The transpiration stream in the leaf apoplast: water and solutes

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Abstract
Flow of the transpiration stream in the lumen apoplast of the xylem appears hydrodynamically orthodox in being approximately described by the Hagen-Poiseuille Law, and by Murray’s Law for branching pipes. Flow may be followed in the major (supply) veins by labelling the stream with dye solutions. Progress of the dye in the stream into the minor (distribution) veins is obscured by surrounding tissues. Observations of the spread of fluorescent tracers from these veins in living leaves gave results that have been seriously misinterpreted to present a false view of the cell wall apoplast. Microscopy of the stabilized water-soluble fluorescent tracers moving out of the minor veins has revealed that: (i) the dye is separated from the water by filtration through cell membranes, and the water moves through the symplast; and (ii) the dye diffuses in the cell wall apoplast at rates 1/100 to 1/10 000 the rate of diffusion in water. As a consequence of (i), high concentrations of dye build up at sites called sumps. In grasses these sumps may be in the intercellular spaces outside the xylem. In dicotyledons these sumps are within the small tracheary elements. In fact, flow in the lumen apoplast is flow through leaky tubes, and is inadequately described by the Hagen-Poiseuille Law. Leaky tubes have a critical radius, below which (for a given pressure gradient) flow cannot occur. As a consequence of this, a wedge of xylem made up of vessels of different radii acts as a unit to concentrate dye tracers in a sump at its apex. Sumps may also be formed by evaporation of the water in the stream, especially at leaf margins. Investigations with the cryo-analytical scanning electron microscope of the natural ions of the transpiration stream reveal high concentrations of K, Cl, P and Ca in the stream in all the sizes of vessel and vein of sunflower leaves. These high concentrations appear to be produced, not by the mechanisms responsible for the formation of sumps of dyes, but by some other processes, probably occurring in the stem. The absence of sump formation by ions at the places where dyes form sumps is probably due to the more rapid penetration of the ions through the cell membranes. Reasons for the discrepancy between these measurements of salt concentrations in the stream and those obtained from sap expressed from leaves by pressure vessels are discussed. Implications of these facts for the design and interpretation of experiments with leaves are presented.