The answer to Gunnar Friedbergs' query lies in the mechanism that trees have
to restore water columns in the xylem after cold winters. I believe the story
is this: In the cold winters of countries such as N. America, Canada and Sweden
the water in the xylem of trees such as maple and birch actually freezes. In
the spring, when the water meltsin the xyelm the water columns contain air bubbles and are no longer continuous i.e. like air-locks in a central heating system
following frosts. But, reserve sugars are mobilized from cells adjacent to
xylem vessels when temp. rise in the spring. These are secreted into the
xylem elemnets, giving them a sulute potential i.e. lowers WP so that water
is absorbed from surrounding cells and the water develops a positive hydrostaticpressure - this dissolves the air bubbles and the water columns are restored.
Sucrose in the xylem at this time of year has another function because the sucrose is moved to the previously dormant buds thus helping their growth - moved
bt the difference in hydrostatic pressure in xylem between trunk and the buds.
- you actually have something loke Munch's pressure flow operating in the xylem at this time of year. All this happens to a smaller extent in the temperate trees e.g willow suggars are 3-5 per ecnt w/v in spring. THIS IS ALSO THE REASON
WHY MAPLE SYRUP IS TAPPED IN SPRING. Ref: Milburn and Zimmermann(1986)
J. Plant Physiol. 124, 331-344. The story is also described In Kaufman's
textbook - Plants, Their Biology and Importance. P. 191. although not
particularly clearly. Have I got the story right?
Dr Paul Clifford, School of Biology and Biochemistry, Queen's University Belfast, Northern Ireland. Email P.Clifford at QUB.AC.UK
