Modelling sugar diffusion across plant leaf cuticles: the effect of free water on substrate availability to phyllosphere bacteria


  • Annemieke van der Wal,

    1. Department of Microbial Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Heteren, the Netherlands.
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  • Johan H. J. Leveau

    Corresponding author
    1. Department of Microbial Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Heteren, the Netherlands.
    2. Department of Plant Pathology, University of California, Davis, CA, USA.
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We present a continuous model for the diffusion of sugars across intact plant leaf cuticles. It is based on the flow of sugars from a source, representing the leaf apoplast, to a sink, in the shape of a hemispherical drop of water on the outside of the cuticle. Flow is a function of the difference between sugar concentrations CSource and CSink, permeability P of the cuticle, volume VSink of the water drop, as well as its contact angle α with the cuticle surface. Using a bacterial bioreporter for fructose, and a two-compartment experimental set-up consisting of isolated cuticles of walnut (Juglans regia) carrying water droplets while floating on solutions with increasing concentrations of fructose, we determined a value of 1 × 10−6 m h−1 for P. Using this value, we explored different scenarios for the leaching of sugars across plant leaf cuticles to reveal in quantitative terms how diffusion takes longer when VSink increases, P decreases or α increases. Bacterial growth was modelled as a function of changes in P, α and VSink and was consistent with observations or suggestions from the literature in relation to the availability of free water on leaves. These results are discussed in the light of bacteria as ecosystem engineers, i.e. with the ability to modify the plant leaf surface environment in favour of their own survival, e.g. by increasing cuticle leakage or leaf wetness. Our model represents a first step towards a more comprehensive model which will enhance our quantitative understanding of the factors that play a role in nutrient availability to bacterial colonizers of the phyllosphere, or plant leaf surface.