Oh, that doesn't look good- if my plants looked like that I'd foliar feed with a complete veg fert and a little epsom salts- even that far into flower- better to add too some N than lose your whole plant- that's a pretty severe deficiency or lockout. From that pic you've got N def and possibly Mg def (that's why epsom salts).
You could also try increasing the concentration of your primary nutrient slightly.

For any more detailed help, wait until someone more hydro-oriented comes along!