I have same issue. Plans are about 8 weeks old about 40cm and grow in rockwool hydro, controlled PH around 5,5 and EC 800-1100. I just turned them from veggie to flower 12/12.
I'm oxynating water and watering 3 times a day 10 min from top and bottom spray to roots
I've attached some pics, lihgt green one the two on the left
Earlier plants suffered from some nute burn, but should be ok now.
Ok, thanks, will try to lower Ec to 600 and keep PH at 5,5. I think no way Nitrogen deficiency bc. been feeding then Bio Nova Pro series with exact recomendation amounts! But list amt. might be too much for this kinda difficult sativa strain.