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.     

Toxicity of nonhost phototoxins to parsnip webworms(Lepidoptera: Oecophoridae)

Summary The parsnip webworm,Depressaria pastinacella (Lepidoptera: Oecophoridae), feeds exclusively on apiaceous hostplants containing furanocoumarins, compounds capable of oxygen-dependent and oxygen-independend photosensitization. Despite high titers of antioxidant enzymes relative to other herbivorous insects, webworms cannot tolerate nonhost photosensitizers such as alpha-terthienyl or beta-carboline alkaloids at dietary concentrations of 0.01% or less. Tolerance of skimmianine, a furano-quinoline alkaloid, may be due to its structural resemblance to furanocoumarins, which are metabolized by cytochrome P450 monooxygenases in this species.