Subject: Sap rising

From: @hursley.ibm.com
Date: 30 Apr 1996 11:14:20 GMT

I asked this before but got no response, so I'm trying again ........	

When a plant is in leaf the transpiration pull creates a pressure differential
between the leaves and the roots, so water and nutrients can rise up the
xylem. At least that's how I understand it. 

But what causes the "sap to rise" in the spring before the buds have broken on
deciduous plants ? Is it to with root pressure ? Perhaps someone might
be able to tell me what root pressure is also.

Thanks for any help.

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From: [email-masked]@nofc.forestry.ca
Date: 1 May 1996 15:16:25 GMT

>be able to tell me what root pressure is also.
It does seem to be root pressure that gets things going in the spring before
leafout. Sugars translocated to the roots during the previous growing season
were condensed into starch which has relatively little effect on the osmotic component
of water potential.   In the spring this starch is hydrolized back to sugar which
drops the osmotic potential dramatically. Water then diffuses into the roots
following the water potential gradient. This results in positive pressure developing
in the roots which drives the xylem sap stream up the stem into the crown where the sugars
and other nutrients still dissolved in the sap support leaf growth. Once the carbohydrates
have been moved out of the roots the osmotic potential rises. This combined with
increasing water demand from the developing crown lowers the pressure in the
system and root pressure disappears for the rest of the season.

I hope this helps.

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From: [email-masked] (Richard L. Boyce)
Date: 1 May 1996 19:02:02 GMT

> be able to tell me what root pressure is also.

I'll take a stab at this, although I'm more of an ecologist than a
physiologist.  Yes, root pressure is responsible for sap rising in some
species, like maples and grapes.  In these species, starch that is
stored in root parenchyma (jargon for storage tissue) is broken down
into sugars in the spring and loaded into xylem, the vascular tissue
normally used for transporting water and nutrients to the shoot from
the roots.  A physiologist would say that this lowers the osmotic
potential, inducing water to flow into the xylem from surrounding root
tissue; another way of looking at would be to say that by increasing
the solute load in the sap, water is "drawn" into the xylem.  Since
xylem is pretty stiff (another name for tree xylem is wood), this
causes the pressure to rise, and this root pressure is enough to push
the sap up to the top of the tree.  Once the leaves come out, water
flow in xylem is due to low water potentials in the atmosphere, but
that is another story...

 =============================
 Richard L. Boyce           
 Department of Biological Sciences     
 University of Denver
 Denver, CO 80208
 303-871-3538   
 303-871-3471 (fax)         
 [email-masked]
 http://www.du.edu/~jfoglema/boyce.html  
 =============================

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From: [email-masked] (Bob.b)
Date: Mon, 13 May 1996 00:21:23 GMT

>deciduous plants ?

The sap is always there, 'though it doesn't contain as much sugar
during the dormant period since there is no need for it.  In spring
the plant mobilises sugars from stored reserves 'cos the new growth
needs the energy.  Add to this the increased metabolic raste due to
higher temperatures and you get an increased flow.

> Is it to with root pressure ? Perhaps someone might

Too complex to go into in accurate detail here, but simplified  it
results from the combined effects of osmosis and selective ionic pumps
working within cell membranes

>Thanks for any help.

regards

Bob

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From: [email-masked] (rick strimbeck)
Date: Tue, 14 May 1996 15:31:06 -0600

> be able to tell me what root pressure is also.

Kramer, P.J. 1983. Water Relations of Plants. Academic press, NY. 489 pp.

Also, search a Bioscience database under M.T. Tyree for some up to date
papers on sap flow in sugar maple.

It's not as simple or a sclear cut as you might think...

-- 
Rick Strimbeck
University of Vermont
School of Natural Resources
Burlington, VT