Nitrogen

It takes 30-35 days for a Mg deficiency to be visible in the leaves, so i don't think this plant is old enough. If you haven't given it any Mg, then you should definitely add some, though. it is a required nutrient for the plant and should be provided at appropriate levels at all times. If the plant is old enough, it's a possibility but you shoudl see spots with intervienal chlorosis, and not only inteveinal chlorosis. if the paling is tip-in and bottom-up progression, then it's most likely an N-deficiency. Though this look a little like interveinal chlorosis to me - sometimes the initial paling of N can look a bit like this. I'd verify your pH is fine and make sure you aren't locking out some trace elements. if this is soilless context, should be giving 75-ish ppm of Mg and 120-ish ppm of N. observe plant and adjust from there. If soil, these numbers may or may not be good for you due to numerous other variables, but for soilless/hydro this is spot on.