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This review discusses how understanding of functional relationships between parasitic plants and their woody hosts have benefited from a range of approaches to their study. Gross comparisons of nutrient content between infected and uninfected hosts, or parts of hosts, have been widely used to infer basic differences or similarities between hosts and parasites. Coupling of nutrient information with additional evidence of key processes such as transpiration, respiration and photosynthesis has helped elucidate host-parasite relationships and, in some cases, the anatomical nature of their connection and even the physiology of plants in general. For example, detailed analysis of xylem sap from hosts and parasites has increased our understanding of the spatial and temporal movement of solutes within plants. Tracer experiments using natural abundance or enriched application of stable isotopes ((15)N, (13)C, (18)O) have helped us to understand the extent and form of heterotrophy, including the effect of the parasite on growth and functioning of the host (and its converse) as well as environmental effects on the parasite. Nutritional studies of woody hosts and parasites have provided clues to the distribution of parasitic plants and their roles in ecosystems. This review also provides assessment of several corollaries to the host-parasite association.

Many members of the tribe Cardueae are invasive weeds, including yellow starthistle (Centaurea solstitialis L.), one of the most important weeds in the Western United States. We examined the root crowns and stems of yellow starthistle and related plants growing in five countries (Armenia, Republic of Georgia, Greece, Russia, and Turkey) where yellow starthistle is native. In its native range, the root crowns and lower stems of yellow starthistle are frequently attacked by the internal feeding larvae of apionid weevils. We present illustrations and a key to the adults of the six apionid species that we reared from yellow starthistle and its relatives: Ceratapion basicorne (Illiger), C. carduorum (Kirby), C. gibbirostre (Gyllenhal), C. onopodri (Kirby), C. orientale (Gerstaecker), and C. penetrans (Germar). The only apionid we reared from yellow starthistle was C. basicorne. In Turkey, where we collected most intensively, 58% of the yellow starthistle at 20 sites had larvae of this weevil, and at sites where C. basicorne was present, there were an average of 1.8 immatures per yellow starthistle plant. This apionid is currently being further researched for its potential as a biological control agent for yellow starthistle.

Background and Aims: A wide variety of plants produce extrafloral nectaries (EFNs) that are visited by predatory arthropods. But very few studies have investigated the relationship between plant genetic variation and EFNs. The presence of foliar EFNs is highly variable among different aspen (Populus tremuloides) genotypes and the EFNs are visited by parasitic wasps and predatory flies. The aim here was to determine the heritability of EFNs among aspen genotypes and age classes, possible trade-offs between direct and indirect defences, EFN induction following herbivory, and the relationship between EFNs and predatory insects., Methods: EFN density was quantified among aspen genotypes in Wisconsin on trees of different ages and broad-sense heritability from common garden trees was calculated. EFNs were also quantified in natural aspen stands in Utah. From the common garden trees foliar defensive chemical levels were quantified to evaluate their relationship with EFN density. A defoliation experiment was performed to determine if EFNs can be induced in response to herbivory. Finally, predatory arthropod abundance among aspen trees was quantified to determine the relationship between arthropod abundance and EFNs., Key Results: Broad-sense heritability for expression (0.74-0.82) and induction (0.85) of EFNs was high. One-year-old trees had 20% greater EFN density than 4-year-old trees and more than 50% greater EFN density than > or =10-year-old trees. No trade-offs were found between foliar chemical concentrations and EFN density. Predatory fly abundance varied among aspen genotypes, but predatory arthropod abundance and average EFN density were not related., Conclusions: Aspen extrafloral nectaries are strongly genetically determined and have the potential to respond rapidly to evolutionary forces. The pattern of EFN expression among different age classes of trees appears to follow predictions of optimal defence theory. The relationship between EFNs and predators likely varies in relation to multiple temporal and environmental factors.