In case anyone cares...
I took 4 strips of cherry cut from the same board; on each strip left
1/3 unfinished, put wb poly on 1/3, and put oil-based poly on 1/3; then
put one strip in sunshine, one strip under incandescent, one strip
under fluorescent, and one strip in dark. After about 4 weeks looked
at darkening to see if poly inhibited darkening.
All portions darkened on the sunlight strip and on the artificial light
strips; however, the oil-based poly darkened the least (hard to
quantify, probably about 1/3 less darkening roughly) and the oil-based
poly clearly had a more yellow tone than the other sections. The strip
in the sunshine clearly darkened more than those in the artificial
light. The strip in the sunshine was not outside, but inside on a
windowsill. The strip left in the dark showed no appreciable darkening
in any of the sections.