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Effects of parasitism by a mistletoe on the structure and functioning of branches of its host

Authors

  • K. U. TENNAKOON,

    1. Department of Botany, University of Western Australia, Nedlands, WA 6907, Austral ia
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  • J. S. PATE

    Corresponding author
    1. Department of Botany, University of Western Australia, Nedlands, WA 6907, Austral ia
      J. S. Pate, Department of Botany, University of Western Australia. Nedlands, WA 6907, Australia.
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J. S. Pate, Department of Botany, University of Western Australia. Nedlands, WA 6907, Australia.

ABSTRACT

Infestation of Acacia acuminata by the xylem-tapping mistletoe Amyema preissii invariably results in inhibition of growth, defoliation and eventual death of host branch parts distal to the mistletoe. Branch sectional areas proximal (P) and distal (D) to mistletoes are used to classify stages of parasitism, with P:D area ratios of 5–6 invariably associated with distal branch senescence. As monopolization of the branch proceeds, mistletoe leaf area increases in parallel with declining host foliage area, and the specific hydraulic conductivity of distal host wood declines sharply relative to that of proximal wood, mineral composition and concentrations of nitrogenous solutes in xylem sap are at no stage appreciably different from those of proximal wood. After the demise of the distal branch parts, the transectional area of the host branch stump increases linearly with increasing mistletoe leaf area, the branch area supporting a unit of mistletoe leaf area always being about 3 times greater than that supporting a unit of host foliage area on unparasitized branches. This differential, compounded with high transpiration rates and selective uptake of host xylem solutes by the haustorium, fosters substantial mineral enrichment of the mistletoe relative to its host. The study provides a background for future investigation of possible cellular mechanisms continuously driving structural and functional changes in favour of the mistletoe.

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