check this out: http://www.drugs-forum.com/growfaq/97.htm

Quote: "Nutrient Solution Burn:
There's a good chance that this leaf was subjected to nutrient solution burn. These symptoms are seen when the EC concentration of hydroponic solutions is too high. These symptoms also appear when strong nutrient solution is splashed onto the leaves under hot HID lamps, causing the leaves to burn under the solution."...." Back off the concentration of the nutrient solution just a touch, and the problem should disappear. Note that if the plants never get any worse than this leaf (figure 3), then the plants are probably just fine" End Quote.
I'm new so I hope that helps! S