Xylem: Differentiation, Water Transport and Ecology


The xylem constitutes the part of plant vascular system which is primarily concerned with the long‐distance transport of water, dissolved minerals and signalling molecules from root to shoot. The development of xylem during tissue differentiation and organ growth is governed in particular by auxins and cytokinins. The mode of transport in xylem is bulk flow, driven by hydrostatic pressure gradients between root and shoot. These gradients involve substantial negative pressures (xylem tensions) during daytime transpiration. As xylem tension increases, so does the possibility of embolism and xylem dysfunction, particularly when water supply is sparse, unless embolism is repaired. The evolution of xylem reflects a combination of demands: optimising the volume flow rate, optimising control over xylem sap composition and minimising the chance of xylem dysfunction through embolism. Strategies to meet these often conflicting demands are reflected through differences in xylem anatomy between species which differ in their ecology.

Key Concepts

  • A key innovation during the evolution of plants was the development of a long‐distance, ‘vascular’, transport system.
  • The mode of long‐distance transport of substances is bulk flow.
  • Bulk flow is driven by gradients in hydrostatic pressure.
  • The xylem constitutes the part of vascular system which is primarily concerned with the transport of substances, including water, between root and shoot.
  • The xylem is made up of cells which are dead at full maturity and fulfil a transport (xylem vessels, tracheids) and mechanical support (fibres) role and cells which are alive and provide metabolic activity (xylem parenchyma).
  • Development of xylem starts from procambial and cambial cells and is governed by auxin and also cytokinins.
  • Daytime transpiration is associated with significant negative pressures (tensions) in xylem; this increases the chance of embolism and (partial) loss of function of xylem.
  • The structural design of xylem of plant species, which differ in their taxonomy and ecology, often reflects differences in the transpirational demand (water flux rates), water availability (and associated likeliness of embolism formation) and soil solution composition (and need to control xylem sap composition) encountered by these species.

Keywords: auxin; bulk flow; embolism; hydrostatic pressure gradient; pit membrane; tension; tracheid; xylem vessel

Figure 1. Diffusion as a mode of transport in biological systems is fast enough to get substances from A to B over short distances, but is too slow to transport substances over longer distances. The time (T1/2) it takes for half of a given species of molecules to move from A to B by diffusion down their gradient in free energy is given through ‘T1/2 = 0.357 × (1/Dj) × ΔX’, whereby Dj is the diffusion coefficient of substance [here: potassium ion in water (being cotransported with chloride ion for electroneutrality), 1.9 × 10−9 m2 s−1], and ΔX is the physical distance in meters between A and B (Nobel, ).
Figure 2. An overview of the occurrence and nature of hydrostatic pressure gradients in the vascular system of plants. Ψ, water potential.
Figure 3. Pipe diameter has a larger‐than expected (based on visual impression) influence on volume flow rate. The rate of water flow through a pipe can be calculated according to Hagen–Poiseuille's Law. For a tube/pipe of given length ‘ΔX’, and pressure difference ‘ΔP’ acting across the tube (from end to end), the volume flow rate ‘VFR’ increases with the fourth power of radius ‘r’ of that pipe; π is about 3.14. In the example shown here, the flow rate through two pipes (r = 10 µm and r = 50 µm), whose radius differs by factor 5, and which are each 200 µm long and subjected to a hydrostatic pressure difference of 0.1 MPa, is calculated. The difference in flow rate between the two pipes is (54) 625‐fold. If we packed 20 of the smaller pipes into a cross‐sectional area equivalent to that of the larger pipe, the difference in flow rate would still amount to 31‐fold. Thus, forming one large xylem vessel per available cross‐sectional area in, for example a root, is a much more efficient use of space in terms of long‐distance water transport compared with forming 20 smaller vessels. The downside, though, is that if the one large vessel becomes dysfunctional, 100% of transport capacity is lost, whereas the dysfunction of one smaller vessel leads to the loss of only 5% of transport capacity.
Figure 4. An overview of some basic challenges faced for the design of xylem and phloem during plant evolution, together with some solutions to master these challenges; PM, plasma membrane.


Brodersen CR and McElrone AJ (2013) Maintenance of xylem network transport activity: a review of embolism repair in vascular plants. Frontiers in Plant Science 4: article 108. DOI: 10.3389/fpls.2013.00108.

Caird MA, Richards JH and Donovan LA (2007) Nighttime stomatal conductance and transpiration in C3 and C4 plants. Plant Physiology 143: 4–10.

Carlquist S (1988) Comparative Wood Anatomy: Systematic, Ecological, and Evolutionary Aspects of Dicotyledonous Wood. Berlin: Springer‐Verlag.

Chaumont F and Tyerman SD (2014) Aquaporins: highly regulated channels controlling plant water relations. Plant Physiology 164: 1600–1618.

Choat B, Cobb AR and Jansen S (2008) Structure and function of bordered pits: new discoveries and impacts on whole‐plant hydraulic function. New Phytologist 177: 608–626.

Esau K (1965) Plant Anatomy, 2nd edn. New York, London, Sydney: Wiley and Sons, Inc..

Fricke W (2002) Botanical briefing review: biophysical limitation of cell elongation in cereal leaves. Annals of Botany 90: 1–11.

Fricke W (2015) The significance of water cotransport for sustaining transpirational water flow in plants – a quantitative approach. Journal of Experimental Botany 66: 731–739.

Hacke UG and Sperry JS (2001) Functional and ecological xylem anatomy. Perspectives in Plant Ecology, Evolution and Systematics 4: 97–115.

Knipfer T, Brodersen CR, Zedan A, et al. (2015a) Patterns of drought‐induced embolisms and spread in living walnut saplings visualised using X‐ray microtomography. Tree Physiology 35: 744–755. DOI: 10.1093/treephys/tpv040.

Knipfer T, Eustis A, Brodersen C, et al. (2015b) Grapevine species from varied native habitats exhibit differences in embolisms formation/repair associated with leaf gas exchange and root pressure. Plant, Cell and Environment 38 (8): 1503–1513. DOI: 10.1111/pce.12497.

Knipfer T, Cuneo IF, Brodersen CR, et al. (2016) In situ visualization of the dynamics in xylem embolism formation and removal in the absence of root pressure: a study on excised grapevine stems. Plant Physiology 17: 1024–1036.

Knoblauch M, Peters WS, Ehlers K, et al. (2001) Reversible calcium‐regulated stopcocks in legume sieve tubes. The Plant Cell 13: 1221–1230.

Kubo M, Udagawa M, Nishikubo N, et al. (2005) Transcription switches for protoxylem and metaxylem vessel formation. Genes and Development 19: 1855–1860.

Mähönen AP, Bonke M, Kauppinen L, et al. (2000) A novel two‐component hybrid molecule regulates vascular morphogenesis of the Arabidopsis root. Genes and Development 14: 2938–2943.

Motose H, Sugiyama M and Fukuda H (2004) A proteoglycan mediates inductive interaction during plant vascular development. Nature 429: 873–878.

Nardini A, Salleo S and Jansen S (2011) More than just a vulnerable pipeline: xylem physiology in the light of ion‐mediated regulation of plant water transport. Journal of Experimental Botany 62: 4701–4718.

Nobel PS (1991) Physicochemical and Environmental Plant Physiology. San Diego: Academic Press, Inc. ISBN 0-12-520021-8.

Ohashi‐Ito K, Oda Y and Fukuda H (2010) Arabidopsis VASCULARRELATED NAC‐DOMAIN6 directly regulates the genes that govern programmed cell death and secondary wall formation during xylem differentiation. The Plant Cell 22: 3461–3473.

Ryu J, Hwang BG and Lee SJ (2016) In vivo dynamic analysis of water refilling in embolized xylem vessels of intact Zea mays leaves. Annals of Botany 118: 1033–1042.

Sachs T (1981) The control of patterned differentiation of vascular tissues. Advances in Botanical Research 9: 151–262.

Sachs T (1991) Pattern Formation in Plant Tissues. Cambridge: Cambridge University Press.

Schuetz M, Smith R and Ellis B (2013) Xylem tissue specification, patterning, and differentiation mechanisms. Journal of Experimental Botany 64: 11–31.

Secchi F, Pagliarani C and Zwieniecki MA (2016) The functional role of xylem parenchyma cells and aquaporins during recovery from severe water stress. Plant, Cell and Environment. DOI: 10.1111/pce.12831.

Taiz L, Zeiger E, Moller IM, et al. (2015) Plant Physiology and Development, 6th edn. Sunderland: Sinauer Associates, Inc. ISBN 10: 1605352551, ISBN 13: 9781605352558.

Tyree MT (1997) The cohesion‐tension theory of sap ascent: current controversies. Journal of Experimental Botany 48: 1753–1765.

Wegner LH (2015) A thermodynamic analysis of the feasibility of water secretion into xylem vessels against a water potential gradient. Functional Plant Biology 42: 828–835.

Zimmermann MH (1983) Xylem Structure and the Ascent of Sap. Berlin: Springer‐Verlag.

Zimmermann U, Meinzer F and Bentrup FW (1995) How does water ascend in tall trees and other vascular plants? Annals of Botany 76: 545–551.

Zhong R, Demura T and Ye ZH (2006) SND1, a NAC domain transcription factor, is a key regulator of secondary wall synthesis in fibers of Arabidopsis. The Plant Cell 18: 3158–3170.

Further Reading

Boyer JS (1985) Water transport. Annual Review of Plant Physiology 36: 473–516.

Bramley H, Turner NC, Turner DW, et al. (2009) Roles of morphology, anatomy, and aquaporins in determining contrasting hydraulic behavior of roots. Plant Physiology 150: 348–364.

Enston DE, Peterson CA and Ma F (2003) Root endodermis and exodermis : structure, function, and responses to the environment. Journal of Plant Growth Regulation 21: 335–351.

Evert RF (1977) Phloem structure and histochemistry. Annual Review of Plant Physiology 28: 199–222.

Hwang BG, Ryu J and Lee SJ (2016) Vulnerability of protoxylem and metaxylem vessels to embolisms and radial refilling in a vascular bundle of maize leaves. Frontiers in Plant Science 7: article 941. DOI: 10.3389/fpls.2016.00941.

Knipfer T, Besse M, Verdeil J‐L, et al. (2011) Aquaporin‐facilitated water uptake in barley (Hordeum vulgare L.) roots. Journal of Experimental Botany 62: 4115–4126.

Kramer PJ (1932) The absorption of water by root systems of plants. American Journal of Botany 19: 148–164.

Pickard WF (2003) The riddle of root pressure. I. Putting Maxwell's demon to rest. Functional Plant Biology 30: 121–134.

Pittermann J (2010) The evolution of water transport in plants: an integrated approach. Geobiology 8: 112–139.

Steudle E (1989) Water flows in plants and its coupling with other processes: an overview. Methods in Enzymology 174: 183–225.

Steudle E (2000) Water uptake by plant roots: an integration of views. Plant and Soil 226: 45–56.

Zeuthen T (2010) Water‐transporting proteins. Journal of Membrane Biology 234: 57–73.

Contact Editor close
Submit a note to the editor about this article by filling in the form below.

* Required Field

How to Cite close
Fricke, Wieland(Jan 2017) Xylem: Differentiation, Water Transport and Ecology. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002076.pub2]