INTERACTIONS BETWEEN THE PARASITIC ANGIOSPERM OROBANCHE AEGYPTIACA AND ITS TOMATO HOST - GROWTH AND BIOMASS ALLOCATION

The influence of the parasitic angiosperm Orobanche aegyptiaca on the growth and shoot/root allometry of tomato plants was studied in two ex periments, In the first, the density of infection was manipulated, wit h host and parasite biomass being measured 56 d after planting (d.a.p. ). In the second, multiple harvests were made from 14 to 91 d.a.p. at one level of infection (20 mg seed dm(-3)). The first experiment demon strated an approximately linear reduction in host biomass up to 10 mg seed dm(-3) soil, beyond which there was no further reduction. The par asite also depressed shoot/root ratio, which changed prior to any decr ease in total biomass. These trends were maintained in the second expe riment, becoming more pronounced with time. Significant reductions wer e observed in host biomass compared with that of uninfected controls f rom 42 d.a.p., following the emergence of the parasite above ground, a nd corresponded with the onset of lower relative growth rates (RGRs) i n the infected plants. Infection also influenced components of RGR:the re was a stimulation of leaf area ratio (LAR) and a depression of unit leaf rate (ULR). The data are discussed with respect to the influence of other parasitic weeds on host growth.