Dude, what ya' growing in, what ya' growing under, how old are they, and how ya' water them (i.e. hydro, or weekly can-o-water,) you nuting them? You growing in a soil with nutes, or you growing in something that needs nutes?
You gotta' include a bit more detail. No, that doesn't look like heat or heat stress. Nor light burn. It looks like an issue you're having with nutes.