Tansley Review No. 22 What becomes of the transpiration stream?

Abstract

SUMMARY Changes of view on the course of the transpiration stream beyond the veins in leaves are followed from the imbibition theory of Sachs, through the (symplastic) endosmotic theory of Pfeffer (which prevailed almost unquestioned until the late 1930s), to Strugger's experiments with fluorescent dye tracers and the epifluorescence microscope. This latter work persuaded many to return to the apoplastic‐(wall)‐path viewpoint, which, despite early and late criticisms that were never rebutted, is still widely held. Tracer experiments of the same kind are still frequently published without consideration of the evidence that they do not reveal the paths of water movement. Experiments on rehydration kinetics of leaves have not produced unequivocal evidence for either path. The detailed destinies of the solutes that reach the leaf in the transpiration stream have received little attention. Consideration of physical principles governing flow and evaporation in a transpiring leaf emphasizes that: (1) Diffusion over interveinal distances at the rates in water will account for substantial solute movement in a few minutes, even in the absence of flow. (2) Diffusion can occur also against opposing now. (3) Volume fluxes in veins are determined by the diameter of the largest leaves examined contain high conductance supply veins which are tapped into by low‐conductance distributing veins. (4) Edges and teeth of leaves will be places of especially rapid evaporation, and they often have high‐conductance veins leading to them. (5) Solutes in the stream will tend to accumulate at leaf margins. On the basis of recent work, the view is maintained that the water of the stream enters the symplast through cell membranes very close to tracheary elements. Also, that this occurs locally over a small area of membrane. Many solutes in the stream are left outside in the apoplast. This produces regions of high solute concentration in the apoplast and an enrichment of solutes in the stream as it perfuses the leaf. Solutes that enter the symplast are not so easily tracked. Suggestions about where some of them may go can be gained from a fluorescent probe that identifies particular cells (scavenging cells) as having H+‐ATPase porter systems to scrub selected solutes from the stream. Unpublished case‐histories are presented which illustrate many aspects of these processes and principles. These are: (1) Maize leaf veins, where the symplastic water path starts at the parenchyma sheath; (2) Lupin veins, where the symplastic path starts at the bundle sheath and where solutes are concentrated in blind terminations; (3) The edges of maize leaves where flow is enhanced by a large vein (open to the apoplast), and solutes are deposited in the apoplast by evaporation; (4) Poplar leaf teeth, which receive strong flows, and where the epithem cells are scavenging cells; (5) Mimosa leaf marginal hairs, which have scavenging cells at their base; (6) Active hydathodes, whose epithem cells are scavenging cells; (7) Pine needle transfusion tissue, which is a site...

Water rise kinetics in refilling xylem after desiccation in a resurrection plant

“…To exclude dye-related artefacts (Canny, 1990(Canny, , 1993, we studied short-and long-term kinetics of water rise without staining. Water-filled xylem regions of cross-sections are visible under appropriate illumination because of a change in the refraction of light.…”

“…The measured dynamic viscosity of the ink solution was comparable with that of pure water. However, the dye concentration might increase owing to radial water extraction from the xylem conduit (Canny, 1990(Canny, , 1993. Ink enrichment is certainly associated with an increase in viscosity.…”

Hydraulic Analysis of Water Flow through Leaves of Sugar Maple and Red Oak

“…Although it is possible that in grasses R outside veins is a larger component of R leaf than in dicotyledonous leaves with more vein orders, modeling the vein system of grasses as a collection of parallel pipes may, to some degree, neglect the actual distribution network. In grass leaves the major longitudinal veins constitute as little as 30% of the total vein density (Canny, 1990). Large veins may dominate axial transport, but smaller longitudinal and transverse veins with conduit diameters and numbers only 20% to 50% of those in the large longitudinal veins may in fact distribute most water to the mesophyll (Colbert and Evert, 1982;Altus and Canny, 1985;Altus et al, 1985;Canny, 1990).…”

Simultaneous recording of xylem pressure and trans‐root potential in roots of intact glycophytes using a novel xylem pressure probe technique

“…Electrical double layers (Amin 1982) and longitudinal potentials (Fensom 1957(Fensom , 1962 which may be formed in the lumen of the vessels (due to xylem loading, water ascent and/or ion accumulation in the leaf vessels, e.g. Schwenke & Wagner 1992;Canny 1990Canny , 1993Canny , 1995Wegner & Raschke 1994) may change upon excision (see also Ginsburg 1972).…”

“…Electrical double layers (Amin 1982) and longitudinal potentials (Fensom 1957(Fensom , 1962 which may be formed in the lumen of the vessels (due to xylem loading, water ascent and/or ion accumulation in the leaf vessels, e.g. Schwenke & Wagner 1992;Canny 1990Canny , 1993Canny , 1995Wegner & Raschke 1994) may change upon excision (see also Ginsburg 1972).The second approach must also be criticized because there is no evidence that the placing of the innermost microelectrode was inside a vessel (without disrupture of the water column). Furthermore, clogging of the microelectrode tip by cellular debris during impalement through the root tissue can lead to erroneous potential recordings (Dunlop 1982), and damage of the tissue may result in a (partial) short-circuit between xylem and medium (see also Anderson & Higinbotham 1975).…”