Flow of sap in Spring

James W. Perry jperry at UWC.EDU
Fri Sep 18 11:15:47 EST 1998

At 10:43 PM 9/16/98 GMT, you wrote:
>How does sap get up from the roots to the buds of a tree in the Spring?
>Is it via phloem or xylem? What is the source of the "pressure"? 
>I have searched in plant physio books and found nothing.
>Could this "flow" be apoplastic? Maple syrup enthusiasts want
>to know! - NH
>---Nancy Harrison, SRJC Life Sciences, Santa Rosa CA 95401
>   http://www.sonic.net/~vulpia/index.html (with link to CNPS in Sonoma 

This has long been a problem that has mystified far too many botanists. A
number of years ago I asked the question of one of the world's most
recognized plant physiologists the mechansim of flow and he told me that no
one really understood it! Here we have one of the oldest recognized sources
of sugar, harvested by Native Americans for centuries, and we could not
figure it out.

Most of sap flow in deciduous trees in the spring is coming from
carbohydrates stored in the xylem over the winter. Storage takes place
throughout the stem. Hence, we tap into the xylem, not the phloem, when
we're harvesting maple sugars.

Now the $64,000 question? How does movement take place in the absence of
transpiring leaves, a seeming requirement for transpiration flow/cohesion
tension mechanism. Consult Am. Scinetist 86: 152-159 (1998). Canny
describes greater acceptance of Zimmerman's Compensating-Pressure Theory of
xylem transport. After reading the article several times, my thoughts
returned to our age-old problem, and it seems to me that
Compensating-Pressure can more easily explain sap rise than any other
theory to date. 

The thrust of C-P as it applies to our question is that living xylem
parenchyma cells generate a positive tissue pressure that pushes water (and
sugars?) from reservoir cells into xylem vessels by reverse osmosis.  The
nice thing about C-P is that it involves root pressure, which seems to have
been interpreted as having nothing to do with long distance xylmen
transport in the past. (Although I am not certain root pressure has
anything to do with sap accension in the spring, other than the fact that
root pressure does occur when no transpiration is taking place.) If this
line of thought is correct, then in the spring these xylem parenchyma cells
are loading the vessels which we are tapping to extract maple sap. 

As a plant anatomist, I would defer to plant physiologists on plant-ed who
may have a better handle on this than I. 

James. W. Perry, CEO/Campus Dean
Professor, Department of Biological Sciences
University of Wisconsin - Fox Valley
1478 Midway Road, P.O. Box 8002
Menasha, Wisconsin 54952-9002
Office: 920.832.2610
FAX: 920.832.2674
jperry at uwc.edu

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