can mean to much water.. but your leaves are curled down and brittle looking..as opposed to overwatering is usually droopy and sad..

SO I am going with ... to much FERT!! or possible wrong PH... or both and over watering..

you see mand's picture, how the leaves are super dark..that is usually the first sign of to much FERT.. second sign will be burnt tips... andthen the curling...

Hang tough.. and don't feed them..

I would recommend a flush but if your soil is already muddy.. you better hold off?

keep us posted

..

How is your soil... muddy?
How is your PH?
What ferts have you been using.. have you been over feeding?

more pics please