Sap ascent in trees

[25362]

I think this query belongs to the "fluid mechanics" forum. This is a question to satisfy my curiosity.
The sap being a water solution of mineral nutrients and aminoacids ascends through the xylem. What is the plausible driving force that explains the upward motion of the sap in tall trees? Surface tension having plainly been discarded by experts.

 

[Montemayor]

25362:

I think the rise of sap in a tree is due to capillary action.

The rise of a liquid in a thin tube, called a capillary tube, is a surface phenomenon closely related to surface tension. The atttractive forces between a molecule in a liquid and other molecules in the liquid are cohesive forces. The force between a liquid molecule and another substance, such as the wall of a thin tube, is an adhesive force.

Art Montemayor
Spring, TX

 

[hacksaw]

osmotic pressure is the usual explanation

 

[quark]

Hacksaw is right. Further, as water evaporates from leaves, the pressure is always lower at the top.

 

[25362]

Art Montemayor

The capillarity effects cannot explain a continuous flow against friction and gravity to a height of more than a few feet.

Hacksaw

Although osmotic pressures are indeed high, osmosis needs a semipermeable membrane to let water and ions move through. Where is this membrane ?

Quark

The fact that more than 90% of the water in the arriving sap evaporates from the leaf cell walls to the surrounding air makes me think you are in the right track. Although the vapor pressure difference cannot explain heights of much more than 10 m (tall trees).

With all due respects: I'm still waiting for a more comprehensive answer.

 

[Scipio]

God help me, I'm a geek! I had no idea, but started digging around the net and found some pretty interesting stuff.

From what I saw, it looks like there is no comprehensive answer, but the current 'theory' regarding circulation the Cohesive-Tension Theory, referred to as the CTT. It's a little involved, so I'm not even going to try summarizing it, I found a great link though,

http://www.plantphys.net/article.php?ch=e&id=99.

It looks like everyone who's had a suggestion has at least a piece of the puzzle. It even turns out plant vessels (xylem)can even experience vascular cavitation if the differential across cellular membranes is too high!

 

[btrueblood]

That explains the roaring noise they make on windy days, it's the water cavitating in their stems!

 

[hacksaw]

25362

don't believe that continuous conduits are involved...there is been quite a bit of progress made in the area over the last 3-5 years

 

[FredGarvin]

The things I have read say that when temps increase above freezing, the sap pressure increases. The other part of that is that sap movement seems to be best when the temperatures go back below freezing during the night times.

 

[quark]

25362,

Any living cell is itself a semipermeable membrane and osmosis is what causes water to go into plant roots from ground. Once the water goes into the roots, it requires some other driving force for its travel in upward direction and plant respiration is the force here.

10 meters is a constraint if you are considering higher pressure as atmospheric pressure but it need not be atmospheric deep down the earth. (why water jets out when we dig bores?)

I think capillary action is also one of the factors because plant xylem is like a thin tube.

I too did some google search today and it seems I am a bit on right track.

http://www.barrygray.pwp.blueyonder.co.uk/Tutoring/PlantN.html#UTM

PS: Sanskrit word for tree 'Padapam' means one which drinks from ones feet. I am simply amazed about the knowledge our earlier generations had. Though it is obvious that a plant should drink water from its feet, considering it as one substantial aspect is what amazes me. Otherwise, there could have been more trivial names.

Regards,

 

[25362]

Quark

It is clear that water can penetrate cellular membranes by the process of osmosis. Would you say the same process pushes it out of the root cells again upwards ? Osmotic pressures at the roots of trees have been measured and found to be quite low.

It seems to me, still, the mechanisms you mentioned cannot explain continuous flow of a quite diluted watery solution at a velocity of, say, 6 m/h up a height of say, 100 ft or more, through vessels of 100 [μ]m in diameter.

I've also read various Google sites mentioning that water is under a kind of tensile stress based on the strength of hydrogen bonds, the sap being moved from a higher (~zero) chemical potential at the roots to a lower (more negative) potential at the vaporizing surface of the leaves. The principal cause being a "sucking" effect from the top.

Estimating the Gibbs free energy H-TS, at both ends one sees a large difference. Is this indeed the reason or is there another property of water that we are missing ?

 

[25362]

I've managed to get some more information on the basics about hydraulics and architecture of the subject under discussion, as follows:

The ascent of sap occurs in conduits formed by dead cells, after having lost their cytoplasm and cytoplasmic membranes.
The cell walls are lignin-incrusted fibrils that provide structural rigidity.

Xylem vessels are neither rectilinear, circular, nor smooth.
In conifers (pine, spruce) elongated cells called tracheids overlap at their ends. Hydraulic continuity is provided by orifices called bordered pits that allow passage of water but trap air bubbles. In birch, flow occurs through partially dissolved end compartments. In oak, water flows through long rigid capillaries composed of squat segments.

Water flows to a tree's leaves during photosynthesis, more than 90% being ultimately transpired to the surrounding air. Thus the flowrate of ascending sap varies during the day in response to photosynthetic activity.

The estimated ideal pressure gradient is:

[Δ]P/L = 8[η]v/r²​

as calculated from the Hagen-Poiseuille equation.

Where [η] is the viscosity, 0.001 kg.(s.m)^-1 for water at 20 deg C, v is the mean velocity in the capillary, and r is the capillary radius.

Experimental correction factors to multiply the H-P estimated pressure drops to reconcile measured pressure gradients, volumetric throughputs and vessel diameters have been reported: 1 for vines, 3 for birch, and 5 for poplar; arriving at pressure gradients typically in the 0.05-0.2 bar/m range. This means that the maximum tension at the top of a 30 m tree will typically be between -1.5 and -6 bar.

A liquid under tension is exposed to a pressure that is lower than its vapor pressure at a given temperature; hence it is superheated, and it appears the liquid is in a metastable state.

Experimental proof of the fact that the sap ascends under tension dates back to 1893 when some botanist fitted a leafy twig to a water-filled glass tubing from which air had been expelled. This tube was immersed in mercury, and the leaf was allowed to transpire, whereupon the mercury was raised more than 760 mm. More recent experiments (1995) have demonstrated the ability of water-filled xylem channels to sustain tensions of up to -35 bar !

If the columns of ascending sap are under tension, hydraulic continuity is constantly in danger of being interrupted by bubble formation, requiring a delicate balance between safety and efficiency.

Evolution is said to having solved this problem by limiting the diameter of xylem vessels to 500[μ]m, and by allowing the ascending sap to follow tangential and radial trajectories.

This is achieved by interconnections between xylem channels and radial rays with extensive interchannel pitting between touching conduits to allow alternative flow paths, insuring the continued ascension of sap even when the tree is severely damaged.

Do you agree with the theory of the sap being under a relatively high tension without evaporating ? How much effort should be applied to a closed-end syringe to slowly pull the plunger over water deprived of dissolved gases until it starts to evaporate ?

 

[quark]

25362,

I think, my superficial knowledge in issues related to fluid dynamics, water properties, thermodynamics and metaphysics(interestingly) is not sufficient even to understand the process to my satisfaction but here are some pointers for further investigation(unlike me, you have a character of going into minute details)

Initially I thought that as all experiments conducted to check osmotic pressures are above ground, we might not be getting the high pressure into observation. The pressure of water below ground includes the pressure due to the weight of soil above it. But experiments already proved that it was not pressure that was moving the water(dye moving on either side of its point of injection)

The tension theory or the CTT may well explain this behaviour but, to my knowledge, this should prove water is highly ductile. Infact, water is brittle and it was proved that when you hit a flowing stream of water with a hammer, fast enough, it will break into pieces like glass does.

Secondly, none of the papers I searched explained how the great tensile force was being created by simple evaporation. They only speak of the surface tension that can hold a water column.

As the radius of meniscus depends upon forces of adhesion and cohesion, this should be fairly constant irrespective of the flowrates. But how a constant radius meniscus can take care of varying velocities(the tensile force should be more due to higher pressure drops at higher flowrates)

Temperature change in sap due to tensile force was not discussed in any of the papers. My understanding is that temperatures should be low at higher tensile forces and thus flow should get reduced, when it is required to be high.

Some quack experiments(atleast, they are called so) indicated that plants could perceive things beyond senses common to human beings. They did identify the difference between cutting a leaf and thinking of cutting a leaf. It can be one of the dramatic mysteries of the creator.

Apart from what Scipio mentioned, here is another good link you might have seen already.

http://fp.uni.edu/berg/pp/Water Relations/waterstories.htm

Perhaps, my thoughts are wrong

Regards,

 

[Leclerc]

I remember from somewhere in the dim and distant, that, when dealing with very fine capillaries, the vapour pressure of the liquid at the surface is effectively lowered by the curvature of the meniscus.

The vapour pressure of a liquid, presenting a flat surface to the vapour space above it, is determined by numbers of liquid molecules leaving the surface for the vapour, and the number of molecules of arriving from the and impacting with the liquid. At equilibrium at any temperature, these two numbers are the same, the rate of molecules leaving liquid is higher (greater velocity) and the rate of vapour molecules arriving at the surface is higher (higher partial pressure). In other words, vapour pressure increases with temperature.

With a highly curved surface, a proportion of molecules will leave the liquid surface, only to impact on the surface at the other side of the meniscus. This has the effect of lowering vapour pressure.

 

[25362]

Water is not totally brittle, it can (elastically) stand tension beyond 3000 psi. Even glass isn't totally brittle.

Whether the CTT theory is right or wrong, it is presently the only one that can explain sap ascent. Would you say the jury is still out on this quandary ?

 

[Scipio]

I think I'd agree that the jury is still out - especially since what appears to be the most commonly accepted theory is still just that, a theory. Too bad, could patent one hell of a metering pump if we could figure it out

 

[25362]

The "water-in-tension" theory based on leaves' transpiration doesn't apply when dealing with deciduous trees before they grow new leaves to replace the shedded ones. It appears that hacksaw's proposed osmotic pressure effect may be the right answer, at least for the case when the sap has already reached the tree top.

Polysaccharides degrade (as needed) to simpler sugars (mono- and di-saccharides) and markedly increase the cell molar concentration to such a level that even a 1 molar solution of sucrose at 25 deg C would produce an osmotic pressure of:

[Π] = (1 mol/L)(0.0821 L.atm/mol.K)(298 K) = 24.4 atm​

sufficient for any tall tree in nature.

Any additional comments welcomed.

 

[unclesyd]

If leaves enter into the equation to any degree it's going to be interesting as some of my trees will only have 10% to 20% of last years branches/leaves to work with. It will be interesting to see how Mother Nature handles the situation of severely damaged large trees.
The sap is starting to rise at this moment.

Sure hope they don't die as my three oaks, 36" to 48" butts, will take $2500 each to remove.

 

[Latexman]

I have not followed this closely, but there is an old saying that goes something like this, "Warm days and cold nights is good for collecting maple syrup". I take it to mean a big difference in day to night temperature swings. Does this support what you have discussed?

Good luck,
Latexman

 

[hydrae]

Extremly tall trees get their water from fog, Redwoods for example only grow in foggy areas, some other conifers take advantage of the fog as well, that is how those trees get around the need to pump in one form or onther water from the base.
Hydrae