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It all depends on where that is happening. If its happening from the bottom up, it could simply be the plant outgrowing the leaves. If the plant is tall and bushy and those are the lower leaves, they might not be getting much light. This is ok, and nothing to worry about. If you are seeing yellowing and dying up the plant, it could be a Nitrogen deficiency. It also may be a nutrient burn if it is happening throughout the plant. But since I am not seeing any burnt or curled edges I would not lean towards nute burn.
I really do not have enough info on your plant and set-up to make a good inference. Maybe if you shared a little more info about the grow you could get a little better help.