Stonerdanswered grow question 4 years ago A Nitrogen (N) deficiency by the looks of it, check the pH and make sure it's in optimal range (5.5-6.5/6.0-6.5 in soil) to prevent nutrient "lockout", and increase the Nitrogen based fertilizer (usually labeled as "Grow" or "Veg") by 50% since it seem pretty severe. Check the plants daily to see how they react to the different diet, adjust your fertilizing accordingly!
Hope this helps! Good luck and happy growing :)