[NAFEX] Maple syrup and further physiology
breen at fedcoseeds.com
Mon Dec 12 20:01:51 EST 2005
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
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
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
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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