Oviposition of carrot fly (Psila rosae) in response to foliage and leaf surface extracts of host plants

Summary. Foliage of twelve host and two non-host species and surrogate leaves treated with the respective leaf extracts were presented to laboratory populations of the carrot fly (Psila rosae) in oviposition choice assays. The stimulatory activity of dichloromethane surface extracts and the diethyl ether fraction of hot water extracts did not reflect accurately the differences in acceptability observed among intact leaves. A better correlation was found using hexane extracts prepared in a microwave oven. Two out of five fractions of this crude hexane extract obtained by silica gel column chromatography stimulated oviposition. The diethyl ether fraction, which contained the previously identified oviposition stimulants (propenylbenzenes, furanocoumarins, polyacetylenes), could account for only a minor part of the variation in the acceptability of host leaves. The preference hierarchy for intact leaves corresponded better to the ranking of species according to activity of the methanolic fraction, which apparently contains unknown stimulatory compounds.? The water fractions of the hot water extracts were shown to reduce egg-laying underneath surrogate leaves treated with a stimulatory extract. This oviposition-deterring effect was particularly strong with the non-preferred species Pimpinella major, which is also highly resistant in the field. Hence, unidentified inhibitory compounds may also contribute to differential accept ability of host plants. It is concluded that antixenotic (non-preference) resistance of host plants to carrot fly attack depends on complex mixtures of semiochemicals. Received 11 June 1997; accepted 26 November 1997.     

Spatial,temporal, and environmental limits on xanthotoxin induction in wild parsnip foliage

Summary Although present constitutively in large amounts, furanocoumarins in leaves of wild parsnip,Pastinaca sativa, are inducible to even higher concentrations by mechanical and insect damage. We conducted several experiments in order to characterize the nature and extent of xanthotoxin inducibility inP. sativa foliage. In order to determine the extent to which induction is localized, we mechanically damaged a single leaflet of a compound leaf on seven plants. Xanthotoxin concentrations increased significantly only in the damaged leaflet and in the half of the terminal leaflet closest to the damaged leaflet; thus, xanthotoxin induction is localized to the immediate vicinity of damage. To determine whether xanthotoxin induction results fromin situ biosynthesis or translocation from other plant parts, we detached individual leaflets from ten plants, damaged half of these detached leaflets, and compared xanthotoxin concentrations after 6 h in damaged and intact leaflets. We found that xanthotoxin concentrations increased 41% in damaged leaflets compared to detached leaflets that were not damaged. We also determined the rapidity and duration of the induction response. In leaflets that were damaged and then harvested after 0, 3, 24, 72, 120 and 168 h, xanthotoxin concentrations increased rapidly compared to undamaged leaflets on the opposite side of the leaf, reaching maximum levels within 24 h. This response was of comparatively short duration; concentrations declined to preinduction levels after seven days. To determine whether availability of resources influences the induced response, we performed two experiments, one in which soil nutrients were manipulated and one in which light level was manipulated. The low nutrient treatment was sufficiently extreme to cause cessation of aboveground growth, and the low light treatment caused etiolation. Extremes of resource limitation notwithstanding, leaflets significantly increased xanthotoxin production (2 to 3-fold increase under nutrient limitation and 3-fold increase under light limitation) in response to damage in both experiments.     

Autotoxic effects of essential oils on photosynthesis in parsley, parsnip, and rough lemon

Summary. Many plant species contain essential oils with allelochemical properties, yet the extent to which these same chemicals can be autotoxic is unclear. In this study, we tested the toxicity of several essential oil components to three species that produce them—Pastinaca sativa and Petroselinum crispum (Apiaceae), and Citrus jambhiri (Rutaceae). The effects of exogenous application of small amounts of essential oil components to the surface of foliage, followed by a pinprick to allow entry into the leaf, were monitored by chlorophyll fluorescence imaging. Rapid and spatially extensive declines in photosynthetic capacity were detected within 200 s. The most toxic constituents were monoterpenes. Two sesquiterpenes, caryophyllene and farnesene, and the phenylpropanoid myristicin, by comparison, were not toxic. Autotoxicity of endogenous essential oil was investigated by slicing through containment structures (glands or tubes); significant toxicity, manifested by reduced photosynthetic activity, was observed in all three species but was most pronounced in P. sativa and P. crispum, both of which possess oil tubes.     

Allelopathic activity of Ammi majus L. fruit waxes

Summary. Ammi majus seeds are known to have a strong allelopathic effect due to coumarins. The coumarins were identified and shown to be present in the seed epicuticular waxes. A simple method to study allelopathic compounds leaching from seeds is reported. Received 8 August 2001; accepted 20 December 2001.     

Comparative electrophysiological analysis of plant odor perception in females of threePapilio species

Summary Antennae of femalePapilio butterflies perceive many volatile plant constituents with widely differing, constituent-specific sensitivities. We compared the responses of threePapilio species to volatiles from host and non-host plants to assess species-specificity and the degree of evolutionary conservatism in olfactory responses.Since previous studies had demonstrated that the polar constituents in odor fromDaucus carota stimulate oviposition behavior inPapilio polyxenes, we collected headspace volatiles fromD. carota, Pastinaca sativa (both Apiaceae) andArtemisia dracunculus (Asteraceae) and separated the polar fraction of these volatiles by gas chromatography. GC-coupled electroantennograms (GC-EAG) were recorded from the speciesPapilio polyxenes, P. machaon hippocrates andP. troilus. In addition, the responses of the three species to five compounds known as generally occurring constituents of plant odor were recorded. The relative sensitivities for these compounds were nearly identical in all threePapilio species. The response spectra to the separated plant volatiles also showed considerable similarities among the species.From the limited set of GC peaks evoking a response in one of the species, 64% (D. carota), 44% (P. sativa) and 29% (A. dracunculus) also evoked a response in both of the other species. The responses of the two closely related Apiaceae feeders (P. polyxenes, P. m. hippocrates) to volatiles fromD. carota were more similar to each other than was either to the response ofP. troilus, which feeds on Lauraceae. However, this was not true for the responses to volatiles fromP. sativa. The least congruence among the three species was found in the responses to volatiles fromA. dracunculus, a non-host for all of them. The differences and similarities found in the response profiles of the threePapilio species are discussed with respect to evolutionary adaptation to host odor versus evolutionary conservatism in adaptation of olfactory receptors.