Que problema tiene la planta ??

LED lights? expect red petioles (=leaf stem) on any that are exposed to light. Even if you are feeding "enough" Mg, it'll turn purple. Some gentics do this normally. Check your guarnateed analysis labels.. how much Mg are you providing? ~80ppm + typical tap water is usually plenty of magnesium. The lower yellow leaves *could be N-deficiency, yes. That will progress from bottom-to-top of plant and from tups of leaves and move inward. Sometimes a plant simply sheds some leaves and it will suck them dry before they fall off, too. I see some "clawing" of leaves, which hints at a slightly overfed plant. So, i'd wager the plant is just shedding a leaf as opposed to a N-deficiency.