A likely cause might be nutritional deficiency. Probably lack of nitrogen, iron, zink, potassium, or calcium. If you are feeding your plants a complete nutrient system, the problem might be nutrient lock out. To remedy this give your plants a good flush. Then, continue with your nutrient schedule. Other causes could be heat, of over watering. Also, look closely to see if there are small dark spots. This could be a sign of disease.
Well guessing the exact nute deficiency/ lockout is hard to do with a few pics and little detail on grow. First make sure your ph is in line, this will be your biggest enemy. It causes nutrients to be locked out or not absorbed if the ph is off. Get that in check, also check your temps, those should be in line as well. One problem it may bay is over fertilization, or "nute burn" What's your nutrient schedule look like?
so I got similiar problems to the feller in the post. \Been having some serious salt build up on my reservoir and havent really been keeping track of the PH or PPM/EC until yesterday. rookie mistake i know. especially with the amount of money/time/energy/sweat/blood ive put into this thing. anyhow i checked my ph befre putting the noots in this time and balanced it out to a smooth 5.2-5.7(it was a high 8 before i added teh ph down) trying to keep it in that goldilocks range. anyhow today no salt on the side of the rez. wondering if the ph swing locked those noots out immediately? and would the nute lockout manifest itself in heavy salt build up in the media/rootzone/res? pretty crazy stuff. loving this norcal stuff tho.