If the stems themselves are completely brown in one area, that will eliminate any transfer of nutrients, so I would remove the leaves as they will only yellow and die. I find that Aponogeton leaves when dying tend to disintegrate from the tips, and sometimes I don't even see that the leaf is dying; in the interim there is a mess of rotting vegetation.
Substrate tabs will help these plants. At least, mine improved dramatically. Though liquid comprehensive fertilizers will provide the nutrients only more slowly. But some nutrients that are only taken up via the leaves of plants need to be in the water, so both liquid and substrate tabs is best here.
Most species also go through a dormant phase too. You may already know this, but thought I'd mention it. Sometimes aquarists wonder why the plant has suddenly "died," not realizing it will return in a few months.
Byron Hosking, BMus, MA
Vancouver, BC, Canada
Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]