Have you used nutes already in the grow??? if so it sounds like nute burn in which case id flush for a few days and stay off the nutes for a few weeks... if not it could be Potassium or Nitrate defficientcy in which case i would still flush for a few days and see what happens.. if the problem remains or gets worse buy some of these:
http://www.lovethegarden.com/product...llpurpose.html... These will ensure the plant is being fed but not stress the plant....
Good luck.. hope they pull through...
Panda
last thought perhaps heat is a problem? burning?