Xylem Function and Evolution

Abstract

Water transporting xylem tissues are among the most obvious and attractive features of land plants. The stems, roots, branches and leaf veins in all vascular plants are largely composed of xylem tissues, and remarkably, most of these xylem cells are dead. The water transport network in plants is closely linked to the capacity for photosynthesis and growth due to the shared pathway for water vapour and photosynthetic carbon dioxide between the leaf and atmosphere. Adaptations in the anatomy and architecture of xylem conduits and leaf vein density enable angiosperms to achieve the highest water transport capacity among land plants. Evolutionary pressure to improve water transport efficiency is counterā€balanced by the need to maintain the integrity of the water column inside the plant, which operates under high tension.

Key Concepts:

  • Land plants trade water for carbon.

  • Using the cohesive nature of water, plants transport huge volumes of water without metabolic cost.

  • Water is transported under tension, leading to cavitation as soil dries.

  • Water transport and photosynthesis are linked by the common stomatal pathway for water and CO2.

  • High rates of photosynthesis require high xylem hydraulic conductivity.

  • Xylem physiology is highly adaptive both in transport efficiency and safety.

  • Angiosperm xylem vessels provide the best compromise of safety and efficiency.

  • Water transport in leaves is limited by the density of veins.

  • Radiation of angiosperms saw a sharp rise in vein density in leaves.

  • Leaf vein density is highly plastic, enabling optimisation of carbon investment.

Keywords: xylem; vessel; hydraulic; venation; cavitation; photosynthesis; angiosperm; fern; stomata; vein density

Figure 1.

Schematic showing how water flows through xylem tissue in the plant to keep leaves hydrated and stomatal pores open so that carbon dioxide can enter the leaf for photosynthetic assimilation. Transpiration is largely a by‐product of opening stomatal pores for photosynthesis, but if water transport is insufficient then leaves begin to dry out, causing stomata to close and photosynthesis ceases.

Figure 2.

Scanning electron microscope images of the pits between xylem elements in typical angiosperm and conifer wood. Far left shows a bordered pit between xylem conduits with the pit membrane removed to show the central aperture where water flows from one conduit to another. In the angiosperm pit, water moves through tiny pores in the pit membrane, whereas in the conifer the margins of the membrane are dissolved to form threads (the margo) that suspend a central plug called a ‘torus’. Stylised drawings (courtesy of John Sperry and Jarmila Pittermann) of angiosperm and conifer pits show how water flows between open pits, and how embolism in a neighbouring conduit seals the pit membrane preventing air from seeding cavitation between conduits. Cavitation occurs when water tension in the functional conduit (blue) becomes sufficiently large to deform the pit membrane and open up pores that allow air to cross between conduits. Far right shows a cross section of a conifer pit with the torus in the sealing position to prevent cavitation moving from the conduit on the left to the right.

Figure 3.

A bleached leaf of Nothofagus illustrating the xylem venation network. The leaf cross section shows the general path for water flowing out of the minor veins (red bundles) through the mesophyll tissue toward the sites of evaporation (the stomata). This final flow pathway is inefficient due to the lack of specialised water conducting tissues, hence the closer minor veins come to the stomata, the shorter the path through the mesophyll, the higher the hydraulic conductance. A strong correlation is observed between leaf hydraulic conductance and photosynthetic rate (µmol CO2 m−2 s−1) because greater water supply allows greater porosity of the stomatal surface of the leaf, enabling greater entry of CO2 for photosynthesis. Diverse species are shown including bryophytes (black), lycopods (white), ferns (green), conifers (red) and angiosperms (blue).

Figure 4.

Vein networks (all at the same magnification) from plant families ranging in age from an ancient fern to a modern angiosperm. A strong pattern of increasing vein density occurs during the evolution of angiosperms, giving the most recently evolved eudicot clade much higher vein densities than any vascular plants in evolutionary history. High vein density in modern angiosperms confers high hydraulic efficiency and maximum photosynthetic rates.

Figure 5.

Examples of the plasticity in veins and stomatal densities in Nothofagus leaves growing under sun and shade conditions. Higher light allows greater photosynthesis but leads to greater transpiration, thus sun plants have more stomata to facilitate CO2 entry, and more veins to supply water to the leaf. Strong co‐ordination between veins and stomata provides an efficient balance between maximum transpiration and investment in leaf venation.

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Further Reading

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Cochard H, Coll L, Le Roux X and Ameglio T (2002) Unraveling the effects of plant hydraulics on stomatal closure during water stress in walnut. Plant Physiology 128(1): 282–290.

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How to Cite close
Brodribb, Tim J(May 2014) Xylem Function and Evolution. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0025290]