Cohesion-tension transport of water is an energetically efficient way to carry large amounts of water from the roots up to the leaves. However, the cohesion-tension mechanism places the xylem water under negative hydrostatic pressure ( P x ), rendering it susceptible to cavitation. There are conflicts among the structural requirements for minimizing cavitation on the one hand vs maximizing efficiency of transport and construction on the other. Cavitation by freeze-thaw events is triggered by in situ air bubble formation and is much more likely to occur as conduit diameter increases, creating a direct conflict between conducting efficiency and sensitivity to freezing induced xylem failure. Temperate ring-porous trees and vines with wide diameter conduits tend to have a shorter growing season than conifers and diffuse-porous trees with narrow conduits. Cavitation by water stress occurs by air seeding at interconduit pit membranes. Pit membrane structure is at least partially uncoupled from conduit size, leading to a much less pronounced trade-off between conducting efficiency and cavitation by drought than by freezing. Although wider conduits are generally more susceptible to drought-induced cavitation within an organ, across organs or species this trend is very weak. Different trade-offs become apparent at the level of the pit membranes that interconnect neighbouring conduits. Increasing porosity of pit membranes should enhance conductance but also make conduits more susceptible to air seeding. Increasing the size or number of pit membranes would also enhance conductance, but may weaken the strength of the conduit wall against implosion. The need to avoid conduit collapse under negative pressure creates a significant trade-off between cavitation resistance and xylem construction cost, as revealed by relationships between conduit wall strength, wood density and cavitation pressure. Trade-offs involving cavitation resistance may explain the correlations between wood anatomy, cavitation resistance, and the physiological range of negative pressure experienced by species in their native habitats.

https://www.ualberta.ca/~hacke/pdf/Perspectives2001.pdf

Functional and ecological xylem anatomy

Since about twenty years, hydraulic architecture (h.a.) is, doubtless, the major trend in the domain of plants (and especially trees) wa- ter relations. This review encompasses the main concepts and results concerning the hydraulic of architecture of trees. After a short paragraph about the definition of the h.a., the qualitative and quantitative characteristics of the h.a. are presented. This is an occasion to discuss the pipe mo- del from the h.a. point of view. The second part starts with the central concept of embolism and give a review of important experimental results and questions concerning summer and winter embolism. The last part deals with the coupling between hydraulic and stomatal conductances. It discusses the theoretical and experimental relationships between transpiration and leaf water potential during a progressive soil drought, the in- crease of soil-root resistance and its consequences in term of xylem vulnerability, the factors controlling the daily maximum transpiration and how stomates can prevent "run away embolism". In conclusion different kinds of unsolved questions of h.a., which can be a matter of future in- vestigations, are presented in addition with a classification of trees behaviour under drought conditions. To end, an appendix recalls the notions of water potential, pressure and tension. hydraulic architecture / cohesion-tension theory / summer embolism / winter embolism / drought resistance

/pdf/hydraulic-architecture-of-trees-main-concepts-and-results-4e0ahxa4wi.pdf

Hydraulic architecture of trees: main concepts and results

Xylem embolism, the reduction of water flow by air-filled vessels, was measured in a stand of 5- to 8-year-old sugar maple (Acer saccharum Marsh.) saplings growing in a nursery bed in northwestern Vermont. Embolism was quantified as percentage loss in hydraulic conductivity of trunk and branch segments relative to maximum values obtained by removing air from vessels by repeated high pressure (173 kPa) perfusions. Ten segments per tree were cut from 6 trees for each of 11 measurement periods spaced at roughly monthly intervals from May 1986 to June 1987. During the 1986 growing season, embolism increased significantly from 11 to 3 1% in the larger branches and trunk (segment diameter ?0.5 cm), but remained at about 10% in twigs (segment diameter <0.5 cm). This was unexpected because the greatest water stess and thus potential for embolism occurs in twigs. During the winter, embolism increased throughout the trees and the trend with diameter was reversed; by February, small twigs were 84% embolized vs. 69% for larger branches and trunk. Dye perfusions showed that winter embolism in trunks was localized on the south side; this may have resulted from water loss by sublimation or evaporation in the absence of water uptake. Beginning in late March, embolism decreased throughout the trees to approximately 20% in June. This decrease was associated with positive xylem pressure of at least 16 kPa which may have originated in the roots, because weather conditions at the time were unfavorable for the generation of stem pressures characteristic of Acer species in early spring. IN THE MEDICAL SCIENCES, embolism is the blockage of a blood vessel caused by a foreign particle or embolus. By analogy, the term can be applied to blockage of water flow in vessels and tracheids ofplants; in this case the embolus is an air bubble. These bubbles can arise from air forced out of solution during freezing of xylem water, or by an unknown mechanism associated with the characteristically negative pressure of the transpiration stream. There is evidence suggesting that air is sucked into vessels from neighboring air spaces via pores in the vessel wall (Zimmermann, 1983; Crombie, Hipkins, and Milburn, 1985; Sperry, Holbrook, and Tyree, 1987). Once an air bubble is inside a vessel, negative pressure causes it to expand and block water flow. There has undoubtedly been selective pressure to minimize xylem embolism, because if unchecked it would block water supply to foliage. Certainly the xylem is well designed in this regard; instead of containing a single open pipe which would conduct water with the least resistance but would become embolized by a single bubble, xylem consists of individual tra

Seasonal occurrence of xylem embolism in sugar maple (acer saccharum)

Due to the exceptional sweetness of its sap, sugar maple (Acer saccharum Marsh.) is economically exploited at a commercial scale for maple syrup production in northeastern North America. Approximately 80% of world production is realised in the province of Quebec, Canada, where it is economically important for rural communities. Despite important financial investments in industrial infrastructure over recent decades, the maple syrup yield (ml of sap/tap/year) has followed a general declining trend over the last 15 years, presumably because of unfavourable climatic conditions. In this study, the relationship between climate and maple syrup yield by tap for the whole province was

Confronting climate change in the U.S. Northeast: science, impacts, and solutions.

We studied the effect of temperature on the carbohydrate status of parenchyma cells during winter in relation to the efflux and influx of sugars between parenchyma cells and xylem vessels in 1-year-old twigs of walnut (Juglans regia L.). The mechanism of sugar transfer between contact cells and vessels was also investigated. We obtained new insights into the possible osmotic role of sugars, particularly sucrose, in stem pressure formation and winter embolism repair. Accumulation of sucrose in the xylem sap during winter was mainly influenced by: (1) abundant conversion of starch to sucrose in the symplast at low temperatures; (2) sucrose efflux into the apoplast at low temperatures (1 degrees C); and (3) inefficient sugar uptake at low temperatures, although efficient sugar uptake occurred at 15 degrees C. We hypothesize that a diethyl pyrocarbonate (DEPC)-sensitive protein mediates facilitated diffusion of sucrose from parenchyma cells to xylem vessels (efflux) in walnut. We discuss the possible occurrence of active H+-sucrose symports and the coexistence of both influx and efflux processes in walnut in winter and the modulation of the relative importance of these flows by temperature.

/pdf/temperature-effects-on-xylem-sap-osmolarity-in-walnut-trees-23i47r809g.pdf

Temperature effects on xylem sap osmolarity in walnut trees: evidence for a vitalistic model of winter embolism repair

concentrations have only slight, if any, effect on permeability. Killing of the tissue results in a very slight increase in permeability. The smallness of the increase indicates that the cell wall rather than the cell membranes is the primary barrier to water movement. Determination of tissue permeability by osmotic means yields rates of water movement essentially identical to those obtained by the isotopic method. The half-time of protoplast expansion or contraction of individual cells located near the surface is however 2 to 3 minutes and in cases in which the cells are verv near to cut tissue edges, 1 minute or less. Cell permeability of Avena coleoptile section tissue is thus greater than the permeabilitv of the tissue as a whole.

/pdf/a-statistical-evaluation-of-some-of-the-factors-responsible-3ydx7xsc5w.pdf

A Statistical Evaluation of Some of the Factors Responsible for the Flow of Sap from the Sugar Maple.

/pdf/temperature-induced-sap-flow-in-excised-stems-of-acer-59b3rxsvst.pdf

Temperature-Induced Sap Flow in Excised Stems of Acer

Use of various carbon sources by isolated maple callus cultures

The total area and number of xylem rays and vessels from tangential and cross sections of twigs of 12 sugar maples (Acer saccharum Marsh.) were determined by the use of an image-analyzing computer. A nested analysis of variance indicated that xylem rays of trees of high sap and sugar yield are more numerous and larger than the rays of other sugar maples. The total area and number of xylem vessels were about the same in all 12 trees.

Image-analyzing computer in plant science: more and larger vascular rays in sugar maples of high sap and sugar yield

Cell shape and cell volume relations in the pith of eupatorium perfoliatum l.

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