How do Trees Really lift Water to their Leaves?

Davlin:
Hello
Leaves Are Plant Food Factories *
Plants and other photosynthesizing organisms have a very special talent. They can turn sunlight into food. It is a pretty neat trick that only photoautotrophs can do (photo=sun; auto=self; troph=feeder).
In order for plants to make food energy, they need water, carbon dioxide (CO2) and sunlight. From this special combination, a plant is able to make its own food, in the form of glucose, a type of sugar. Plants then use the glucose as food energy to live and grow. In order to harvest sunlight energy, plants have a green pigment called chlorophyll. This pigment is what makes a plant's leaves appear green.

* Shutting Down Operations for the Winter *

As winter approaches, the days get shorter and cooler. This change in day length and temperature triggers some trees to go dormant, essentially hibernating for the winter. A tree's woody roots, branches and twigs can endure freezing temperatures, but most leaves are not so tough.

It is also very energetically expensive for a tree to run its leafy food factories in the winter, when there is often little sunlight and freezing temperatures make water transport (from the ground into the tree's trunk and leaves) a problem. So it's more energy efficient for a leafy tree to close down operations in the winter and go dormant.

* How Leaves are 'Told' to Drop *

A tree is full of vascular cells that transport water and sap throughout, from root to leaf tip. As the amount of sunlight decreases in autumn, the veins that transport sap into and out of a leaf slowly close off. Then a layer of cells, called the separation or abscission layer, develops at the base of the leaf's stem. When this layer is completely formed, the leaf falls off.

This process happens in all deciduous trees (trees that annually shed their foliage), with oak leaves as a notable exception. In oaks, the separation layer doesn't fully allow the oak leaves to detach. That's why most dead oak leaves remain on the tree through winter and even into early spring (much to the perpetual leaf-raking consternation of home owners with oak trees on their property

Thanks for sharing
« Last Edit: 28/10/2009 17:03:37 by BenV »

Hello Davlin

Your welcome

The thread deals with what happens to the glucose and dissolved minerals in the sap after evaporation at the leaf. Current theory relies on transpiration to pull water up from the ground. However, when the leaves have fallen very little transpiration if any is taking place so clearly another method of fluid transport is relied upon.

Stefan:
Why would trees that have lost their leaves need any significant water uptake at all? Climates in which deciduous trees evolved are cold in winter, so tree metabolism and water-loss is reduced. Sap can even freeze. Any slight water loss could probably be compensated for by stomata in the stems. So what's the problem?

Also, how does photosynthate transport affect your hypotheses?

Trees inevitably evaporate water even when leaves are not present and water will leave the tree and enter the soil if the circulation within is interrupted.

How does the current literature deal with this problem in deciduous trees? After all there are no leaves to effect this imagined magical pull on each of the water molecules yet the water inside the naked tree is happy to circulate to the highest twigs and branches?

A big clue is the migration of minerals and sugars to the roots over the winter period.

Gravity cannot be ignored no matter how inconvenient the truth is.

The fact that the solutes have migrated to the lower part of the tree means that the more dilute sap must have been drawn and pushed up to replace the falling sap. This will inevitably apply tension to the soil water molecules and draw in water and diluted minerals which in turn alter the density of the sap at the roots causing a density imbalance which must be corrected by rising up the tree caused by the tension from the falling sap! Affording circulation to continue unhindered when leaves have fallen.

en.wikipedia.org/wiki/Transpirational_pull
The Incoherent Cohesion Tension Hypothesis.

Stefan:
Do you accept that transpiration is the major factor in water transport in leaved trees?

Have you demonstrated experimentally and mathematically that density changes are enough to cause water transport?

In winter, metabolism slows down and sugar production at the top of the tree stops. So once that sugar (and other nutrients produced in the leaves) have sunk to the bottom of the tree, the tree's water transport power is significantly reduced. What does the tree do then? Are dissolved minerals really enough?

Trees have thick bark and waxy coatings that prevent water loss. Can you demonstrate that any water loss that does occur is significant enough to endanger the tree in winter? Or if water that's lost is replaced by your proposed mechanism, can you demonstrate that this is the case?

I would appreciate direct answers

Thanks for the questions Stefan.

Stated many times that evaporation is required to alter the density of the sap in the upper part of the tree, releasing pulses of salts down the phloem to induce a positive pressure in front of the falling sap and a negative pressure / tension on the xylem sap affording bulk flow back to the leaf. So clearly transpiration is important when leaves are on the tree but not required for inducing the circulation when there are no leaves on the tree. Yet the stored solutes in the upper parts of the tree as you rightly state move down the tree over the winter towards the roots and this cannot take place without affecting a return flow of dilute sap back up the tree.

Even when the tree has shed it's leaves root growth requires there to be circulation and the change in pressures brought about by shedding the leaves would undoubtedly influence a downward growth at the roots providing an increase in pressure within the roots.

Have already demonstrated circulation inside tubular experiments using tea, liberated from tea leaves, urine, milk, juiced fruit, juiced leafy vegetables, demonstrating slight changes in density at an elevated point will cause both a downward flow and a return flow.
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Mathematical calculations designed to show it can't happen or can happen appears a little pointless when we can show it taking place experimentally and reliably so. Has anyone done the maths on the Atlantic conveyor system? If you feel this can contribute anything please feel free to share your results.

Over a prolonged winter when hypothetically all of the sugars and salts have reached the roots which incidentally could not happen without a continual flow and return circulation taking place, all that would be required to trigger circulation in the spring would be a density change in the sap. Warming the outer part of the trunk first from the seasonal change in temperature would provide such a density change and induce circulation together with an increase in head of water at the tips of the branches to induce bud burst and the blooms of blossom and leaves.

Also, the last ice age that is believed to have been started by a sudden influx of salt free water flowing onto the ocean surface and causing the Atlantic Conveyor system to shut down. See film: After the Warming. Offers an understanding of the sudden increase in rainfall in the Autumn diluting the sugars and solutes in the tree and effectively washing out the nutrients from the leaves returning them to the trunk and branches and in doing so altering the circulation causing the leaves to wilt and fall.

The fact that the leaves fall from the tree indicates they are in danger given that the deciduous trees that normally shed leaves hang on to them when planted in warmer dryer conditions. And when a tree is in trouble from water stress it is generally the upper parts of the tree that die back again indicating a reduction in water within the tree.

Andrew

Rosy:
Yes, Andrew, but current theory can explain your observations in loops closed at the top. I've yet to hear of a system in which you've managed to extract the water (and therefore the kinetic energy) once it reaches the top of the system.

Have you done the experiment? Explained how it might work in a tree?
It still looks like a perpetual motion machine to me...

Stefan:
Thanks for your reply Andrew.

I second Rosy's post.

What I meant to convey when I asked what does the tree do when its photosynthates have sunk, was, what if this occurs long before Spring arrives?
And is the concentration of salts normally found in water enough? Do you use the correct (natural) concentrations, as well as tube diameter, in your experiments?

Hi Rosy

The Atlantic Conveyor system does not require tubes, yet evaporates water from the "top" and circulates a phenomenal amount of sea water without much problem.

The tubular experiments are as I have stated many times a method of showing the circulation caused by the density differences, not a method of showing a tree. If we needed to show this we only have to go back to Strasburger's experiments with picric acid to see that the tree structure is capable of both evaporation and circulation for 3-4 weeks following the complete death of the tree.

The cohesion tension hypothesis you are defending does not have any working model.