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rising tree sap

Kathleen Archer kathleen.archer at trincoll.edu
Tue Apr 12 10:01:21 EST 2005

Dear Plant Ed Folks,
I have tapped my maple tree in the yard to collect sap for maple syrup, and
in doing this I noticed a couple of things.
First, the volume of sap is astonishing - the fluid practically gushes out.
Second, the flow rate is closely linked to sunshine.  On cloudy days, the
sap flow is strikingly reduced, as is the flow at night.

The second observation calls into question something I have been told by
others, namely that the driving force of upward sap flow is root
pressure.  I don't see how this can be.  Root pressure is what causes
guttation droplets to form on the leaves of plants, and this is most
typically seen in the early  morning.  That suggests to me that root
pressure should be operating at night.  Further, I don't see how cloudy
days or sunshine could affect root pressure.

Naturally, since I am lecturing now about water transport in xylem, and
phloem transport, students are questioning me about maple sap.  I'm
guessing we may not know as much about how this works as we would
like.  But if anyone has good solid knowledge of what governs the flow
upward of tree sap in the spring, I would be grateful if you would share it
with the group.

By the way, tapping a single tree for several weeks yielded 4 - 5 cups of
truly delicious maple syrup!
Kathleen Archer

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