It's hungry. That plant is using up N too fast. Make sure your runoff pH is not too low, and if you have more than 3 weeks to harvest (looks like it), you can give ONE feeding with a quarter strength grow fert to slow down the yellowing.
What have you been feeding it in the last couple weeks? Just bloom, right? Does that bloom fert show a N number on the label at all, and if so, what?

EDIT- not K, Mg, or Zn def. N and Ca def or lockout. What was the date of your last re-pot?