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  • From: Heron Breen <breen@fedcoseeds.com>
  • To: nafex@lists.ibiblio.org
  • Subject: [NAFEX] Maple syrup and further physiology
  • Date: Mon, 12 Dec 2005 19:01:51 -0600


Hi sportsfans,
Offlist, Jerry Lehman asked me about how my previous posts about plant
physiology
relate to Maple Sugaring. I think explaining the conditions around the human
production of maple syrup will continue the cycle of physiology we started.
So here
goes:

All summer long, the tree is transporting sugar solution down the phloem, and
parenchyma cells are sucking as much as they can hold as the fluid descends.
The
solution is sugar and some water. The cambium layer is using a lot of this
sugar to
divide and create new Phloem parts and new xylem parts, as well as new
"hungry"
parenchyma cells within the xylem. Meanwhile water is "pumping" up from the
roots
to the leaves for transpiration/photosynethesis.
When I previously posted, I seperated the dead water transport vessels in the
xylem
(wood) and the parenchyma sugar storage/transport cells. These cells do
interact,
actually, but it is good to keep them clear in your mind before melding them.
As
the sugar solution is transported into the wood, parenchyma and other cells,
"fill"
up and/or use some of the sugar and water for cell functions. Sometimes, if
the
tree is really going great and has lots of leaves and sun, the parenchyma
can't
store all the sugar and water at their disposal. This would be similar to our
bodies only being able to absorb so much vitamin C and hour, regardless of
intake.
Many parenchyma cells abut the water transport vessels and "leftover" water
regularly returns to the vessels for another journey to the top. Sometimes a
small
amount of leftover sugar gets released into the vessels as well. But this is
a very
very little amount of sugar and it gets put back into use by cambium up top.
This
description might encourage one to think of these vessels and parenchyma as
able
and readily willing to share their goods regularly. This is not the case. For
more
than 75% of the year, they are mostly seperate.
This changes in the Fall. Almost all the stored sugar is meant for the spring
push
of buds, flowers, leaves. Throughout the Fall, parenchyma are turning the
sugars
into starch (long-term storage form of sugar). A lot of this starch is
"dumped"
(see below paragraphs for "dumped" explanation) into the vessels and pumped
by the
water upward and outward. Buds formed that summer are given a massive amount
of
starch build-up in the wood behind the bud to draw from in the spring.
Remember
photosynthesis is barely happening during spring (a little from the cortex)
and
trees come out of dormancy and begin cambial activity from the top down, so
the
starch needs to be already in place for a bud to grow either leaf or flower.
Most
buds are on wood that have a limited number (sometimes only 1 or 2) of growth
rings
worth of xylem backing it up for fuel, so parenchyma from more thick xylem
send
theirs. This sugar release into the vessels from the parenchyma takes place
after
the leaves have fallen usually. This is why collecting scionwood and dormant
cuttings in the fall can have variable results and must generally be done in
late
November into December at the earliest here in Maine.
This brings us to maple syrup. Along with parenchyma and vessels, there are
lots of
fiber cells in the wood. They store water as well as adding the structural
integrity of the tree. Fiber cells need to be dead to function structurally,
and
their walls are made up of criss=crossing layers of bundled cellulose (which
is in
turn bundled starch chains) and lignin which is the equivalent of tree
lanolin. The
cambium makes all these cells. The interior of the cell is called the lumen.
The
interior and exterior can store huge amounts of water, and this determines
the
weight of a piece of wood "dry" or "green". They fill and empty depending on
many
factors. After feeding the living cells around them, their excess water would
also
end up in the transport vessels.
These fiber cells go through the winter with water in their lumens
(interiors). As
it gets cold, this water freezes into pure crystals. In maple sugaring time,
the
cold nights followed by warm days freezes then thaws this water. As the water
thaws, vapor/gas is produced. We've all seen this from ice cubes in the
freezer and
snowbanks. I do not honestly know why ice does this, so someone else will
have to
fill in here. Nor do I know what makes up the vapor. This vapor goes through
the
walls of the fiber cell into the rest of the xylem. There it exerts pressure
on the
rest of the cells present, the parenchyma and xylem. The parenchyma are
squeezed
and some more of their starch goes the only place in can: into the water
transport
vessels. The water transport vessels themselves are under pressure and, if we
didn't tap the tree, this starch would go up to help buds prepare for winter.
This
process is going on all late fall and winter, as freezing and thawing take
place
within the tree. It is a fairly passive process, compared to phloem transport
and
photosynthesis. The starch stops travelling when it gets to the buds and
collects
there ready for Spring usage.
When we tap the tree, we are releasing that pressure. The tree, if punctured,
is
literally willing to squirt its "sap" out whatever whole. Tapping in the
trunk
yields more "sap" because more parenchyma cells and transport vessels are
present
due to thicker xylem compared to farther up the tree. This freezing and
thawing
period of time is the only time that enough starch is being dumped into the
vessels
for that solution to be called "sap". If a tree is tapped in the summer, it
will
not yield much if any syrup, just plain water with a drop of sugar per gallon
probably.
So, my fingers are worn out and probably your eyes, so I'll stop there for
now.
Elaborate, question, and correct at your leisure. Pardon my sentence
structure and
spelling.
sincerely,
Heron Breen
zone 4, Maine
PS Next time: the short course on a tree going "dormant" from September
onward,
including the theories of why rootstocks influence early or late dormancy and
winter hardiness. Maybe by next week after my fingers rest.






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