Long-distance transport of gases in plants: aperspective on internal aeration and radial oxygen loss from roots

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    Abstract

    Internal transport of gases is crucial for vascular plantsinhabiting aquatic, wetland or flood-prone environments.Diffusivity of gases in water is approximately 10000 timesslower than in air; thus direct exchange of gases betweensubmerged tissues and the environment is stronglyimpeded. Aerenchyma provides a low-resistance internalpathway for gas transport between shoot and root extremities.By this pathway, O2is supplied to the roots and rhizosphere,while CO2, ethylene, and methane move from thesoil to the shoots and atmosphere. Diffusion is the mechanismby which gases move within roots of all plant species,but significant pressurized through-flow occurs in stems andrhizomes of several emergent and floating-leaved wetlandplants. Through-flows can raise O2concentrations in therhizomes close to ambient levels. In general, rates of floware determined by plant characteristics such as capacity togenerate positive pressures in shoot tissues, and resistanceto flow in the aerenchyma, as well as environmental conditionsaffecting leaf-to-air gradients in humidity and temperature.O2diffusion in roots is influenced by anatomical,morphological and physiological characteristics, and environmentalconditions. Roots of many (but not all) wetlandspecies contain large volumes of aerenchyma (e.g. rootporosity can reach 55%), while a barrier impermeable toradial O2loss (ROL) often occurs in basal zones. Thesetraits act synergistically to enhance the amount of O2diffusingto the root apex and enable the development of anaerobic rhizosphere around the root tip, which enhancesroot penetration into anaerobic substrates. The barrier toROL in roots of some species is induced by growth in stagnantconditions, whereas it is constitutive in others. Aninducible change in the resistance to O2across the hypodermis/exodermis is hypothesized to be of adaptive significanceto plants inhabiting transiently waterlogged soils.Knowledge on the anatomical basis of the barrier to ROLin various species is scant. Nevertheless, it has been suggestedthat the barrier may also impede influx of: (i) soilderivedgases, such as CO2, methane, and ethylene; (ii)potentially toxic substances (e.g. reduced metal ions) oftenpresent in waterlogged soils; and (iii) nutrients and water.Lateral roots, that remain permeable to O2, may be the main surface for exchange of substances between the rootsand rhizosphere in wetland species. Further work isrequired to determine whether diversity in structure andfunction in roots of wetland species can be related to variousniche habitats.
    Original languageEnglish
    Pages (from-to)17-36
    JournalPlant, Cell and Environment
    Volume26
    Issue number1
    DOIs
    Publication statusPublished - 2003

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