why is the leaf yellow?

It delends is it orange?? If it is then u over fed it nutes.try a ppm of 750 or so. If its light yellow then could be your light is too close that is if its a top leaf. If its a bottom leaf then sounds loke your in soil or using organics. No need to defoliate using organics the leaves will just turn yellow and die. Good lick

Yes it could but only if the light is very close to the plant. Where was the leaf was it on the top of the plant or the bottom if its top then its probably right but if its a bottom leaf it could be a Nitrogen deficiency.