It looks like a possible Boron defficiency. I'd say you are in the early stages of it.
The photo is from the Marijuana Garden Saver handbook. I'd say yours is in the very early stages, as the new growth hasn't started to brown off yet. I would foliar feed with a mix of water and eye wash that contains boric acid. The mix is about 1/2 tsp per gal of water and is fast acting. If you see no results in 24 hours, use one tsp per gallon on subsequent foliar feedings. A longer term solution is to use bone meal.
Anyone that would like to view my reference material, I'm using the Garden Saver Handbook and am on page 37.