During the plant's growth, it is natural for it to shed the bottom leafs as growth progresses. The leaf that is completely yellow is the first three tiered leaf from germ right? Roots coming out of the bottom can lead to bigger problems. If left in run-off from watering, fungus and mold can develope on them stunting growth serverly. And serious root problems can kill your plant (not to scare you, I don't think your there). Your pics do not show how large the plant is overall so I cannot determine if it may be rootbound. You may need to transplant to a bigger pot. How long has it been in veg? 3-4 weeks? and how large is the container? The yellowing tips is a different condition that may have several factors. Check the ph... If the ph is off add dolimite lime to correct this. This turned out to be a solution for my problem. It may also be fert burn from over feeding or nute lock out, or Mg defficencey, or even heat stress (but I don't think your lights are causing this. They don't appear to be charred but yellowing from the inside). So give us some feed back... Tell us what's going on... good luck to you!!