This looks to be the start of nitrogen toxicity ... autos don't like a whole bunch of nutes early in the game! I'm growing some now (autos are ALL I grow) that have reacted badly to ANY nutes even at 4 weeks and in flower! Each strain is going to be different and the best thing to do with them is to wait until they're about 3 weeks old to introduce nutes and then ONLY about 1/4 strength. If they tolerate that, you can up them to 1/2 strength the next week and go full strength in week 5... This one seems to be telling you they're not happy with the amount of N they're getting - but it's VERY minor and probably not worth a flush. You don't mention what kind of soil they're in but if the soil itself has any nutes in it, what I've said goes double!
All in all, this look great with just a hint that you're giving too much nitrogen ... if you back off on that, those leaves will NOT show any improvement but what you want to be looking at and checking are the new leaves ... THEY will show you if you've sorted the issue.
Good luck and enjoy the journey!