Hermann M. Niemeyer

Hydroxamic acids (4-hydroxy-1,4-benzoxazin-3-ones), defence chemicals in the Gramineae.

Abstract Hydroxamic acids of the type 4-hydroxy-1,4-benzoxazin-3-ones constitute one of the most extensively studied secondary metabolites in relation to host plant resistance to pests and diseases. They play a major role in the defence of cereals against insects, fungi and bacteria, in the detoxification of herbicides and in allelopathic efrects of the crop. Although other mechanisms have also a bearing on these effects, more intensive exploitation of hydroxamic acids in cereal crops is indicated.

Role of hydroxamic acids in the resistance of cereals to aphids.

Abstract Hydroxamic acid concentration in Gramineae, both natural and incorporated, correlates with resistance to the aphid Metopolophium dirhodum . 2,4-Dihydroxy-7-methoxy-2H-1,4-benzoxazin-3(4H)-one, a hydroxamic acid isolated from corn extracts, is deleterious to aphids fed on artificial diets. It is proposed that hydroxamic acids act as naturally-occurring protective factors against M. dirhodum .

Effect of content and distribution of hydroxamic acids in wheat on infestation by the aphid Schizaphis graminum

Abstract The content of hydroxamic acids in wheat plants shows substantial variations between different species and cultivars (1.0–6.3 mmol/kg fr. wt). It also varies with the age of the plant and the organ assayed. The maximum concentration is reached by the fourth day in epicotyls and roots. The amount in leaves at different plant ages is always higher in the younger leaves. Based on feeding and infestation experiments, it is proposed that the distribution of aphids on leaves of different ages is regulated by the hydroxamic acid content.

Semiochemicals mediating spacing behavior of bird cherry-oat aphid, Rhopalosiphum padi feeding on cereals.

Olfactometry using an apterous individual of Rhopalosiphum padi (L.) showed an arresting effect by volatiles from a wheat seedling and a repellent effect by volatiles from a wheat seedling infested with aphids at a high population density (ca. 9 aphids/cm2). Four compounds, 6-methyl-5-hepten-2-one, (−)- and (+)-6-methyl-5-hepten-2-ol, and 2-tridecanone, were identified by GC-MS in air entrainments from the wheat seedlings with high aphid density but not from the wheat seedlings alone. The mixture of the four compounds in the natural proportion counteracted the attractivity of the volatiles from the intact uninfested wheat seedling. The likely role of these compounds in the spacing behavior of this aphid species, when present in high densities on wheat, is discussed.

Differences in Effects of Pyrrolizidine Alkaloids on Five Generalist Insect Herbivore Species

The evolution of the diversity in plant secondary compounds is often thought to be driven by insect herbivores, although there is little empirical evidence for this assumption. To investigate whether generalist insect herbivores could play a role in the evolution of the diversity of related compounds, we examined if (1) related compounds differ in their effects on generalists, (2) there is a synergistic effect among compounds, and (3) effects of related compounds differed among insect species. The effects of pyrrolizidine alkaloids (PAs) were tested on five generalist insect herbivore species of several genera using artificial diets or neutral substrates to which PAs were added. We found evidence that structurally related PAs differed in their effects to the thrips Frankliniella occidentalis, the aphid Myzus persicae, and the locust Locusta migratoria. The individual PAs had no effect on Spodoptera exigua and Mamestra brassicae caterpillars. For S. exigua, we found indications for synergistic deterrent effects of PAs in PA mixtures. The relative effects of PAs differed between insect species. The PA senkirkine had the strongest effect on the thrips, but had no effect at all on the aphids. Our results show that generalist herbivores could potentially play a role in the evolution and maintenance of the diversity of PAs.

TOXICITY AND FEEDING DETERRENCY OF HYDROXAMIC ACIDS FROM GRAMINEAE IN SYNTHETIC DIETS AGAINST THE GREENBUG, SCHIZAPHIS GRAMINUM

2,4‐Dihydroxy‐7‐methoxy‐1,4‐benzoxazin‐3‐one (DIMBOA), the main hydroxamic acid isolated from maize extracts, increased the mortality of Schizaphis graminum when fed in artificial diets. Electrically‐monitored feeding assays showed that DIMBOA acted as a feeding deterrent at concentrations as low as 1 mM. On 12 mM DIMBOA diets, feeding by aphids was completely inhibited. Additional feeding experiments showed that when DIMBOA was ingested there was an increase in aphid mortality relative to that of aphids which did not ingest the compound. Thus, the deleterious effects of DIMBOA on aphids are due to feeding deterrency and toxicity. The 2‐ß‐0 glucoside of DIMBOA (DIMBOA‐Glc), the form in which DIMBOA naturally occurs in Gramineae, had a slight effect on lowering aphid survival and an appreciable feeding‐deterrent effect on diet‐fed aphids. The relevance of the effects of DIMBOA and DIMBOA‐Glc on aphids to resistance of certain graminaceous crops against aphids is discussed.

Hydroxamic acid content in wild and cultivated gramineae

Abstract The content of two hydroxamic acids, 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) and 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), in cultivated and wild species of Gramineae was determined. Zea mays and Triticum durum contained both DIBOA and DIMBOA, the latter being in greater concentrations. Secale cereale and Arundo donax contained only DIBOA, while Elymus gayanus and Chusquea cumingii contained only DIMBOA. Poa annua, Bromus unioloides, Dactylis glomerate, Phalaris canariense, Lolium perenne, Hordeum species, Setaria verticilata, Cynodon dactylon and a Sorghum hybrid lacked these hydroxamic acids. The maximum concentration of hydroxamic acid in A. donax was found at the end of summer, and the minimum at the beginning of winter. In annual plants, such as wheat, while neither acid was found in the fruits, their concentrations in coleoptiles and leaves increased rapidly reaching a maximum 4 days after germination and decreasing gradually afterwards. DIBOA and DIMBOA had toxic and feeding deterrent effects on the greenbug Schizaphis graminum at concentrations similar to those found in both cultivated and wild Gramineae.

Changes in hydroxamic acid levels of wheat plants induced by aphid feeding

Abstract Seedlings of four wheat cultivars were infested with Metopolophium dirhodum nymphs. After aphids had fed for 40 hr on the plants, the levels of the defense metabolite 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one had changed. The changes depended on the cultivar and the portion of the leaf examined. The increase was greater in cultivars Naofen (45.0% at the tip of the leaf, 96.2% at the base where aphids were feeding) and Quilafen (14.7 and 35.8% respectively). The increase was not significant in cultivars Huenufen and Sonka. A simple and sensitive high performance liquid chromatographic method is described for the quantitation of the above benzoxazinone and its demethoxylated analogue using small amounts of plant tissue.

Chemical composition of precloacal secretions of Liolaemus lizards.

Interspecific chemical variation of precloacal pore secretions of Liolaemus lizards was characterized in 20 species, and intraspecific chemical variation was characterized using nine individuals of L. bellii. The latitude (Chile, 18° to 33° South latitude) and altitude (100 to 4350 m.a.s.l.) of the capture sites were recorded, as well as the number of precloacal pores of each lizard. Secretions were analyzed by GC-MS. A total of 49 compounds were found distributed among the 20 species of Liolaemus. Different chemical patterns occurred at intra- and interspecific levels. Compounds belonged to three main families: n-alkanes, long chain carboxylic acids, and steroids. Cholesterol and five carboxylic acids (tetradecanoic, hexadecanoic, hexadecenoic, octadecanoic, and Z-9-octadecenoic) appeared in all species. The number of precloacal pores correlated positively with altitude and negatively with latitude, suggesting that lizards produce more secretions under harsh environments.

Eremophilane, germacrane and shikimic acid derivatives from chilean Senecio species

Abstract From 11 Chilean Senecio species eight new furoeremophilanes and eremophilanolides, eight eremophilane, one cadinane, five germacrane, one oplopanone and five shikimic acid derivatives were isolated in addition to known compounds. The structures were elucidated by high field NMR techniques and the chemotaxonomic aspects are discussed briefly.