Piotr M. Górski

Polymorphism in Plant Defense Against Herbivory: Constitutive and Induced Resistance in Cucumis sativus

Theory predicts that plant resistance to herbivores is determined by both genetic and environmentally induced components. In this study, we demonstrate that the phenotypic expression of plant resistance to spider mite herbivory in Cucumis sativus is determined by genetic and environmental factors and that there is an interaction between these factors. Previous feeding by spider mites induced systemic resistance to subsequent attack over several spatial scales within plants, reducing the population growth of mites compared to that on control plants. Effects of induction were effective locally over the short term, but resulted in local increased susceptibility to spider mite attack after several days. However, this local induced susceptibility on the damaged leaf was associated with induced resistance on newer leaves. Induced resistance was correlated with increases in cucurbitacin content of leaves, but was not associated with changes in the density of leaf trichomes. Induced resistance to herbivory was not detected in plants of a genotype lacking constitutive expression of cucurbitacins, which were in general highly susceptibile to mite attack. Allocation trade-offs between growth and defense are often invoked to explain the maintenance of variation in the levels of plant resistance. Contrary to current thinking, neither constitutive nor herbivore-induced plant resistance were associated with reductions in plant allocation to root and shoot growth. However, plants that had high levels of induced resistance to spider mites were the most susceptible to attack by a specialist beetle. Such ecological trade-offs between resistance to generalist herbivores and susceptibility to specialist herbivores may be important in the maintenance of variation of plant resistance traits. In summary, C. sativus exhibits strong genetic variation for constitutive and induced resistance to spider mites, and this variation in resistance is associated with ecological trade-offs.

Fate of male-derived cucurbitacins in spotted cucumber beetle females.

Using the spotted cucumber beetle, Diabrotica undecimpunctata howardi, as a model species, we begin an examination of the role of dietary cucurbitacins, the bitter tetracyclic triterpenes produced by cucurbits, in the reproductive behavior of luperine chrysomelids that seek these compounds for purposes other than host-plant recognition or nutrition. By manipulating the cucurbitacin content of adult diets, we determine: (1) how males and females partition cucurbitacins between reproductive organs and other tissues, (2) the extent to which males pass cucurbitacins to females during copulation, and (3) if females increase their cucurbitacin stores via multiple matings. After two days of exposure to dietary cucurbitacins, 8.3 times more cucurbitacins per gram dry weight were sequestered in male spermatophores than in the aggregate of remaining male tissues. These cucurbitacin-rich spermatophores were passed to females during mating, and 78.8% of the transferred cucurbitacin was deposited in eggs, 11.8% was irreversibly sequestered in other female tissues, and 9.4% was apparently excreted. Females copulated with 0–15 males prior to accepting a complete spermatophere, but did not accept measurable amounts of cucurbitacin from rejected males. Regardless of when it was received, females lost subsequent receptivity to additional males after accepting a complete spermatophore. The implications of these data are discussed in terms of sexual selection hypotheses and mating behavior in cucumber beetles.

Production and biological activity of saponins and canavanine in alfalfa seedlings

The saponin and canavanine concentrations and pattern were analyzed in growing alfalfa seedlings (Medicago sativa L.). Accumulation of saponins and canavanine was found to follow different time courses. During the first eight days, saponin concentration rose from zero in alfalfa seeds to 8.7% in roots and 1.8% in shoots on eighth day and then slowly decreased to 7.6% in roots and 0.8% in shoots present on the 24th day. Canavanine was found in seeds at a concentration of 1 % then increased to 3.2% in seedlings on the sixth day and rapidly decreased to 0.2% per dry mass in roots and shoots on the 24th day. The effect of saponins-medicagenic acid sodium salt and medicagenic acid glycosides-on the growth ofAmaranthus andLepidium in Petri dishes and tomato (Lycopersicon) cell growth in tissue culture also was investigated. In contrast to medicagenic acid glycosides, a very strong inhibition of plant and cell growth was found as an effect of medicagenic acid.

Flavonoids from the bird's foot trefoil seeds (Lotus corniculatus L.)

In the seeds of birds foot trefoil ( Lotus corniculatus L.) the presence of quercetin- and kaempferol heterosides was found. Eight crystalline compounds and one chromatographically homogeneous fraction were obtained from the flavonoid mixture using column chromatography on polyamide and cellulose. By means of TL-chromatography, elemental analysis and UV-spectrometry seven of the isolated compounds were identified to be quercetin, quercetin 3-arabopyranoside (guajaverin), quercetin 3-galactoside, quercetin 3-rhamnoside, kaempferol 3,7-diglucoside, kaempferol 3,7-dirhamnoside kaempferol 3-glucosyl-7-rhamnoside and two remaining ones were only partly characterized a snearer indefinite kaempferol heterosides.