Under what environmental condition is water transport through the
xylem reduced to near zero?

Transpiration in plants depends on the evaporation of water from the
leaf surface.  This means that the stomata ? small pores in the leaf
surface ? have to be open, and that water has to be evaporating from
the surface of the leaves.  Water doesn't evaporate in conditions of
very high humidity, so I'd answer that transpiration stops in
conditions of 100% humidity ? when it is raining!

Transpiration can also be slowed down when the stomata close.

I hope this helps.  Please let me know if you need more information.
--neurogeek

Search strategy:
Google ?transpiration?
http://en.wikipedia.org/wiki/Transpiration
Wikipedia has a reasonable treatment of transpiration.

http://extension.oregonstate.edu/mg/botany/transpir.html
Link from Wikipedia