Flow of sap in Spring

Dave Haas dhaas at uncfsu.campus.mci.net
Fri Sep 18 15:12:56 EST 1998

In article < at uwc.edu>, jperry at UWC.EDU 
>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. 

Not being inhibited by any facts I wonder if the sap is really rising in 
the xylem.  If there is no transpiration and if the turgor of the cells is 
increasing wouldn't you simply increase pressure in the vessels?  When one 
taps into these they would be under positive pressure.  As buds turn to 
leaves the ray parenchyma could load the sieve tube members in phloem so 
that the sugar could ascend to the meristems.  I would imagine that sugar 
in the transpiration stream would tend to be lost (could this be the source 
of all that goo that collects on my windshield when parked under a maple?)  
I wonder if anyone has tapped a tree at its top? (or close to it) It would 
be interesting to see how much pressure/sugar is present.  I can't believe 
that no one has tried this.  

D. Haas 

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