(14)C-Labelled alkaloid precursors (arginine, putrescine, spermidine) fed to Senecio vulgaris plants via the root system were rapidly taken up and efficiently incorporated into the pyrrolizidine alkaloid senecionine N-oxide (sen-Nox) with total incorporations of 3-6%. Considerable amounts of labelled sen-Nox were translocated into the shoot and were directed mainly into the inflorescences, the major sites of pyrrolizidine-alkaloid accumulation. Detached shoots of S. vulgaris were unable to synthesize pyrrolizidine alkaloids, indicating that the roots are the site of their biosynthesis. Further evidence was obtained from studies with in-vitro systems established from S. vulgaris: root cultures were found to synthesize pyrrolizidine alkaloids but not cell-suspension cultures, tumor cultures or shoot-like teratomas obtained by transformation with Agrobacterium tumefaciens. Studies on transport of [(14)C]sen-Nox, which was fed either to detached shoots or to the root system of intact plants, indicate that the alkaloid N-oxide does not simply follow the transpiration stream but is specifically channelled to the target tissues such as epidermal stem tissue and flower heads. Exogenously applied [(14)C]senecionine is rapidly N-oxidized. If the phloem path along the stem is blocked by a "steam girdle" translocation of labelled sen-Nox is blocked as well. Root-derived sen-Nox accumulated below the girdle and only trace amounts were found in the tissues above. It is most likely that the root-to-shoot transport of sen-Nox occurs mainly if not exclusively via the phloem. In accordance with previous studies the polar, salt-like N-oxides, which are often considered to be artifacts, were found to be the real products of pyrrolizidine-alkaloid biosynthesis as well as the physiological forms for long-distance transport, tissue-specific distribution and cellular accumulation.

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The pronotal and elytral defensive secretions of 10Oreina species were analyzed. Species feeding on Apiaceae, i.e.,O. frigida andO. viridis, or on Cardueae (Asteraceae), i.e.,O. bidentata, O. coerulea, andO. virgulata, produce species-specific complex mixtures of autogenous cardenolides.O. melanocephala, which feeds onDoronicum clusii (Senecioneae, Asteraceae), devoid of pyrrolizidine alkaloids (PAs) in its leaves, secretes, at best, traces of cardenolides. Sequestration of host-plant PAs was observed in all the other species when feeding on Senecioneae containing these alkaloids in their leaves.O. cacaliae is the only species that secretes host-derived PA N-oxides and no autogenous cardenolides. Differences were observed in the secretions of specimens collected in various localities, because of local differences in the vegetation. The other species, such asO. elongata, O. intricata, andO. speciosissima, have a mixed defensive strategy and are able both to synthesize de novo cardenolides and to sequester plant PA N-oxides. This allows a great flexibility in defense, especially inO. elongata andO. speciosissima, which feed on both PA and non-PA plants. Populations of these species were found exclusively producing cardenolides, or exclusively sequestering PA N-oxides, or still doing both, depending on the local availability of food-plants. Differences were observed between species in their ability to sequester different plant PA N-oxides and to transform them. Therefore sympatric species demonstrate differences in the composition of their host-derived secretions, also resulting from differences in host-plant preference. Finally, within-population individual differences were observed because of local plant heterogeneity in PAs. To some extent these intrapopulation variations in chemical defense are tempered by mixing diet and by the long-term storage of PA N-oxides in the insect body that are used to refill the defensive glands.

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