Is this a form of heat stress? leaves curling.

Did you recently water them?

Looks like water stress to me also. But give us the run down on your grow. Soil/hydro-nutes ect...(this helps when asking for any diagnoses) But what we need to know most is it soil or hydro. Watering habits as asked by PSUAGRO and have you fed and if so what. These things will help a lot to helping you get a viable answer.

And get rid of the foil over the container. You medium needs to breath as much as your plant...

what is the difference between the plant in the background and the one in the pic, in-terms of set up and what above said, i thinks there a few things wrong over been the word used

One was covered with foil and reported ill. The one that was in the background looked healthy and in soil. Do not forget kiddies...never assume anything....