Yellowing of leaves

yellowing from below is sign it's mobile, so likely N - if not related to pH, which doesn't seem to be the case. if you've been light on ferts, dial it up a bit. those first couple sets of blades tend to be abandoned quicker by the plant than others. Once it is covered, it may just be getting ignored a bit. the tip burn on same leaf isn't consistent with N-def, though. This could be a K-deficiency or even Mg def or Fe.... It's a mobile nutrient, if that helps... So, dial back a bit without starving, if in hydro substrate, and let it give a more specific answer for you to react to, before tosing kerosene on a fire by accident. Try a slightly higher pH closer to 6.5 than 6, if you are confident about how much you have fed it so far. that will help with the chemistry involving N in the plant.