EFFECTS OF PARASITISM BY A MISTLETOE ON THE STRUCTURE AND FUNCTIONINGOF BRANCHES OF ITS HOST

Infestation of Acacia acuminata by the xylem-tapping mistletoe Amyema preissii invariably results in inhibition of growth, defoliation and e ventual death of host branch parts distal to the mistletoe, Branch sec tional areas proximal (P) and distal (D) to mistletoes are used to cla ssify stages of parasitism, with P:D area ratios of 5-6 invariably ass ociated with distal branch senescence, As monopolization of the branch proceeds, mistletoe leaf area increases in parallel with declining ho st foliage area, and the specific hydraulic conductivity of distal hos t wood declines sharply relative to that of proximal wood, Mineral com position and concentrations of nitrogenous solutes in xylem sap are at no stage appreciably different from those of proximal mood, 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 tr anspiration rates and selective uptake of host xylem solutes by the ha ustorium, fosters substantial mineral enrichment of the mistletoe rela tive to its host, The study provides a background for future investiga tion of possible cellular mechanisms continuously driving structural a nd functional changes in favour of the mistletoe.