The flow of xylem sap in spring before the leaves have grown

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Settia
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What is the mechanism that causes xylem sap to flow during the early spring when the leaves have not grown yet? Is the only mechanism pressure in the roots from when the soil solution passes the endodermis and enters the xylem? Is there any bulk flow at this time of year?
 
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Well from what I know about Xylem, it moves water in a passive transport (meaning it doesn't require energy) and is simply a matter of water evaporating through the leaves and the water molecules pulling each other in a "train" up through the plant. As for sap, that is generally (depending on the species) an excess of "waste" created by the plants, and they are really only clogged up xylem tissue.
So where are you seeing the sap "flow"? Is it seeping out somewhere or are you just wondering if it moves at all?
 
Xylem sap is a mixture/solution of water and minerals that comes from the roots and moves up the plant through the xylem. What causes it to move upward is the cohension-tension theory along with transpiration. So, my question is fairly simple. When a tree does not have leaves in the early spring, is there any flow of xylem sap? There is minor movement of xylem sap from pressure being created when the roots transport soil solution past the endomeris and into the vascular bundle, but its extremely slow. I do not recall how slow the flow of xylem sap is by root pressure alone, but it is extremely insignificant in the flow of xylem sap. Therefore, it seems that there is no flow of xylem sap during the time a tree has no leaves. I am wondering if this is correct.
 
Hmm I see. Well trees go dormant during the winter (as with most plants) so perhaps some of the metabolic changes the tree undergoes in order to prepare for producing new leaves in the spring causes some movement in the sap channels of xylem. But as I stated earlier sap is generally referring to clogged xylem tissue, which would entail that is does very little conducting if any at all. So I think it would be safe to infer that almost no sap flow occurs once it has set there, regardless of the presence of leaves. Although it would be safe to infer that root pressure changes could affect the flow of sap, simply through a mechanical function.
 
PVastro said:
But as I stated earlier sap is generally referring to clogged xylem tissue, which would entail that is does very little conducting if any at all.

I don't think you're really answering the question that Settia is asking.

By what mechanisms (if any) does xylem sap rise before transpirational pull is possible?

My best answer is that xylem vessels do not always drain over winter, so it seems like normally the sap is sitting there ready to rock in the spring. Osmotic root pressure, in most cases, will get the sap back up there if it happens to drain.
 
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In general xylem does remain full of sap during winter months and water conduction can begin again in spring with little effort because of the cohesion tension theory that was mentioned.

From what i understand the mechanism is a combination of osmotic root pressure and increased soil water pressure caused by higher soil water volume during late winter and early spring.

There will be some bulk movement as trees are not totally dormant during winter, at this time they consume stored energy instead of energy manufactured by leaves.