Which is this symptom?

Since this is occurring in the new leaves, this is usually an indication of a deficiency of minor nutrients (trace elements) which are immobile, meaning they can not be "taken" from any older growth and be redistributed to where it is needed, unlike nitrogen, potassium, phosphorous, calcium and magnesium. A dose or two of soluble trace element mix available from any garden centre should restore the balance in the substrate and best of all, the affected leaves will recover too. Follow up doses with the soluble trace element mix ever 3-4 weeks will keep your plant in top condition. Hope this helps, Organoman.